PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Cancer Epidemiol Biomarkers Prev. Author manuscript; available in PMC 2017 September 26.
Published in final edited form as:
PMCID: PMC5614602
NIHMSID: NIHMS896787

A Review of Pulmonary Toxicity of Electronic Cigarettes In The Context of Smoking: A Focus On Inflammation

Abstract

The use of electronic cigarettes (e-cigs) is increasing rapidly, but their effects on lung toxicity are largely unknown. Smoking is a well-established cause of lung cancer and respiratory disease, in part through inflammation. It is plausible that e-cig use might affect similar inflammatory pathways. E-cigs are used by some smokers as an aid for quitting or smoking reduction, and by never smokers (e.g., adolescents and young adults). The relative effects for impacting disease risk may differ for these groups. Cell culture and experimental animal data indicate that e-cigs have the potential for inducing inflammation, albeit much less than smoking. Human studies show that e-cig use in smokers is associated with substantial reductions in blood or urinary biomarkers of tobacco toxicants when completely switching and somewhat for dual use. However, the extent to which these biomarkers are surrogates for potential lung toxicity remains unclear. The FDA now has regulatory authority over e-cigs and can regulate product and e-liquid design features such as nicotine content and delivery, voltage, e-liquid formulations, and flavors. All of these factors may impact pulmonary toxicity. This review summarizes current data on pulmonary inflammation related to both smoking and e-cig use, with a focus on human lung biomarkers.

Keywords: inflammation, flavors, nicotine, biomarkers, gene expression, metabolomics

INTRODUCTION

The category of electronic cigarettes (e-cigs) includes a wide variety of products that result in aerosolizing (vaporizing) nicotine and/or flavors for inhalation, along with a carrier (1). Some e-cigs look like cigarettes that have LED lights opposite the mouthpiece (known as a “cig-alike”), some have e-liquid cartridges or refillable tanks, and others are hookah-like. All of these products are battery powered with electronic heating elements that aerosolize carrier liquids that usually contain nicotine. The carriers are vegetable glycerol (VG) and/or propylene glycol (PG). The use of e-cigs and similar products is rapidly rising, with sales totaling more than $3.7 billion per year. All of the major tobacco manufacturers are marketing these products (2). The rates of e-cig use among youth are now higher than cigarette use, although the estimate of use may vary depending on the method of survey (35). Nonetheless, many youth with no history of cigarette use are using e-cigs. In 2015, the prevalence of never-smokers using e-cigs was as high as 19% among youths, and about 10% for adults. About 5% of college students who have never smoked are using e-cigs (6). Fifty percent of adult smokers in the US have tried e-cigs, and 23% currently use both cigarettes and e-cigs (termed dual users) (5, 79). For adults and youth who use multiple tobacco products, the most common combination is cigarettes and e-cigs (5). The reasons for adult e-cig use vary and include hoping to quit smoking, health concerns, and convenience (10). Contributing to the popularity of e-cigs is the availability of many e-liquid flavors, which are attractive to a variety of smokers and non-smokers. However, there is concern that the availability of flavors may promote uptake of other tobacco products among non-smokers and possibly hinder cessation among smokers (11).

There has been significant controversy in the public health community regarding the risks and benefits of e-cigs, resulting in confusion among health care practitioners and the general population (1, 1220). Despite the paucity of human data, there is a growing perception among lay adults that e-cigs are as risky as cigarettes (2123). Most professional organizations have been cautious in their assessment of what is known regarding benefits and risks of e-cigs (2427), reflecting the lack of data regarding e-cigs’ toxicity, particularly relative to that of cigarette smoke. Adding to the difficulty of providing evidence based policy recommendations is the considerable diversity of products in terms of devices, flavors, and solvents. Thus, there is considerable need for studies on e-cig use, behavior, and toxicity (14, 22, 24).

In 2016, the Food and Drug Administration (FDA) Center for Tobacco Products finalized a “deeming” regulation extending its tobacco-related regulatory authority to e-cigs that contain nicotine derived from tobacco, and its current research priorities include the study of e-cig toxicity (1). However, some have voiced concern that increased regulation too soon would hinder an emerging market with the promise for a positive health impact, and also impair long-term observational research needed to assess the risks of e-cigs use at the population level (28). At this time, much of the evidence regarding effects of e-cigs comes from cell culture and animal studies. Biomarkers from the lung, e.g., sputum, exhaled air, and samples collected by bronchoscopy (inserting a scope through the mouth or nose into the lung for bronchial alveolar lavage [BAL], bronchial brushings and biopsies) provide direct evidence for assessing lung toxicity in humans. Although the study of biomarkers in the sputum and exhaled air are useful because they are non-invasive, they also provide more conflicting data and their relevance to lung toxicity is not well understood (29). In contrast, bronchoscopy specimens measure physiological changes directly from lung samples and not subject to factors such as sputum production or gases exhaled that circulated through the body.

When making policy, the FDA based its decisions on likely population-level public health impact of its decisions. Thus, when available, regulatory judgments about e-cigs should be informed by human toxicity data, which ideally considers the heterogeneity in the population, e.g., smoking history (current smokers using e-cigs to quit, former smokers at risk for future cancers and smoking relapse, and never-smokers including adolescents or young adults), age, gender, and rural vs. urban. It also needs to consider patterns of use, including whether e-cigs are being used concurrently with cigarettes or other tobacco products. The FDA has not clarified what evaluation frameworks and risk assessment methods it will use, there are available frameworks to consider that include a robust research agenda for human studies (30).

In this review, we summarize the available bronchoscopy evidence regarding lung inflammation associated with smoking and e-cig used. We focus on inflammation because this pathway is plausibly affected by e-cigs and is important in the etiology of lung cancer and chronic obstructive pulmonary disease (COPD). While there is an extensive literature for the relationship of inflammation to lung cancer and respiratory disease developed from the laboratory (3136), this review will focus on human studies of cigarette smokers and e-cig users. The data reviewed focus on methods for considering a validated biomarker for inflammation that reflects differences between smokers and non-smokers, shows a dose-response relationship with smoking, identifies changes in levels after quitting towards that of a non-smoker, and has the sensitivity to show differences when switching to a less harmful product (37).

Smoking, Inflammation, and the Human Lung

Cigarette smoking is the major cause of lung cancer and COPD, accounting for about 90% of all cases (3840). The smoke contains numerous toxicants that promote inflammatory responses that contribute to the risk for these diseases (31, 32, 34, 4042). Inflammation is considered a hallmark of cancer (43) and COPD (31, 32). The pro-inflammatory effects on the lung are observable in healthy smokers before the onset of disease (36). Cigarette smoke activates alveolar macrophages and airway epithelial cells to release proinflammatory cytokines, resulting in the recruitment of infiltrating inflammatory cells from the blood to the lung. At the same time, normal protective mechanisms for adequate tissue repair by fibroblasts are hindered by cigarette smoke: pro-inflammatory pathways are upregulated and anti-inflammatory ones are down-regulated. Key inflammatory cytokines (e.g., TNF-α, interleukins [IL], and interferons) and cytotoxic mediators such as reactive oxygen species, metalloproteinases and soluble mediators of cell death are induced by smoking with chronic inflammation promoting unregulated cell proliferation, cell invasion, and angiogenesis and genomic instability (34, 44). Smoking drives KRAS oncogenesis (frequently mutated in lung cancer) via inflammation induced by the activation of NF-κB and STAT3, and stimulating lung cell survival (31, 4547). In experimental animals, chemopreventive agents that inhibit inflammation reduce lung tumorigenesis (48). In humans, there is some evidence that non-steroidal anti-inflammatory agents reduce lung cancer risk, although not consistently (34, 4952). COPD is a known risk factor for lung cancer, indicating some shared mechanisms that include an effect on inflammation, although each may have pathways that are not shared (5359).

There are numerous biomarkers that have been used for sampling the lung for inflammation. These will be reviewed below. Each have the potential for assessing inflammatory responses from e-cigs.

Inflammatory cell infiltrates

There are numerous studies indicating that induced sputum has higher inflammatory cell content (e.g., neutrophils) in smokers compared to non-smokers (29, 34, 60); counts tend to be increased with increased smoking exposure. Sputum neutrophils decreased after 6 weeks of smoking cessation (61, 62) in two studies; in a small sputum study there was not a change 4 weeks after quitting (63). Macrophages decrease as early as 1 week following smoking cessation (64). Based on bronchoscopy data, total cell counts, macrophages, lymphocytes, neutrophils, eosinophils and basophils, are much higher in smokers compared to non-smokers (6575). For example, in a study with 132 smokers and 295 never-smokers who underwent bronchoscopy, the smokers had increased numbers of inflammatory cells in BAL samples, most noticeably for macrophages with lesser effects on neutrophils and lymphocytes in a dose dependent manner associated with smoking status (76). Results are similar for studies of bronchial biopsies; e.g., 45 asymptomatic smokers compared to never-smokers had statistically higher numbers of neutrophils, eosinophils, mast cells, and macrophages, with means differing 2–4 fold (70). Important evidence comes from smoking cessation studies. In a study of 28 smokers who underwent bronchoscopy, 12 months after quitting they had reduced numbers of inflammatory cells compared to those who continued smoking (77). Reducing cigarettes per day by more than 50% was also associated with decreased BAL macrophages and neutrophils at 2 months (78).

Inflammatory cytokines

Lung cytokines also are affected by smoking (e.g., IL6, IL8, IL10, and IL33); these cytokines have been shown to be associated with risk of lung cancer and other lung diseases (65, 72, 7986). In sputum, an exposure-response gradient with increased numbers of packs per day has been reported (60, 87). For example, in a bronchial biopsy study of 45 asymptomatic smokers and never-smokers, smokers had 2- to 4-fold higher IL8 compared to never smokers (70). In another study that used bronchial biopsies and immunohistochemistry in 47 subjects, IL6 was associated with smoking (85). Inflammatory cytokines, such as IL8, are higher in patients with emphysema (79). While in one cross-sectional study, there was no difference between smokers and non-smokers in IL6 and IL8 (88), a smoking cessation study reported statistically significant reductions at 12 months for IL8 (65). The reliability of repeated measures for BAL cytokines has been demonstrated, but it also should be noted that blood cytokines are not a good surrogate for lung cytokines (75).

mRNA expression

Differences in mRNA expression for smokers versus non-smokers have been well described. These differences, including those related to inflammation, are used for the early detection of lung cancer (8996). Expression profiles in the lung for genes that are up- and down-regulated have been described and shown to cluster with smoking status (90). In comparisons of 16 smokers and 17 non-smokers, genes coding for inflammatory cytokines and innate immunity, and response to oxidants and xenobiotics were differentially expressed (91). Dose-response mRNA expression changes to urine cotinine have been identified in 121 subjects who were smoking the equivalent of only a few cigarettes per day (95). In this large cross-sectional study, pathway analysis implicated genes involved in the metabolism of xenobiotics, eicosanoid metabolism, and oxidative stress responses.

MicroRNAs (miRNAs)

MiRNAs are short non-coding single-stranded RNA transcripts that negatively regulate mRNA expression at the post-transcriptional level. There are many studies linking smoking and COPD via changes in miRNA expression and inflammation pathways, for example miR-146a altered by smoking (97101). In vitro studies using cigarette smoke condensate (CSC) on human bronchial epithelial cell lines show up-regulation of miR-101 and miR-144, which target the cystic fibrosis transmembrane conductance regulator found to mitigate airway cell inflammation, and also are found to be up-regulated in COPD (102, 103). Other changes in vitro include a decrease in miR-200c, related to NF-κB-mediated inflammation and thought to increase epithelial to mesenchymal transition (EMT) associated with tissue remodeling and cigarette smoking in COPD (104107). Experimental animal models for cigarette smoke exposure have identified altered expression of several miRNAs including, miR-146a, miR-92a-2*, miR-147, miR-21 miR-20 and miR-181. Both miR21 and miR-181a are involved in chronic systemic inflammation (108) and have been reported to be affected by smoking in humans (109). Cross-sectional studies assessing the sputum of smokers and non-smokers identified let-7c as over-expressed and inversely correlated with tumor necrosis factor receptor type II, implicated in COPD and inflammation pathogenesis and a predicted target gene of let-7c) was inversely correlated with the sputum levels of let-7c (29, 110, 111), and alveolar macrophages alter expression of miR-210, miR-150, miR-146b-3p, and miR-452 (112). The latter miRNA targets matrix metalloproteinase-12, which is increased in the sputum of patients with COPD and contributes the development of emphysema (113, 114). In a recent study of 19 subjects in a 3-month smoking cessation trial, 34 miRNAs in bronchial brushings were differentially expressed between the smokers and baseline non-smokers, and 22 of these decreased with smoking cessation (115). The major function of both the up- and down-regulated miRNAs was inflammation, with several targets associated with NF-κB pathway. There are other examples of miRNAs related to cigarette smoke and inflammation considered to be involved in COPD, such as effects in smooth muscle, fibroblasts, macrophages and neutrophils, and specific miRNA changes in bronchial epithelia of smokers versus non-smokers (97, 116).

Untargeted metabolomic profiles

Metabolomics is an emerging technology that is being used to identify new biomarkers of tobacco smoke exposure (117125), and for studying COPD (126128). The assay can be used to identify thousands of small molecules (<1500 Daltons) reflective of exogenous exposures and cellular responses to those exposures. Metabolomics is now being widely applied to evaluate disease and disease causation (129132). In the case of smoking, metabolomic screening can reveal changes induced by cigarette smoke constituents as well as those due to endogenous cellular responses to cigarette smoke. In an animal model, BAL metabolomics have mapped with emphysema progression, identifying a lung specific L-carnitine as a central metabolite (133). In our studies, we have 1) demonstrated the feasibility for assessing smoking-related biomarkers in blood and urine (134); 2) identified novel biomarkers related to smoking (e.g., glycophospholipids and pathways related to inhibition of cAMP), including some that differ by gender and race (117); and, 3) identified the presence of menthol metabolites (117). We are not aware of metabolomics studies in the lung for smoking-related changes, but metabolomics have shown changes in smokers’ sputum (135), and have been used in a bronchoscopy study for air pollution (136).

Nitric oxide

Fractional exhaled nitric oxide (FeNO) is a validated marker of lower airway inflammation that is simple to assess, non-invasive and reproducible (137, 138). It is used for the diagnosis and treatment of asthma in children (139143). Nitric oxide (NO) is synthesized in the lung by NO synthase (NOS) and the oxidation of L-arginine to L-citrulline. The inducible NOS (iNOS) is transcriptionally regulated by pro-inflammatory cytokines in epithelial cells and macrophages in the airways (144). FeNO has been shown to be decreased by almost 50% in smokers in several cross-sectional studies (145148), possibly related to the large amount of NO in cigarette smoke (146). The reduction in FeNO also is thought to be related to nitric oxide synthase inhibition due to cigarette smoke carbon monoxide and/or oxygen free radicals (146, 149). Reduced FeNO has been reported to be significantly associated with increased neutrophilic inflammation (150).

E-Cig Toxicity

While there are numerous recent reviews for the risks and benefits of e-cigs, there are substantial research gaps in our knowledge of the effects of e-cigs on inflammation (20, 22). There is some evidence that they do affect inflammation as indicated below. However, there are only a few studies that provide data related to lung inflammation; most human studies assess cigarette smoke exposure biomarkers. This section reviews recent studies that support the hypothesis that e-cigs might affect inflammation in the human lung.

E-cig aerosol constituents

E-liquids, in addition to nicotine, are composed mostly of PG, VG, and flavors. When used in foods and skin products, these carriers and flavors are “generally regarded as safe” by the FDA (151, 152). However, it is unknown what happens to the lung when these constituents are heated and inhaled. E-cig heated PG can be converted to propylene oxide (1, 153), which is an irritant and an International Agency for Research on Cancer group 2b carcinogen (possibly carcinogenic to humans) (154). Heated VG and PG can be converted to acrolein, acetaldehyde, and formaldehyde, which also are known strong irritants that affect inflammation (155157). In addition, the e-cig aerosols include many chemical constituents in e-cig flavors, including glycidol, acetol, and diacetyl (158) as well as tobacco specific nitrosamines (TSNAs), aromatic hydrocarbons, acetone, and volatile organic compounds (VOC) (e.g., benzaldehyde, propionaldehyde, crotonaldehyde) (1, 22, 157, 159176). A recent study using mass spectroscopy identified over 115 VOCs in e-cig aerosol, many that were not present in the unheated liquids (160), while another identified trace quantities of benzene, methyl ethyl ketone, toluene, xylene, styrene, and acetic acid (177). However, their presence is substantially reduced compared to cigarette smoke.

The amount of aerosol and constituent levels in e-cig aerosols can greatly increase under different heating conditions that occur when using higher voltages of the device. For example, increasing temperature overall increases the overall amount of aerosol of flavor-free liquids, as well as total aldehydes, formaldehyde, acetaldehyde and acrolein, and the release of inflammatory cytokines, as much as 10-fold with higher voltages (157, 158, 178182).

Laboratory Studies

There has been some toxicology testing for e-cig liquids and aerosols, but these are limited and the relationship to human disease risk is unclear (12, 183, 184). Existing studies suggest that the toxicological responses are qualitatively similar to smoking, e.g., exposing cell lines and cultures to the aerosols induces a pro-inflammatory effect (185, 186), disruption to epithelia barriers (187), oxidative stress (188), cytotoxicity (189), neutrophil inflammatory response (190) and DNA damage (191, 192). However, the magnitude of effect is low compared to cigarette smoke and aerosols were not found to be mutagenic (193). Normal human bronchial epithelial (NHBE) cells exposed to e-cig aerosols, with or without nicotine, increase IL-6 and IL-8 cytokine levels (194). Another study reported a change in the gene expression pattern of NHBE cells with silenced p53 and activated KRAS when exposed to e-cig aerosol (153). Separately, e-cig liquid was assessed in NHBE cells in parallel with a knock-out mouse model; there were increased rates of infection, inflammatory markers and altered gene expression (195). Metals present in e-cig aerosol are capable of causing cell injury and inflammatory cytokine induction, e.g., in human lung fibroblasts (196). There have been some studies of gene expression in cultured human bronchial epithelial cells showing changes in profiles that are much less than smoking but clearly distinctive (197). The pathways that have been implicated in these studies include phospholipid and fatty acid triacylglycerol metabolism, with enrichment of cell cycle associated functions (e.g., cell cycle checkpoint regulation, control of mitosis) and immune system function.

In vitro studies using human bronchial epithelial cells demonstrate that increasing voltage decreases cell viability and increases the release of inflammatory cytokines (IL-1β, IL-6, IL-10,CXCL1, CXCL2 and CXCL10) (178). Experimental animal studies have also shown that there are some toxic effects in the lungs of e-cig aerosols, which includes pro-inflammatory responses (12, 184, 198). While in vivo studies indicate that aerosolized PG or VG alone only have slight toxic effects in the lung (199202), more recent data using e-cig devices are identifying various effects on inflammatory and other responses. For example, mice exposed to e-cig aerosols with or without nicotine showed increased lung macrophages, neutrophils and lymphocytes (194). Separately, mice exposed to e-cig aerosol intratracheally had an increased rate of inflammatory infiltrate and cytokines, and IgE production (203). Other studies report lung oxidant reactivity and reactive oxygen species increasing inflammatory cytokines (i.e., increasing IL-8), changes in lung fibroblasts thought to be part of COPD pathogenesis, and altered redox balance (204). There also is evidence that e-cig aerosols may promote oxidative damage, mitochondrial reactive oxygen species, a dose-dependent loss of lung epithelial barrier function and increased inflammation-related intracellular ceramides and myosin light chain phosphorylation (198). A recent animal study showed measurable effects on inflammation and lung injury for both cigarette smoke and e-cigs, but much less for the latter (186).

Human Studies

Important information about potential toxic exposures from e-cigs can be learned from human biomarker studies. There are several studies that indicate that e-cig users have substantially less toxicant exposure than cigarettes, depending on either complete quitting or the amount of smoking reduction, both for clinical symptoms and by reducing exposure to cigarette smoke exposure biomarkers. The studies are either cross-sectional studies or clinical trials that assess complete switching or dual use, but these studies are all small. The most informative studies are the ones that are published most recently, because they provide data for the most advanced generation e-cigs. All of the published studies that we are aware of use peripheral biomarkers (e.g., urine and blood) or exhaled air, and not those collected directly from the lung. They also represent only short term exposures, lacking direct data for the long term consequences, if any, of e-cig use.

In humans, e-cig acute health effects are minimal and short-lived (27, 205212). The most common adverse effects reported across studies were nausea, headache, cough, and mouth/throat irritation, which were similar or less compared to nicotine patches. Although adolescents using e-cigs reported an overall increased rate of chronic bronchitis symptoms (213), smokers with COPD who switched to e-cigs had a reduction in symptoms and an improved quality of life (214, 215).

In studies of smokers completely switching to e-cigs, there are substantial reductions in such exposures. In a 2016 trial of 419 smokers randomized to an e-cig or continued smoking over 12 weeks, Cravo et al. (209), reported that assignment to e-cigs was associated with statistically significant decreases in urinary metabolites of acrolein (3-HPMA), benzene (S-PMA) and NNAL (a pulmonary carcinogen) compared to controls. Another important measure in that study was urinary PG, which almost doubled after one month of e-cig use, indicating that this could be a biomarker for exposure generally to e-cigs. In another recent study of 20 smokers switched for only two weeks, authors reported reductions for a large panel of biomarkers, including a 50% reduction in acrolein metabolites (carbon monoxide [CO], NNAL and all measured VOCs and PAHs) (216). McRobbie et al. (217) reported that among 40 smokers switched to e-cigs use, there was a statistically significant decrease in acrolein exposure after 4 weeks. Pulvers and co-workers (2016) studied 40 smokers switched to e-cigs and reported substantial reductions (to non-smoking levels) for urinary NNAL, but only for 2 (benzene and acrylonitrile) of 8 VOCs (218). CO also was substantially reduced. O’Connel et al. (219, 220), reported on a five day trial of 105 subjects confined to a clinical facility; they found similar reductions in the urinary biomarkers and CO. Lastly, a one-year clinical trial reported significant reductions in exhaled CO (221). Thus, compared to smoking, there appears to be a significant overall reduction in biomarkers for persons completely switching to e-cigs, but it is not known if these peripheral biomarkers reflect effects in the lung.

There are 3 studies for e-cig use that includes smokers who dually use e-cigs (217, 222, 223). A cross-sectional study was published by Shahab and coworkers (2017), where 5 groups of long-term smokers or former smokers were recruited for a total n of 181 subjects (222). These groups were long term e-cig users, long term NRT users, smokers, and smokers who dually used either e-cigs or NRT. All groups had similar total nicotine equivalents, indicating that the products chosen by the smokers or former smokers all were able to deliver the particular levels of nicotine needed by the smoker. However, the levels were numerically higher compared to smokers for the e-cig dual users (157%), not being statistically different perhaps due to the small numbers of subjects. TSNAs were substantially and statistically significantly lower for the NRT-only (12% of smokers) and the e-cig-only groups (3% of smokers), and they were also statistically lower for the smoker-NRT dual users (57%). However, the levels were not statistically lower for the smoker-e-cig dual users (81%), also perhaps due to the small numbers. It may also be due to lower cigarettes per day, and while not statistically different, the mean numbers were 13.9 for the smokers, 10.8 for the smoker-NRT dual users and 11.9 for the smoker-e-cig dual users. The dual users with NRT or e-cigs, compared to smokers had similar acrolein levels (107% and 91%, respectively), and the exclusive NRT and e-cig users had similar levels (35% and 33%, respectively). The similar acrolein levels for the exclusive NRT and e-cig users indicates that there was no measurable increase in levels from e-cig aerosols. Other volatile organics had similar results, where there were clear decreases for complete switching to NRT or e-cigs, but there were not for the dual users. Thus, although the data is cross-sectional in nature, the results are consistent with substantial reductions in smoke toxicants when exclusively switching to e-cigs, but a reduction in dual use is more modest and likely depends on the amount of smoking reduction that can be achieved. Somewhat consistent with this cross-sectional study, McRobbie and coworkers (2015) reported that dual users after 4 weeks had reductions in cotinine, CO and acrolein compared to smokers based on the reduction in numbers of cigarettes used per day (217). Using a novel study design, Jorenby and coworkers (2017) studied long term smokers and e-cig dual users (n=74) and smokers (n=74) (223). Both groups were asked to reduce their cigarettes per day by 75% over 2 weeks, allowed to resume their regular use and then asked to quit smoking for 3 days. The e-cig users were free to increase their e-cig use using whatever e-cig device they normally used, and were found to have increased their vaping by more than 4 times while reducing smoking or quitting. CO substantially decreased during reduction and quitting, although the levels for the two groups did not differ from each other.

Four switching studies showed a decrease FeNO (219, 221, 224, 225) (including a 1-year trial), while another found no difference (226), and another with methodological limitations (i.e., e-cigs and controls were tested on different days) reported an increase (227).

Flavors

Most e-cig users indicate that their first and usual e-cigs are flavored, with non-tobacco flavors used by a strong majority of college students (95%) and young adult (71%) e-cigs users, but a minority (44%) of adults (228). In most cases, non-tobacco flavors are fruit and candy flavors, especially among never-smokers and former smokers who take up e-cigs, without any discernible patterns for type of fruit or candy flavor. A 2016 study showed that adults prefer menthol, mint, and fruit, followed by candy and chocolate (229). A recent review by Hoffman et al. (230), provided similar results, including preferences for cherry, candy, strawberry, orange, apple and cinnamon, with these higher preferences in adolescents than adults. The choice among youth and former smokers typically is a fruit or candy flavor, while among smokers it is a tobacco flavor (228).

There are data that some flavorings may induce lung inflammation. For example, diacetyl present in many e-cig liquids (found in caramel, butterscotch, watermelon, pina colada, and strawberry) has received widespread attention because it is a cause of bronchiolitis obliterans (popcorn lung) in the occupational setting (231, 232). Additional research has indicated that some flavors may be a source of aldehydes (233). For example, cherry flavored e-cig liquids yield increased amount of benzaldehyde, a key ingredient for many fruit flavors (176). There are a few in vitro and in vivo studies for the effects of flavors in the context of e-cig aerosols (in contrast to food uses where they are generally regarded as safe). Using a high through-put screening method based on cell death endpoints, 7 flavors used in e-cigs showed positive results, such as the chocolate flavoring 2,5-dimethylpyrazine (234). Using a different cell culture model for cytotoxicity that assesses vapors from e-liquids (volatility of the liquid, not the aerosols emitted from an e-cig), cinnamon-flavorings had the most cytotoxicity among 36 different e-liquids and confirmed among sources from multiple manufacturers; the constituents in the cinnamon-flavored liquids thought to be responsible for the cytotoxicity were cinnamaldehyde (CAD) and 2-methoxycinnamaldehyde (2MOCA) (235, 236). In vivo, one study reported no effect in rats, but they chose a mixture of flavors with constituents not known to cause cell damage or inflammation (237). Menthol is a flavor of concern for enhancing the abuse liability in cigarettes (238). Although there are some toxic effects of menthol, there are no data for the human lung (239). Menthol flavorings for e-liquids may also have diacetyl (231). A recent study has demonstrated that several flavorings induce expression of inflammatory cytokines in lung cell cultures, where acetoin and maltol are among the most potent (240).

Nicotine

Nicotine content can be regulated by the FDA and some considerations for this will be affected by the addictiveness (i.e., abuse liability) of the product, but toxicity considerations may also apply. Nicotine content varies widely among e-cigs, and users can formulate e-liquids with their own choice of nicotine concentration. It is well established that nicotine is highly bioactive in that it induces proliferation, inhibits apoptosis, promotes the epithelial to mesenchymal transition (EMT), and promotes angiogenesis (55, 241). All of these are important components of cancer and COPD development (55, 198). To date, nicotine is not considered a carcinogen for humans, as nicotine replacement therapy (NRT) and low-TSNA smokeless tobacco (snus) have not demonstrated increased risks of cancer (242). Regarding inflammation, nicotine is both pro- and anti-inflammatory, and therefore theoretically able to affect cancer and COPD pathogenesis in different ways (241, 243248). In cell culture studies of human bronchial epithelial cells, while cigarette smoke condensate increases inflammatory cytokine production, nicotine alone does not, and pretreatment with nicotine reduced the condensate effects (244). In a study of wound healing in smokers, compared to continued smoking and quitting with or without nicotine, it was observed that NRT reduced inflammation and macrophage infiltration, but not angiogenesis (243). In human nasal epithelial cells, in contrast to cigarette smoke and acrolein, nicotine induced inflammatory cytokine response (249). In vivo, nicotine was able to inhibit acute lung injury in mice through anti-inflammatory effects (248). The anti-inflammatory effect may be through the stimulation of nicotinic receptors present in lung and other cells, and there is data that nicotinic receptor agonists reduce acute lung injury (245, 250, 251). There are nicotinic receptors on macrophages that reduce pro-inflammatory cytokines while having no effect on anti-inflammatory cytokines (252). In contrast to data for nicotine reducing inflammation, other data, using different experimental models, indicate that nicotine may increase inflammatory response because of its toxic effects on the lung epithelium (187, 195). Pro-inflammatory effects have been observed in cell culture models of vascular smooth muscles and in atherogenesis, because nicotine can induce oxidative damage (253, 254). It also has been reported that nicotinic receptors both increase and decrease inflammation pathways in human lung and lung cells, depending on the experimental model and receptor subunits (but better lung function (250)) (255258). Because of the potential anti-inflammatory effect of nicotine, NRT has been explored as a treatment for inflammatory disease, such as ulcerative colitis, but results have been inconclusive to date (247, 259).

Summary and Research Gaps

Numerous studies demonstrate that cigarette smoking induces pulmonary inflammation in humans, as measured by cellular infiltrates, altered cytokines, and changes in gene expression. Importantly, these are biomarkers of effect, rather than biomarkers of exposure, and many can be considered as validated for assessing smoking and harm reduction. Inflammation is considered important for the development of both lung cancer and COPD. There is sufficient data about e-cig aerosols to also indicate a pro-inflammatory effect that warrants further investigation, given the toxicant and irritant constituents in e-cig aerosols. The bronchoscopic biomarkers discussed in this review represent direct evidence for the inflammatory effects in the human lung, the target organ for lung cancer and COPD. The studies also indicate that they are valid markers of tobacco smoke exposure because of the identified differences between smokers and non-smokers, the dose-response with smoking levels, and the reversal of effects with cessation and smoking reduction (37). Thus, assessing inflammation for e-cig toxicity is feasible. An important research gap for currently available studies are the lack of assessing long term chronic effects; all studies to date assess short term exposures and acute changes in health effects or biomarkers of recent exposures. Thus, studies of longer clinical trials and observational cohort studies with repeated measures are needed. Focusing on the lung provides some data for more chronic effects, but definitive data would need to come longer term observational studies and clinical trials.

E-cigs may have the potential for supporting smoking cessation, although current data is not yet sufficient to support specific recommendations for their use (24, 260, 261). Whether or not the efficacy of e-cigs becomes established for assisting smoking cessation, their safety profile also needs to be determined. An important consideration about safety is the context of the e-cig user. While e-cigs are likely less toxic than smoking given the lack of most combustible tobacco constituents and evidence by human biomarker studies, the amount of reduced toxicity that may occur in the lung remains unknown both for a long-term user who quits smoking and for dual users. For dual users, the extent of harm reduction, if any, will likely depend on the amount of smoking reduction. At the other end of the spectrum, while the conceptual effects of e-cig aerosols promoting inflammation may be much less than smoking, it also is unknown if the use of e-cigs in never smokers with naïve lungs (e.g., adolescents who become nicotine dependent with e-cigs) would have a clinically significant impact on future disease risk.

Given the chemical complexity of the e-cig aerosol, and that cigarette smoking induces pulmonary inflammation, studies for e-cig lung effects in both smokers and never-smokers are needed. While cross-sectional studies provide relevant information, they are subject to bias and confounding, and do not demonstrate causal relationships. In contrast, clinical trials for both smokers and never-smokers can provide better evidence for the uptake of e-cigs and related exposures. The studies to date, however, only measure blood and urine biomarkers, where it is unknown if these biomarkers are suitable surrogates for lung inflammation and disease risk. This could only be determined for humans using biomarkers obtained from lung sampling, i.e., bronchoscopy.

While bronchoscopy is an invasive procedure, research bronchoscopies are commonly done for healthy smokers and non-smokers to understand the effects of smoking, and are considered sufficiently safe for the research of healthy subjects (6573, 76, 77, 86, 89, 94, 95, 115, 262269). The risk of the procedure increases with the number of lavaged segments. For persons with reactive airway disease there can be wheezing and bronchospasm. Non-invasive tests are available to assess pulmonary inflammation, such as induced sputum, but these studies also have complications (e. g., inducing bronchospasm) and the results are less consistent than bronchoscopy studies. FeNO, however, is a validated marker with utility to assess e-cig use and lung effects.

The induction of inflammation by e-cigs may differentially impact lung cancer and COPD risk, because e-cig aerosols do not have the complexity of carcinogen exposure found in cigarette smoke. While it is entirely speculative at this point, it may be that long-term e-cig use heightens one’s risk for COPD; whether the inflammatory effect is sufficient to increase risk in never smokers, or in smokers with existing lung damage, is an open research question. It may be that the risk for an individual smoker who switches to e-cigs may decrease, but as overall use in the population increases, including use by never smokers and former smokers, population-level risks might increase. (270, 271). Risk assessment models are being developed to estimate these possible effects (272274). The role of nicotine also needs to be considered, as it has both pro- and anti-inflammatory potential, making it unclear how nicotine content may mediate the effects of the other aerosol constituents.

A methodological challenge to studying e-cigs and their health effects are the almost countless brands on the market of differing design and performance. There has been a successive generation of manufactured devices that have generally improved on use and nicotine delivery. Thus, the generalizability of studies that assess one type of e-cig may not be reflective of the marketplace, and which device was used is an important consideration. Another challenge to the researcher when studying particular products is that the manufacturer may alter the design or withdraw the product from the market, which may affect the research results. These issues however, are somewhat addressed by the recently developed National Institutes of Drug Abuse production of a standardized research electronic cigarette (SREC; https://www.drugabuse.gov/funding/supplemental-information-nida-e-cig) that can be used for both laboratory and human studies. While this advancement will provide sustainability and allow for comparing data from different research studies, the generalizability would still be a continued limitation.

The FDA now has the regulatory authority to regulate e-cig product design and e-liquid formulations. Subjects for further research and possible regulation include voltage, flavors, and nicotine content. Voltage and higher temperatures have been shown to increase the toxicity of e-cig aerosol content. Flavors are not all one type of chemical constituent, and different flavors may impact morbidity risk differently. And nicotine content may play a protective or adverse effect that can be additive or synergistic. As indicated above, there is an urgent and broad research agenda to identify the magnitude of effect for e-cig pulmonary toxicity, and how that magnitude impacts the risk for never-smokers and smokers.

Table 1
Summary of Human Biomarker Studies

Acknowledgments

Financial support: Research reported in this publication was supported by grant numbers P50CA180908 and U19CA157345 from the National Cancer Institute of the National Institutes of Health (NIH) and the Food and Drug Administration (FDA) Center for Tobacco Products. Research reported in this publication also was supported by the National Cancer Institute of the National Institutes of Health under Award Number K07CA197221 (MB). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH or the Food and Drug Administration.

Footnotes

Disclosures: Drs. Shields and Brasky serve or have served as an expert witness in tobacco company litigation on behalf of plaintiffs. The other authors declare that they have no conflict of interest.

Literature Citations

1. Grana R, Benowitz N, Glantz SA. E-Cigarettes: A Scientific Review. Circulation. 2014;129(19):1972–86. doi: 10.1161/CIRCULATIONAHA.114.007667. [PMC free article] [PubMed] [Cross Ref]
2. Adams S. E-Cigarette Manufacturers Say New Regulations Will Devastate The Industry. Forbes. 2016
3. Singh T, Kennedy S, Marynak K, Persoskie A, Melstrom P, King BA. Characteristics of Electronic Cigarette Use Among Middle and High School Students - United States, 2015. MMWR Morb Mortal Wkly Rep. 2016;65(5051):1425–9. doi: 10.15585/mmwr.mm655051a2. [PubMed] [Cross Ref]
4. Singh T, Arrazola RA, Corey CG, Husten CG, Neff LJ, Homa DM, et al. Tobacco Use Among Middle and High School Students--United States, 2011–2015. MMWR Morb Mortal Wkly Rep. 2016;65(14):361–7. doi: 10.15585/mmwr.mm6514a1. [PubMed] [Cross Ref]
5. Kasza KA, Ambrose BK, Conway KP, Borek N, Taylor K, Goniewicz ML, et al. Tobacco-Product Use by Adults and Youths in the United States in 2013 and 2014. N Engl J Med. 2017;376(4):342–53. doi: 10.1056/NEJMsa1607538. [PMC free article] [PubMed] [Cross Ref]
6. Spindle TR, Hiler MM, Cooke ME, Eissenberg T, Kendler KS, Dick DM. Electronic cigarette use and uptake of cigarette smoking: A longitudinal examination of U.S. college students. Addictive Behaviors. 2017;67:66–72. doi http://dx.doi.org/10.1016/j.addbeh.2016.12.009. [PubMed]
7. Huang LL, Kowitt SD, Sutfin EL, Patel T, Ranney LM, Goldstein AO. Electronic Cigarette Use Among High School Students and Its Association With Cigarette Use And Smoking Cessation, North Carolina Youth Tobacco Surveys, 2011 and 2013. Prev Chronic Dis. 2016;13:E103. doi: 10.5888/pcd13.150564. [PMC free article] [PubMed] [Cross Ref]
8. Weaver SR, Majeed BA, Pechacek TF, Nyman AL, Gregory KR, Eriksen MP. Use of electronic nicotine delivery systems and other tobacco products among USA adults, 2014: results from a national survey. Int J Public Health. 2016;61(2):177–88. doi: 10.1007/s00038-015-0761-0. [PMC free article] [PubMed] [Cross Ref]
9. McMillen RC, Gottlieb MA, Shaefer RM, Winickoff JP, Klein JD. Trends in Electronic Cigarette Use Among U.S. Adults: Use is Increasing in Both Smokers and Nonsmokers. Nicotine Tob Res. 2015;17(10):1195–202. doi: 10.1093/ntr/ntu213. [PubMed] [Cross Ref]
10. Patel D, Davis KC, Cox S, Bradfield B, King BA, Shafer P, et al. Reasons for current E-cigarette use among U.S. adults. Prev Med. 2016;93:14–20. doi: 10.1016/j.ypmed.2016.09.011. [PubMed] [Cross Ref]
11. Smith DM, Bansal-Travers M, Huang J, Barker D, Hyland AJ, Chaloupka F. Association between use of flavoured tobacco products and quit behaviours: findings from a cross-sectional survey of US adult tobacco users. Tob Control. 2016;25(Suppl 2):i73–ii80. doi: 10.1136/tobaccocontrol-2016-053313. [PubMed] [Cross Ref]
12. Dinakar C, O’Connor GT. The Health Effects of Electronic Cigarettes. N Engl J Med. 2016;375(14):1372–81. doi: 10.1056/NEJMra1502466. [PubMed] [Cross Ref]
13. Bareham D, Ahmadi K, Elie M, Jones AW. E-cigarettes: controversies within the controversy. Lancet Respir Med. 2016;4(11):868–9. doi: 10.1016/s2213-2600(16)30312-5. [PubMed] [Cross Ref]
14. Correa JB, Ariel I, Menzie NS, Brandon TH. Documenting the emergence of electronic nicotine delivery systems as a disruptive technology in nicotine and tobacco science. Addict Behav. 2017;65:179–84. doi: 10.1016/j.addbeh.2016.10.021. [PubMed] [Cross Ref]
15. Fairchild RB AL. Smoke and fire over e-cigarettes, Science. Public Health. 2015 [PubMed]
16. McKee M, Chapman S, Daube M, Glantz S. The debate on electronic cigarettes. The Lancet. 2014;384(9960):2107. doi: 10.1016/S0140-6736(14)62366-7. [PubMed] [Cross Ref]
17. Hajek P. Electronic cigarettes have a potential for huge public health benefit. BMC Medicine. 2014;12(1) doi: 10.1186/s12916-014-0225-z. [PMC free article] [PubMed] [Cross Ref]
18. Oh AY, Kacker A. Do electronic cigarettes impart a lower potential disease burden than conventional tobacco cigarettes?: Review on e-cigarette vapor versus tobacco smoke: Review on E-Cigarette Vapor Versus Tobacco Smoke. The Laryngoscope. 2014;124(12):2702–6. doi: 10.1002/lary.24750. [PubMed] [Cross Ref]
19. McKee M. Electronic cigarettes: peering through the smokescreen. Postgraduate Medical Journal. 2014;90(1069):607–9. doi: 10.1136/postgradmedj-2014-133029. [PubMed] [Cross Ref]
20. Rowell TR, Tarran R. Will chronic e-cigarette use cause lung disease? Am J Physiol Lung Cell Mol Physiol. 2015;309(12):L1398–409. doi: 10.1152/ajplung.00272.2015. [PubMed] [Cross Ref]
21. Majeed BA, Weaver SR, Gregory KR, Whitney CF, Slovic P, Pechacek TF, et al. Changing Perceptions of Harm of E-Cigarettes Among U.S. Adults, 2012–2015. American Journal of Preventive Medicine. doi: 10.1016/j.amepre.2016.08.039. [PubMed] [Cross Ref]
22. Kaisar MA, Prasad S, Liles T, Cucullo L. A decade of e-cigarettes: Limited research & unresolved safety concerns. Toxicology. 2016;365:67–75. doi: 10.1016/j.tox.2016.07.020. [PMC free article] [PubMed] [Cross Ref]
23. Xu Y, Guo Y, Liu K, Liu Z, Wang X. E-Cigarette Awareness, Use, and Harm Perception among Adults: A Meta-Analysis of Observational Studies. PLoS One. 2016;11(11):e0165938. doi: 10.1371/journal.pone.0165938. [PMC free article] [PubMed] [Cross Ref]
24. Brandon TH, Goniewicz ML, Hanna NH, Hatsukami DK, Herbst RS, Hobin JA, et al. Electronic nicotine delivery systems: a policy statement from the American Association for Cancer Research and the American Society of Clinical Oncology. Clin Cancer Res. 2015;21(3):514–25. doi: 10.1158/1078-0432.CCR-14-2544. [PubMed] [Cross Ref]
25. Schraufnagel DE, Blasi F, Drummond MB, Lam DCL, Latif E, Rosen MJ, et al. Electronic Cigarettes. A Position Statement of the Forum of International Respiratory Societies. American Journal of Respiratory and Critical Care Medicine. 2014;190(6):611–8. doi: 10.1164/rccm.201407-1198PP. [PubMed] [Cross Ref]
26. McCarthy M. American Medical Association calls for stricter regulation of electronic cigarettes. BMJ. 2014 [PubMed]
27. Tomashefski A. The perceived effects of electronic cigarettes on health by adult users: A state of the science systematic literature review. Journal of the American Association of Nurse Practitioners. 2016;28(9):510–5. doi: 10.1002/2327-6924.12358. [PubMed] [Cross Ref]
28. Sarewitz D. Allow use of electronic cigarettes to assess risk. Nature. 2014;512(7515):349. doi: 10.1038/512349a. [PubMed] [Cross Ref]
29. Van Pottelberge GR, Mestdagh P, Bracke KR, Thas O, van Durme YM, Joos GF, et al. MicroRNA expression in induced sputum of smokers and patients with chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2011;183(7):898–906. doi: 10.1164/rccm.201002-0304OC. [PubMed] [Cross Ref]
30. Berman ML, Connolly G, Cummings KM, Djordjevic MV, Hatsukami DK, Henningfield JE, et al. Providing a Science Base for the Evaluation of Tobacco Products. Tob Regul Sci. 2015;1(1):76–93. doi: 10.18001/TRS.1.1.8. [PMC free article] [PubMed] [Cross Ref]
31. Caramori G, Kirkham P, Barczyk A, Di Stefano A, Adcock I. Molecular pathogenesis of cigarette smoking-induced stable COPD. Ann N Y Acad Sci. 2015;1340:55–64. doi: 10.1111/nyas.12619. [PubMed] [Cross Ref]
32. Crotty Alexander LE, Shin S, Hwang JH. Inflammatory Diseases of the Lung Induced by Conventional Cigarette Smoke: A Review. Chest. 2015;148(5):1307–22. doi: 10.1378/chest.15-0409. [PubMed] [Cross Ref]
33. Garvey C. Recent updates in chronic obstructive pulmonary disease. Postgraduate medicine. 2016;128(2):231–8. doi: 10.1080/00325481.2016.1118352. [PubMed] [Cross Ref]
34. Gomes M, Teixeira AL, Coelho A, Araujo A, Medeiros R. The role of inflammation in lung cancer. Adv Exp Med Biol. 2014;816:1–23. doi: 10.1007/978-3-0348-0837-8_1. [PubMed] [Cross Ref]
35. Okada F. Inflammation-related carcinogenesis: current findings in epidemiological trends, causes and mechanisms. Yonago acta medica. 2014;57(2):65–72. [PMC free article] [PubMed]
36. Zhou Z, Chen P, Peng H. Are healthy smokers really healthy? Tob Induc Dis. 2016;14:35. doi: 10.1186/s12971-016-0101-z. [PMC free article] [PubMed] [Cross Ref]
37. Hatsukami DK, Benowitz NL, Rennard SI, Oncken C, Hecht SS. Biomarkers to assess the utility of potential reduced exposure tobacco products. Nicotine Tob Res. 2006;8(4):599–622. [PubMed]
38. UDoHaH, editor. Services. Atlanta: US Department of Health and Human Services, Centres for Disease Control and Prevention, National Centre for Chronic Disease Prevetnion and Health Promotion, Office on Smoking and Health; 2014. The Health Consequences of Smoking: 50 Years of Progress: A Report of the Sugeon General.
39. The health consequences of smoking: A report of the Surgeon General. Atlanta, GA: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2004.
40. Services. USDoHaH. A Report of the Surgeon General. Rockville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2010. How Tobacco Smoke Causes Disease: The Biology and Behavioral Basis for Smoking-Attributable Disease. [PubMed]
41. U.S. Department of Health and Human Services. The Health Consequences of Smoking -- 50 Years of Progress: A Report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2014.
42. Malkinson AM. Role of inflammation in mouse lung tumorigenesis: a review. Exp Lung Res. 2005;31(1):57–82. [PubMed]
43. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell. 2011;144(5):646–74. [PubMed]
44. Coussens LM, Werb Z. Inflammation and cancer. Nature. 2002;420(6917):860–7. [PMC free article] [PubMed]
45. Takahashi H, Ogata H, Nishigaki R, Broide DH, Karin M. Tobacco smoke promotes lung tumorigenesis by triggering IKKbeta- and JNK1-dependent inflammation. Cancer Cell. 2010;17(1):89–97. doi: 10.1016/j.ccr.2009.12.008. [PMC free article] [PubMed] [Cross Ref]
46. Kitajima S, Thummalapalli R, Barbie DA. Inflammation as a driver and vulnerability of KRAS mediated oncogenesis. Seminars in cell & developmental biology. 2016;58:127–35. doi: 10.1016/j.semcdb.2016.06.009. [PMC free article] [PubMed] [Cross Ref]
47. Schuliga M. NF-kappaB Signaling in Chronic Inflammatory Airway Disease. Biomolecules. 2015;5(3):1266–83. doi: 10.3390/biom5031266. [PMC free article] [PubMed] [Cross Ref]
48. Hecht SS, Kassie F, Hatsukami DK. Chemoprevention of lung carcinogenesis in addicted smokers and ex-smokers. Nat Rev Cancer. 2009;9(7):476–88. [PMC free article] [PubMed]
49. Suthar SK, Sharma M. Recent Developments in Chimeric NSAIDs as Anticancer Agents: Teaching an Old Dog a New Trick. Mini reviews in medicinal chemistry. 2016;16(15):1201–18. [PubMed]
50. Baik CS, Brasky TM, Pettinger M, Luo J, Gong Z, Wactawski-Wende J, et al. Nonsteroidal Anti-Inflammatory Drug and Aspirin Use in Relation to Lung Cancer Risk among Postmenopausal Women. Cancer Epidemiol Biomarkers Prev. 2015;24(5):790–7. doi: 10.1158/1055-9965.epi-14-1322. [PMC free article] [PubMed] [Cross Ref]
51. Brasky TM, Baik CS, Slatore CG, Potter JD, White E. Non-steroidal anti-inflammatory drugs and small cell lung cancer risk in the VITAL study. Lung Cancer. 2012;77(2):260–4. doi: 10.1016/j.lungcan.2012.04.015. [PMC free article] [PubMed] [Cross Ref]
52. McCormack VA, Hung RJ, Brenner DR, Bickeboller H, Rosenberger A, Muscat JE, et al. Aspirin and NSAID use and lung cancer risk: a pooled analysis in the International Lung Cancer Consortium (ILCCO) Cancer Causes Control. 2011;22(12):1709–20. doi: 10.1007/s10552-011-9847-z. [PMC free article] [PubMed] [Cross Ref]
53. Sekine Y, Hata A, Koh E, Hiroshima K. Lung carcinogenesis from chronic obstructive pulmonary disease: characteristics of lung cancer from COPD and contribution of signal transducers and lung stem cells in the inflammatory microenvironment. General thoracic and cardiovascular surgery. 2014;62(7):415–21. doi: 10.1007/s11748-014-0386-x. [PubMed] [Cross Ref]
54. Takiguchi Y, Sekine I, Iwasawa S, Kurimoto R, Tatsumi K. Chronic obstructive pulmonary disease as a risk factor for lung cancer. World journal of clinical oncology. 2014;5(4):660–6. doi: 10.5306/wjco.v5.i4.660. [PMC free article] [PubMed] [Cross Ref]
55. Yang IA, Relan V, Wright CM, Davidson MR, Sriram KB, Savarimuthu Francis SM, et al. Common pathogenic mechanisms and pathways in the development of COPD and lung cancer. Expert opinion on therapeutic targets. 2011;15(4):439–56. doi: 10.1517/14728222.2011.555400. [PubMed] [Cross Ref]
56. Koshiol J, Rotunno M, Consonni D, Pesatori AC, De Matteis S, Goldstein AM, et al. Chronic obstructive pulmonary disease and altered risk of lung cancer in a population-based case-control study. PLoS One. 2009;4(10):e7380. doi: 10.1371/journal.pone.0007380. [PMC free article] [PubMed] [Cross Ref]
57. Zaynagetdinov R, Sherrill TP, Gleaves LA, Hunt P, Han W, McLoed AG, et al. Chronic NF-kappaB activation links COPD and lung cancer through generation of an immunosuppressive microenvironment in the lungs. Oncotarget. 2016;7(5):5470–82. doi: 10.18632/oncotarget.6562. [PMC free article] [PubMed] [Cross Ref]
58. Barreiro E, Bustamante V, Curull V, Gea J, Lopez-Campos JL, Munoz X. Relationships between chronic obstructive pulmonary disease and lung cancer: biological insights. J Thorac Dis. 2016;8(10):E1122–e35. doi: 10.21037/jtd.2016.09.54. [PMC free article] [PubMed] [Cross Ref]
59. Vermaelen K, Brusselle G. Exposing a deadly alliance: novel insights into the biological links between COPD and lung cancer. Pulm Pharmacol Ther. 2013;26(5):544–54. doi: 10.1016/j.pupt.2013.05.003. [PubMed] [Cross Ref]
60. Kuschner WG, D’Alessandro A, Wong H, Blanc PD. Dose-dependent cigarette smoking-related inflammatory responses in healthy adults. Eur Respir J. 1996;9(10):1989–94. [PubMed]
61. Chaudhuri R, Livingston E, McMahon AD, Lafferty J, Fraser I, Spears M, et al. Effects of smoking cessation on lung function and airway inflammation in smokers with asthma. Am J Respir Crit Care Med. 2006;174(2):127–33. doi: 10.1164/rccm.200510-1589OC. [PubMed] [Cross Ref]
62. Westergaard CG, Porsbjerg C, Backer V. The effect of smoking cessation on airway inflammation in young asthma patients. Clin Exp Allergy. 2014;44(3):353–61. doi: 10.1111/cea.12243. [PubMed] [Cross Ref]
63. Hogman M, Holmkvist T, Walinder R, Merilainen P, Ludviksdottir D, Hakansson L, et al. Increased nitric oxide elimination from the airways after smoking cessation. Clinical science (London, England : 1979) 2002;103(1):15–9. doi 10.1042/ [PubMed]
64. Swan GE, Hodgkin JE, Roby T, Mittman C, Jacobo N, Peters J. Reversibility of airways injury over a 12-month period following smoking cessation. Chest. 1992;101(3):607–12. [PubMed]
65. Willemse BW, ten Hacken NH, Rutgers B, Lesman-Leegte IG, Postma DS, Timens W. Effect of 1-year smoking cessation on airway inflammation in COPD and asymptomatic smokers. Eur Respir J. 2005;26(5):835–45. doi: 10.1183/09031936.05.00108904. [PubMed] [Cross Ref]
66. Ravensberg AJ, Slats AM, van Wetering S, Janssen K, van Wijngaarden S, de Jeu R, et al. CD8(+) T cells characterize early smoking-related airway pathology in patients with asthma. Respir Med. 2013;107(7):959–66. doi: 10.1016/j.rmed.2013.03.018. [PubMed] [Cross Ref]
67. O’Shaughnessy TC, Ansari TW, Barnes NC, Jeffery PK. Inflammation in bronchial biopsies of subjects with chronic bronchitis: inverse relationship of CD8+ T lymphocytes with FEV1. Am J Respir Crit Care Med. 1997;155(3):852–7. doi: 10.1164/ajrccm.155.3.9117016. [PubMed] [Cross Ref]
68. Costabel U, Bross KJ, Reuter C, Ruhle KH, Matthys H. Alterations in immunoregulatory T-cell subsets in cigarette smokers. A phenotypic analysis of bronchoalveolar and blood lymphocytes. Chest. 1986;90(1):39–44. [PubMed]
69. Lofdahl JM, Wahlstrom J, Skold CM. Different inflammatory cell pattern and macrophage phenotype in chronic obstructive pulmonary disease patients, smokers and non-smokers. Clin Exp Immunol. 2006;145(3):428–37. doi: 10.1111/j.1365-2249.2006.03154.x. [PubMed] [Cross Ref]
70. Amin K, Ekberg-Jansson A, Lofdahl CG, Venge P. Relationship between inflammatory cells and structural changes in the lungs of asymptomatic and never smokers: a biopsy study. Thorax. 2003;58(2):135–42. [PMC free article] [PubMed]
71. Lams BE, Sousa AR, Rees PJ, Lee TH. Subepithelial immunopathology of the large airways in smokers with and without chronic obstructive pulmonary disease. Eur Respir J. 2000;15(3):512–6. [PubMed]
72. Barnes PJ. Inflammatory mechanisms in patients with chronic obstructive pulmonary disease. J Allergy Clin Immunol. 2016;138(1):16–27. doi: 10.1016/j.jaci.2016.05.011. [PubMed] [Cross Ref]
73. Skold CM, Lundahl J, Hallden G, Hallgren M, Eklund A. Chronic smoke exposure alters the phenotype pattern and the metabolic response in human alveolar macrophages. Clin Exp Immunol. 1996;106(1):108–13. [PubMed]
74. Hunninghake GW, Gadek JE, Kawanami O, Ferrans VJ, Crystal RG. Inflammatory and immune processes in the human lung in health and disease: evaluation by bronchoalveolar lavage. Am J Pathol. 1979;97(1):149–206. [PubMed]
75. Ropcke S, Holz O, Lauer G, Muller M, Rittinghausen S, Ernst P, et al. Repeatability of and relationship between potential COPD biomarkers in bronchoalveolar lavage, bronchial biopsies, serum, and induced sputum. PLoS One. 2012;7(10):e46207. doi: 10.1371/journal.pone.0046207. [PMC free article] [PubMed] [Cross Ref]
76. Karimi R, Tornling G, Grunewald J, Eklund A, Sköld CM. Cell Recovery in Bronchoalveolar Lavage Fluid in Smokers Is Dependent on Cumulative Smoking History. PLoS ONE. 2012;7(3):e34232. doi: 10.1371/journal.pone.0034232. [PMC free article] [PubMed] [Cross Ref]
77. Willemse BWM. Effect of 1-year smoking cessation on airway inflammation in COPD and asymptomatic smokers. European Respiratory Journal. 2005;26(5):835–45. doi: 10.1183/09031936.05.00108904. [PubMed] [Cross Ref]
78. Rennard SI, Daughton D, Fujita J, Oehlerking MB, Dobson JR, Stahl MG, et al. Short-term smoking reduction is associated with reduction in measures of lower respiratory tract inflammation in heavy smokers. Eur Respir J. 1990;3(7):752–9. [PubMed]
79. Tanino M, Betsuyaku T, Takeyabu K, Tanino Y, Yamaguchi E, Miyamoto K, et al. Increased levels of interleukin-8 in BAL fluid from smokers susceptible to pulmonary emphysema. Thorax. 2002;57(5):405–11. [PMC free article] [PubMed]
80. Emami Ardestani M, Zaerin O. Role of Serum Interleukin 6, Albumin and C-Reactive Protein in COPD Patients. Tanaffos. 2015;14(2):134–40. [PMC free article] [PubMed]
81. Zhang L, Cheng Z, Liu W, Wu K. Expression of interleukin (IL)-10, IL-17A and IL-22 in serum and sputum of stable chronic obstructive pulmonary disease patients. COPD. 2013;10(4):459–65. doi: 10.3109/15412555.2013.770456. [PubMed] [Cross Ref]
82. Bhavani S, Tsai CL, Perusich S, Hesselbacher S, Coxson H, Pandit L, et al. Clinical and Immunological Factors in Emphysema Progression. Five-Year Prospective Longitudinal Exacerbation Study of Chronic Obstructive Pulmonary Disease (LES-COPD) Am J Respir Crit Care Med. 2015;192(10):1171–8. doi: 10.1164/rccm.201504-0736OC. [PMC free article] [PubMed] [Cross Ref]
83. McEvoy JW, Nasir K, DeFilippis AP, Lima JA, Bluemke DA, Hundley WG, et al. Relationship of cigarette smoking with inflammation and subclinical vascular disease: the Multi-Ethnic Study of Atherosclerosis. Arterioscler Thromb Vasc Biol. 2015;35(4):1002–10. doi: 10.1161/ATVBAHA.114.304960. [PMC free article] [PubMed] [Cross Ref]
84. Shiels MS, Katki HA, Freedman ND, Purdue MP, Wentzensen N, Trabert B, et al. Cigarette smoking and variations in systemic immune and inflammation markers. J Natl Cancer Inst. 2014;106(11) doi: 10.1093/jnci/dju294. [PMC free article] [PubMed] [Cross Ref]
85. Herfs M, Hubert P, Poirrier AL, Vandevenne P, Renoux V, Habraken Y, et al. Proinflammatory cytokines induce bronchial hyperplasia and squamous metaplasia in smokers: implications for chronic obstructive pulmonary disease therapy. Am J Respir Cell Mol Biol. 2012;47(1):67–79. doi: 10.1165/rcmb.2011-0353OC. [PubMed] [Cross Ref]
86. Willemse BW, ten Hacken NH, Rutgers B, Postma DS, Timens W. Association of current smoking with airway inflammation in chronic obstructive pulmonary disease and asymptomatic smokers. Respir Res. 2005;6:38. doi: 10.1186/1465-9921-6-38. [PMC free article] [PubMed] [Cross Ref]
87. Hacievliyagil SS, Mutlu LC, Temel I. Airway inflammatory markers in chronic obstructive pulmonary disease patients and healthy smokers. Nigerian journal of clinical practice. 2013;16(1):76–81. doi: 10.4103/1119-3077.106771. [PubMed] [Cross Ref]
88. Kunz LI, Lapperre TS, Snoeck-Stroband JB, Budulac SE, Timens W, van Wijngaarden S, et al. Smoking status and anti-inflammatory macrophages in bronchoalveolar lavage and induced sputum in COPD. Respir Res. 2011;12:34. doi: 10.1186/1465-9921-12-34. [PMC free article] [PubMed] [Cross Ref]
89. Steiling K, Kadar AY, Bergerat A, Flanigon J, Sridhar S, Shah V, et al. Comparison of proteomic and transcriptomic profiles in the bronchial airway epithelium of current and never smokers. PLoS One. 2009;4(4):e5043. [PMC free article] [PubMed]
90. Tilley AE, O’Connor TP, Hackett NR, Strulovici-Barel Y, Salit J, Amoroso N, et al. Biologic phenotyping of the human small airway epithelial response to cigarette smoking. PLoS One. 2011;6(7):e22798. doi: 10.1371/journal.pone.0022798. [PMC free article] [PubMed] [Cross Ref]
91. Harvey BG, Heguy A, Leopold PL, Carolan BJ, Ferris B, Crystal RG. Modification of gene expression of the small airway epithelium in response to cigarette smoking. Journal of molecular medicine (Berlin, Germany) 2007;85(1):39–53. doi: 10.1007/s00109-006-0103-z. [PubMed] [Cross Ref]
92. Whitney DH, Elashoff MR, Porta-Smith K, Gower AC, Vachani A, Ferguson JS, et al. Derivation of a bronchial genomic classifier for lung cancer in a prospective study of patients undergoing diagnostic bronchoscopy. BMC Med Genomics. 2015;8:18. doi: 10.1186/s12920-015-0091-3. [PMC free article] [PubMed] [Cross Ref]
93. Vachani A, Whitney DH, Parsons EC, Lenburg M, Ferguson JS, Silvestri GA, et al. Clinical Utility of a Bronchial Genomic Classifier in Patients With Suspected Lung Cancer. Chest. 2016;150(1):210–8. doi: 10.1016/j.chest.2016.02.636. [PubMed] [Cross Ref]
94. Beane J, Vick J, Schembri F, Anderlind C, Gower A, Campbell J, et al. Characterizing the Impact of Smoking and Lung Cancer on the Airway Transcriptome Using RNA-Seq. Cancer Prevention Research. 2011;4(6):803–17. doi: 10.1158/1940-6207.CAPR-11-0212. [PMC free article] [PubMed] [Cross Ref]
95. Strulovici-Barel Y, Omberg L, O’Mahony M, Gordon C, Hollmann C, Tilley AE, et al. Threshold of Biologic Responses of the Small Airway Epithelium to Low Levels of Tobacco Smoke. American Journal of Respiratory and Critical Care Medicine. 2010;182(12):1524–32. doi: 10.1164/rccm.201002-0294OC. [PMC free article] [PubMed] [Cross Ref]
96. Wang G, Xu Z, Wang R, Al-Hijji M, Salit J, Strulovici-Barel Y, et al. Genes associated with MUC5AC expression in small airway epithelium of human smokers and non-smokers. BMC Med Genomics. 2012;5:21. doi: 10.1186/1755-8794-5-21. [PMC free article] [PubMed] [Cross Ref]
97. De Smet EG, Mestdagh P, Vandesompele J, Brusselle GG, Bracke KR. Non-coding RNAs in the pathogenesis of COPD. Thorax. 2015;70(8):782–91. doi: 10.1136/thoraxjnl-2014-206560. [PubMed] [Cross Ref]
98. Perry MM, Moschos SA, Williams AE, Shepherd NJ, Larner-Svensson HM, Lindsay MA. Rapid changes in microRNA-146a expression negatively regulate the IL-1beta-induced inflammatory response in human lung alveolar epithelial cells. J Immunol. 2008;180(8):5689–98. [PMC free article] [PubMed]
99. Sato T, Liu X, Nelson A, Nakanishi M, Kanaji N, Wang X, et al. Reduced miR-146a increases prostaglandin E(2)in chronic obstructive pulmonary disease fibroblasts. Am J Respir Crit Care Med. 2010;182(8):1020–9. doi: 10.1164/rccm.201001-0055OC. [PMC free article] [PubMed] [Cross Ref]
100. Zago M, Rico de Souza A, Hecht E, Rousseau S, Hamid Q, Eidelman DH, et al. The NF-kappaB family member RelB regulates microRNA miR-146a to suppress cigarette smoke-induced COX-2 protein expression in lung fibroblasts. Toxicol Lett. 2014;226(2):107–16. doi: 10.1016/j.toxlet.2014.01.020. [PubMed] [Cross Ref]
101. Taganov KD, Boldin MP, Chang KJ, Baltimore D. NF-kappaB-dependent induction of microRNA miR-146, an inhibitor targeted to signaling proteins of innate immune responses. Proc Natl Acad Sci U S A. 2006;103(33):12481–6. doi: 10.1073/pnas.0605298103. [PubMed] [Cross Ref]
102. Hassan F, Nuovo GJ, Crawford M, Boyaka PN, Kirkby S, Nana-Sinkam SP, et al. MiR-101 and miR-144 regulate the expression of the CFTR chloride channel in the lung. PLoS One. 2012;7(11):e50837. doi: 10.1371/journal.pone.0050837. [PMC free article] [PubMed] [Cross Ref]
103. Hallows KR, Fitch AC, Richardson CA, Reynolds PR, Clancy JP, Dagher PC, et al. Up-regulation of AMP-activated kinase by dysfunctional cystic fibrosis transmembrane conductance regulator in cystic fibrosis airway epithelial cells mitigates excessive inflammation. J Biol Chem. 2006;281(7):4231–41. doi: 10.1074/jbc.M511029200. [PubMed] [Cross Ref]
104. Zhao Y, Xu Y, Li Y, Xu W, Luo F, Wang B, et al. NF-kappaB-mediated inflammation leading to EMT via miR-200c is involved in cell transformation induced by cigarette smoke extract. Toxicol Sci. 2013;135(2):265–76. doi: 10.1093/toxsci/kft150. [PubMed] [Cross Ref]
105. Shen HJ, Sun YH, Zhang SJ, Jiang JX, Dong XW, Jia YL, et al. Cigarette smoke-induced alveolar epithelial-mesenchymal transition is mediated by Rac1 activation. Biochim Biophys Acta. 2014;1840(6):1838–49. doi: 10.1016/j.bbagen.2014.01.033. [PubMed] [Cross Ref]
106. Milara J, Peiro T, Serrano A, Cortijo J. Epithelial to mesenchymal transition is increased in patients with COPD and induced by cigarette smoke. Thorax. 2013;68(5):410–20. doi: 10.1136/thoraxjnl-2012-201761. [PubMed] [Cross Ref]
107. Sohal SS, Walters EH. Role of epithelial mesenchymal transition (EMT) in chronic obstructive pulmonary disease (COPD) Respir Res. 2013;14:120. doi: 10.1186/1465-9921-14-120. [PMC free article] [PubMed] [Cross Ref]
108. Rippo MR, Olivieri F, Monsurro V, Prattichizzo F, Albertini MC, Procopio AD. MitomiRs in human inflamm-aging: a hypothesis involving miR-181a, miR-34a and miR-146a. Exp Gerontol. 2014;56:154–63. doi: 10.1016/j.exger.2014.03.002. [PubMed] [Cross Ref]
109. Xie L, Wu M, Lin H, Liu C, Yang H, Zhan J, et al. An increased ratio of serum miR-21 to miR-181a levels is associated with the early pathogenic process of chronic obstructive pulmonary disease in asymptomatic heavy smokers. Mol Biosyst. 2014;10(5):1072–81. doi: 10.1039/c3mb70564a. [PubMed] [Cross Ref]
110. Yu JH, Long L, Luo ZX, Li LM, You JR. Anti-inflammatory role of microRNA let-7c in LPS treated alveolar macrophages by targeting STAT3. Asian Pacific journal of tropical medicine. 2016;9(1):72–5. doi: 10.1016/j.apjtm.2015.12.015. [PubMed] [Cross Ref]
111. Murugan V, Peck MJ. Signal transduction pathways linking the activation of alveolar macrophages with the recruitment of neutrophils to lungs in chronic obstructive pulmonary disease. Exp Lung Res. 2009;35(6):439–85. [PubMed]
112. Graff JW, Powers LS, Dickson AM, Kim J, Reisetter AC, Hassan IH, et al. Cigarette smoking decreases global microRNA expression in human alveolar macrophages. PLoS One. 2012;7(8):e44066. doi: 10.1371/journal.pone.0044066. [PMC free article] [PubMed] [Cross Ref]
113. Bosse Y, Postma DS, Sin DD, Lamontagne M, Couture C, Gaudreault N, et al. Molecular signature of smoking in human lung tissues. Cancer Res. 2012;72(15):3753–63. doi: 10.1158/0008-5472.CAN-12-1160. [PubMed] [Cross Ref]
114. Trojanek JB, Cobos-Correa A, Diemer S, Kormann M, Schubert SC, Zhou-Suckow Z, et al. Airway mucus obstruction triggers macrophage activation and matrix metalloproteinase 12-dependent emphysema. Am J Respir Cell Mol Biol. 2014;51(5):709–20. doi: 10.1165/rcmb.2013-0407OC. [PubMed] [Cross Ref]
115. Wang G, Wang R, Strulovici-Barel Y, Salit J, Staudt MR, Ahmed J, et al. Persistence of smoking-induced dysregulation of miRNA expression in the small airway epithelium despite smoking cessation. PLoS One. 2015;10(4):e0120824. doi: 10.1371/journal.pone.0120824. [PMC free article] [PubMed] [Cross Ref]
116. Osei ET, Florez-Sampedro L, Timens W, Postma DS, Heijink IH, Brandsma CA. Unravelling the complexity of COPD by microRNAs: it’s a small world after all. Eur Respir J. 2015;46(3):807–18. doi: 10.1183/13993003.02139-2014. [PubMed] [Cross Ref]
117. Hsu PC, Lan RS, Brasky TM, Marian C, Cheema AK, Ressom HW, et al. Metabolomic profiles of current cigarette smokers. Mol Carcinog. 2016 doi: 10.1002/mc.22519. [PubMed] [Cross Ref]
118. Hsu PC, Lan RS, Brasky TM, Marian C, Cheema AK, Ressom HW, et al. Menthol Smokers: Metabolomic Profiling and Smoking Behavior. Cancer Epidemiol Biomarkers Prev. 2016 doi: 10.1158/1055-9965.EPI-16-0124. [PubMed] [Cross Ref]
119. Hsu PC, Zhou B, Zhao Y, Ressom HW, Cheema AK, Pickworth W, et al. Feasibility of identifying the tobacco-related global metabolome in blood by UPLC-QTOF-MS. J Proteome Res. 2013;12(2):679–91. doi: 10.1021/pr3007705. [PMC free article] [PubMed] [Cross Ref]
120. Mathe EA, Patterson AD, Haznadar M, Manna SK, Krausz KW, Bowman ED, et al. Noninvasive urinary metabolomic profiling identifies diagnostic and prognostic markers in lung cancer. Cancer Res. 2014;74(12):3259–70. doi: 10.1158/0008-5472.CAN-14-0109. [PMC free article] [PubMed] [Cross Ref]
121. Gu F, Derkach A, Freedman ND, Landi MT, Albanes D, Weinstein SJ, et al. Cigarette smoking behaviour and blood metabolomics. Int J Epidemiol. 2015 doi: 10.1093/ije/dyv330. [PMC free article] [PubMed] [Cross Ref]
122. Garcia-Perez I, Lindon JC, Minet E. Application of CE-MS to a metabonomics study of human urine from cigarette smokers and non-smokers. Bioanalysis. 2014;6(20):2733–49. doi: 10.4155/bio.14.136. [PubMed] [Cross Ref]
123. Muller DC, Degen C, Scherer G, Jahreis G, Niessner R, Scherer M. Metabolomics using GC-TOF-MS followed by subsequent GC-FID and HILIC-MS/MS analysis revealed significantly altered fatty acid and phospholipid species profiles in plasma of smokers. J Chromatogr B Analyt Technol Biomed Life Sci. 2014;966:117–26. doi: 10.1016/j.jchromb.2014.02.044. [PubMed] [Cross Ref]
124. Xu T, Holzapfel C, Dong X, Bader E, Yu Z, Prehn C, et al. Effects of smoking and smoking cessation on human serum metabolite profile: results from the KORA cohort study. BMC Med. 2013;11:60. doi: 10.1186/1741-7015-11-60. [PMC free article] [PubMed] [Cross Ref]
125. Kaluarachchi MR, Boulange CL, Garcia-Perez I, Lindon JC, Minet EF. Multiplatform serum metabolic phenotyping combined with pathway mapping to identify biochemical differences in smokers. Bioanalysis. 2016;8(19):2023–43. doi: 10.4155/bio-2016-0108. [PubMed] [Cross Ref]
126. Ghosh N, Dutta M, Singh B, Banerjee R, Bhattacharyya P, Chaudhury K. Transcriptomics, proteomics and metabolomics driven biomarker discovery in COPD: an update. Expert review of molecular diagnostics. 2016;16(8):897–913. doi: 10.1080/14737159.2016.1198258. [PubMed] [Cross Ref]
127. Chen Q, Deeb RS, Ma Y, Staudt MR, Crystal RG, Gross SS. Serum Metabolite Biomarkers Discriminate Healthy Smokers from COPD Smokers. PLoS One. 2015;10(12):e0143937. doi: 10.1371/journal.pone.0143937. [PMC free article] [PubMed] [Cross Ref]
128. Ren X, Zhang J, Fu X, Ma S, Wang C, Wang J, et al. LC-MS based metabolomics identification of novel biomarkers of tobacco smoke-induced chronic bronchitis. Biomed Chromatogr. 2016;30(1):68–74. doi: 10.1002/bmc.3620. [PubMed] [Cross Ref]
129. Beebe K, Kennedy AD. Sharpening Precision Medicine by a Thorough Interrogation of Metabolic Individuality. Comput Struct Biotechnol J. 2016;14:97–105. doi: 10.1016/j.csbj.2016.01.001. [PMC free article] [PubMed] [Cross Ref]
130. Tebani A, Abily-Donval L, Afonso C, Marret S, Bekri S. Clinical Metabolomics: The New Metabolic Window for Inborn Errors of Metabolism Investigations in the Post-Genomic Era. Int J Mol Sci. 2016;17(7) doi: 10.3390/ijms17071167. [PMC free article] [PubMed] [Cross Ref]
131. Guo L, Milburn MV, Ryals JA, Lonergan SC, Mitchell MW, Wulff JE, et al. Plasma metabolomic profiles enhance precision medicine for volunteers of normal health. Proc Natl Acad Sci U S A. 2015;112(35):E4901–10. doi: 10.1073/pnas.1508425112. [PubMed] [Cross Ref]
132. Snyder NW, Mesaros C, Blair IA. Translational metabolomics in cancer research. Biomark Med. 2015;9(9):821–34. doi: 10.2217/bmm.15.52. [PMC free article] [PubMed] [Cross Ref]
133. Conlon TM, Bartel J, Ballweg K, Gunter S, Prehn C, Krumsiek J, et al. Metabolomics screening identifies reduced L-carnitine to be associated with progressive emphysema. Clinical science (London, England : 1979) 2016;130(4):273–87. doi: 10.1042/CS20150438. [PubMed] [Cross Ref]
134. Hsu PC, Zhou B, Zhao Y, Ressom HW, Cheema AK, Pickworth W, et al. Feasibility of identifying the tobacco-related global metabolome in blood by UPLC-QTOF-MS. J Proteome Res. 2012 [PMC free article] [PubMed]
135. Cameron SJ, Lewis KE, Beckmann M, Allison GG, Ghosal R, Lewis PD, et al. The metabolomic detection of lung cancer biomarkers in sputum. Lung Cancer. 2016;94:88–95. doi: 10.1016/j.lungcan.2016.02.006. [PubMed] [Cross Ref]
136. Surowiec I, Karimpour M, Gouveia-Figueira S, Wu J, Unosson J, Bosson JA, et al. Multi-platform metabolomics assays for human lung lavage fluids in an air pollution exposure study. Analytical and bioanalytical chemistry. 2016;408(17):4751–64. doi: 10.1007/s00216-016-9566-0. [PubMed] [Cross Ref]
137. Malerba M, Montuschi P. Non-invasive biomarkers of lung inflammation in smoking subjects. Current medicinal chemistry. 2012;19(2):187–96. [PubMed]
138. See KC, Christiani DC. Normal values and thresholds for the clinical interpretation of exhaled nitric oxide levels in the US general population: results from the National Health and Nutrition Examination Survey 2007-2010. Chest. 2013;143(1):107–16. doi: 10.1378/chest.12-0416. [PubMed] [Cross Ref]
139. Smith AD, Cowan JO, Taylor DR. Exhaled nitric oxide levels in asthma: Personal best versus reference values. J Allergy Clin Immunol. 2009;124(4):714–8. doi: 10.1016/j.jaci.2009.07.020. e4. [PubMed] [Cross Ref]
140. Smith B, D’Costa J. Review: medication adjustment based on fractional exhaled nitric oxide did not prevent asthma exacerbations. Evid Based Med. 2009;14(1):8. doi: 10.1136/ebm.14.1.8. [PubMed] [Cross Ref]
141. Smith AD, Cowan JO, Brassett KP, Herbison GP, Taylor DR. Use of exhaled nitric oxide measurements to guide treatment in chronic asthma. N Engl J Med. 2005;352(21):2163–73. doi: 10.1056/NEJMoa043596. [PubMed] [Cross Ref]
142. Petsky HL, Kew KM, Chang AB. Exhaled nitric oxide levels to guide treatment for children with asthma. Cochrane Database Syst Rev. 2016;11:Cd011439. doi: 10.1002/14651858.CD011439.pub2. [PubMed] [Cross Ref]
143. Kim JK, Jung JY, Kim H, Eom SY, Hahn YS. Combined use of fractional exhaled nitric oxide and bronchodilator response in predicting future loss of asthma control among children with atopic asthma. Respirology. 2016 doi: 10.1111/resp.12934. [PubMed] [Cross Ref]
144. Redington AE. Modulation of nitric oxide pathways: therapeutic potential in asthma and chronic obstructive pulmonary disease. Eur J Pharmacol. 2006;533(1–3):263–76. doi: 10.1016/j.ejphar.2005.12.069. [PubMed] [Cross Ref]
145. Hillas G, Kostikas K, Mantzouranis K, Bessa V, Kontogianni K, Papadaki G, et al. Exhaled nitric oxide and exhaled breath condensate pH as predictors of sputum cell counts in optimally treated asthmatic smokers. Respirology. 2011;16(5):811–8. doi: 10.1111/j.1440-1843.2011.01984.x. [PubMed] [Cross Ref]
146. Pietropaoli AP, Perillo IB, Perkins PT, Frasier LM, Speers DM, Frampton MW, et al. Smokers have reduced nitric oxide production by conducting airways but normal levels in the alveoli. Inhal Toxicol. 2007;19(6–7):533–41. doi: 10.1080/08958370701260673. [PubMed] [Cross Ref]
147. Kharitonov SA, Robbins RA, Yates D, Keatings V, Barnes PJ. Acute and chronic effects of cigarette smoking on exhaled nitric oxide. Am J Respir Crit Care Med. 1995;152(2):609–12. doi: 10.1164/ajrccm.152.2.7543345. [PubMed] [Cross Ref]
148. Xu X, Hu H, Kearney GD, Kan H, Carrillo G, Chen X. A population-based study of smoking, serum cotinine and exhaled nitric oxide among asthmatics and a healthy population in the USA. Inhal Toxicol. 2016;28(14):724–30. doi: 10.1080/08958378.2016.1264502. [PubMed] [Cross Ref]
149. Jones KL, Bryan TW, Jinkins PA, Simpson KL, Grisham MB, Owens MW, et al. Superoxide released from neutrophils causes a reduction in nitric oxide gas. Am J Physiol. 1998;275(6 Pt 1):L1120–6. [PubMed]
150. Rytila P, Rehn T, Ilumets H, Rouhos A, Sovijarvi A, Myllarniemi M, et al. Increased oxidative stress in asymptomatic current chronic smokers and GOLD stage 0 COPD. Respir Res. 2006;7:69. doi: 10.1186/1465-9921-7-69. [PMC free article] [PubMed] [Cross Ref]
151. Lerner CA, Sundar IK, Watson RM, Elder A, Jones R, Done D, et al. Environmental health hazards of e-cigarettes and their components: Oxidants and copper in e-cigarette aerosols. Environ Pollut. 2015;198:100–7. doi: 10.1016/j.envpol.2014.12.033. [PMC free article] [PubMed] [Cross Ref]
152. Administration UFaD. Generally Recognized as Safe (GRAS) 2016 http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/
153. Park SJ, Walser TC, Perdomo C, Wang T, Pagano PC, Liclican EL, et al. Abstract B16: The effect of e-cigarette exposure on airway epithelial cell gene expression and transformation. Clinical Cancer Research. 2014;20(2 Supplement):B16–B. doi: 10.1158/1078-0432.14AACRIASLC-B16. [Cross Ref]
154. Sinjewel A, Swart EL, Lingeman H, Wilhelm AJ. LC Determination of Propylene Glycol in Human Plasma After Pre-Column Derivatization with Benzoyl Chloride. Chromatographia. 2007;66(1–2):103–5. doi: 10.1365/s10337-007-0231-9. [Cross Ref]
155. Holčapek M, Virelizier H, Chamot-Rooke J, Jandera P, Moulin C. Trace Determination of Glycols by HPLC with UV and Electrospray Ionization Mass Spectrometric Detections. Analytical Chemistry. 1999;71(13):2288–93. doi: 10.1021/ac981087y. [PubMed] [Cross Ref]
156. McIntosh TS, HMD, Matthews DE. A liquid chromatography-mass spectrometry method to measure stable isotopic tracer enrichments of glycerol and glucose in human serum. Anal Biochem. 2002;300(2002):163–69. doi: 10.1006/abio20015455. [PubMed] [Cross Ref]
157. Kosmider L, Sobczak A, Fik M, Knysak J, Zaciera M, Kurek J, et al. Carbonyl compounds in electronic cigarette vapors: effects of nicotine solvent and battery output voltage. Nicotine Tob Res. 2014;16(10):1319–26. doi: 10.1093/ntr/ntu078. [PMC free article] [PubMed] [Cross Ref]
158. Sleiman M, Logue JM, Montesinos VN, Russell ML, Litter MI, Gundel LA, et al. Emissions from Electronic Cigarettes: Key Parameters Affecting the Release of Harmful Chemicals. Environ Sci Technol. 2016;50(17):9644–51. doi: 10.1021/acs.est.6b01741. [PubMed] [Cross Ref]
159. Uchiyama S, Senoo Y, Hayashida H, Inaba Y, Nakagome H, Kunugita N. Determination of Chemical Compounds Generated from Second-generation E-cigarettes Using a Sorbent Cartridge Followed by a Two-step Elution Method. Analytical sciences : the international journal of the Japan Society for Analytical Chemistry. 2016;32(5):549–55. doi: 10.2116/analsci.32.549. [PubMed] [Cross Ref]
160. Herrington JS, Myers C. Electronic cigarette solutions and resultant aerosol profiles. Journal of chromatography A. 2015;1418:192–9. doi: 10.1016/j.chroma.2015.09.034. [PubMed] [Cross Ref]
161. Flora JW, Meruva N, Huang CB, Wilkinson CT, Ballentine R, Smith DC, et al. Characterization of potential impurities and degradation products in electronic cigarette formulations and aerosols. Regul Toxicol Pharmacol. 2016;74:1–11. doi: 10.1016/j.yrtph.2015.11.009. [PubMed] [Cross Ref]
162. Department of Health and Human Services. FDA Federal Register. 2014
163. Conference of the Parties to the WHO Framework Convention on Tobacco Control. Report by WHO. 2014
164. Grana RA, Popova L, Ling PM. A Longitudinal Analysis of Electronic Cigarette Use and Smoking Cessation. JAMA Internal Medicine. 2014;174(5):812. doi: 10.1001/jamainternmed.2014.187. [PMC free article] [PubMed] [Cross Ref]
165. Adzersen KH, Becker N, Steindorf K, Frentzel-Beyme R. Cancer mortality in a cohort of male German iron foundry workers. Am J Ind Med. 2003;43(3):295–305. [PubMed]
166. WHO | World Health Assembly Resolution 561, (2015) [accessed March 17, 2015];2015 http://wwwwhoint/tobacco/framework/final_text/en/printhtml.
167. Cressey D. E-cigarettes: The lingering questions. Nature. 2014;513(7516):24–6. doi: 10.1038/513024a. [PubMed] [Cross Ref]
168. More on Hidden Formaldehyde in E-Cigarette Aerosols. New England Journal of Medicine. 2015;372(16):1575–7. doi: 10.1056/NEJMc1502242. [PubMed] [Cross Ref]
169. Cheng T. Chemical evaluation of electronic cigarettes. Tobacco Control. 2014;23(suppl 2):i11–ii7. doi: 10.1136/tobaccocontrol-2013-051482. [PMC free article] [PubMed] [Cross Ref]
170. Burstyn I. Peering through the mist: systematic review of what the chemistry of contaminants in electronic cigarettes tells us about health risks. BMC Public Health. 2014;14(1) doi: 10.1186/1471-2458-14-18. [PMC free article] [PubMed] [Cross Ref]
171. Goniewicz ML, Knysak J, Gawron M, Kosmider L, Sobczak A, Kurek J, et al. Levels of selected carcinogens and toxicants in vapour from electronic cigarettes. Tobacco Control. 2014;23(2):133–9. doi: 10.1136/tobaccocontrol-2012-050859. [PMC free article] [PubMed] [Cross Ref]
172. Orr MS. Electronic cigarettes in the USA: a summary of available toxicology data and suggestions for the future: Table 1. Tobacco Control. 2014;23(suppl 2):i18–ii22. doi: 10.1136/tobaccocontrol-2013-051474. [PMC free article] [PubMed] [Cross Ref]
173. Tayyarah R, Long GA. Comparison of select analytes in aerosol from e-cigarettes with smoke from conventional cigarettes and with ambient air. Regul Toxicol Pharmacol. 2014;70(3):704–10. doi: 10.1016/j.yrtph.2014.10.010. [PubMed] [Cross Ref]
174. Bekki K, Uchiyama S, Ohta K, Inaba Y, Nakagome H, Kunugita N. Carbonyl compounds generated from electronic cigarettes. Int J Environ Res Public Health. 2014;11(11):11192–200. doi: 10.3390/ijerph111111192. [PMC free article] [PubMed] [Cross Ref]
175. Hutzler C, Paschke M, Kruschinski S, Henkler F, Hahn J, Luch A. Chemical hazards present in liquids and vapors of electronic cigarettes. Archives of Toxicology. 2014;88(7):1295–308. doi: 10.1007/s00204-014-1294-7. [PubMed] [Cross Ref]
176. Kosmider L, Sobczak A, Prokopowicz A, Kurek J, Zaciera M, Knysak J, et al. Cherry-flavoured electronic cigarettes expose users to the inhalation irritant, benzaldehyde. Thorax. 2016;71(4):376–7. doi: 10.1136/thoraxjnl-2015-207895. [PMC free article] [PubMed] [Cross Ref]
177. Kim YH, Kim KH. A novel method to quantify the emission and conversion of VOCs in the smoking of electronic cigarettes. Sci Rep. 2015;5:16383. doi: 10.1038/srep16383. [PMC free article] [PubMed] [Cross Ref]
178. Leigh NJ, Lawton RI, Hershberger PA, Goniewicz ML. Flavourings significantly affect inhalation toxicity of aerosol generated from electronic nicotine delivery systems (ENDS) Tob Control. 2016;25(Suppl 2):i81–ii7. doi: 10.1136/tobaccocontrol-2016-053205. [PubMed] [Cross Ref]
179. Gillman IG, Kistler KA, Stewart EW, Paolantonio AR. Effect of variable power levels on the yield of total aerosol mass and formation of aldehydes in e-cigarette aerosols. Regul Toxicol Pharmacol. 2016;75:58–65. doi: 10.1016/j.yrtph.2015.12.019. [PubMed] [Cross Ref]
180. Jensen RP, Luo W, Pankow JF, Strongin RM, Peyton DH. Hidden formaldehyde in e-cigarette aerosols. N Engl J Med. 2015;372(4):392–4. [PubMed]
181. Geiss O, Bianchi I, Barrero-Moreno J. Correlation of volatile carbonyl yields emitted by e-cigarettes with the temperature of the heating coil and the perceived sensorial quality of the generated vapours. Int J Hyg Environ Health. 2016;219(3):268–77. doi: 10.1016/j.ijheh.2016.01.004. [PubMed] [Cross Ref]
182. Havel CM, Benowitz NL, Jacob P, 3rd, St Helen G. An Electronic Cigarette Vaping Machine for the Characterization of Aerosol Delivery and Composition. Nicotine Tob Res. 2016 doi: 10.1093/ntr/ntw147. [PubMed] [Cross Ref]
183. Pisinger C, Døssing M. A systematic review of health effects of electronic cigarettes. Preventive Medicine. 2014;69:248–60. doi: 10.1016/j.ypmed.2014.10.009. [PubMed] [Cross Ref]
184. Hiemstra PS, Bals R. Basic science of electronic cigarettes: assessment in cell culture and in vivo models. Respir Res. 2016;17(1):127. doi: 10.1186/s12931-016-0447-z. [PMC free article] [PubMed] [Cross Ref]
185. Misra M, Leverette R, Cooper B, Bennett M, Brown S. Comparative In Vitro Toxicity Profile of Electronic and Tobacco Cigarettes, Smokeless Tobacco and Nicotine Replacement Therapy Products: E-Liquids, Extracts and Collected Aerosols. International Journal of Environmental Research and Public Health. 2014;11(11):11325–47. doi: 10.3390/ijerph111111325. [PMC free article] [PubMed] [Cross Ref]
186. Husari A, Shihadeh A, Talih S, Hashem Y, El Sabban M, Zaatari G. Acute Exposure to Electronic and Combustible Cigarette Aerosols: Effects in an Animal Model and in Human Alveolar Cells. Nicotine Tob Res. 2016;18(5):613–9. doi: 10.1093/ntr/ntv169. [PubMed] [Cross Ref]
187. Schweitzer KS, Chen SX, Law S, Van Demark M, Poirier C, Justice MJ, et al. Endothelial disruptive proinflammatory effects of nicotine and e-cigarette vapor exposures. Am J Physiol Lung Cell Mol Physiol. 2015;309(2):L175–87. doi: 10.1152/ajplung.00411.2014. [PubMed] [Cross Ref]
188. Scheffler S, Dieken H, Krischenowski O, Forster C, Branscheid D, Aufderheide M. Evaluation of E-cigarette liquid vapor and mainstream cigarette smoke after direct exposure of primary human bronchial epithelial cells. Int J Environ Res Public Health. 2015;12(4):3915–25. doi: 10.3390/ijerph120403915. [PMC free article] [PubMed] [Cross Ref]
189. Scheffler S, Dieken H, Krischenowski O, Aufderheide M. Cytotoxic Evaluation of e-Liquid Aerosol using Different Lung-Derived Cell Models. Int J Environ Res Public Health. 2015;12(10):12466–74. doi: 10.3390/ijerph121012466. [PMC free article] [PubMed] [Cross Ref]
190. Higham A, Rattray NJ, Dewhurst JA, Trivedi DK, Fowler SJ, Goodacre R, et al. Electronic cigarette exposure triggers neutrophil inflammatory responses. Respir Res. 2016;17(1):56. doi: 10.1186/s12931-016-0368-x. [PMC free article] [PubMed] [Cross Ref]
191. Yu V, Rahimy M, Korrapati A, Xuan Y, Zou AE, Krishnan AR, et al. Electronic cigarettes induce DNA strand breaks and cell death independently of nicotine in cell lines. Oral Oncol. 2016;52:58–65. doi: 10.1016/j.oraloncology.2015.10.018. [PMC free article] [PubMed] [Cross Ref]
192. Holliday R, Kist R, Bauld L. E-cigarette vapour is not inert and exposure can lead to cell damage. Evidence-based dentistry. 2016;17(1):2–3. doi: 10.1038/sj.ebd.6401143. [PubMed] [Cross Ref]
193. Thorne D, Crooks I, Hollings M, Seymour A, Meredith C, Gaca M. The mutagenic assessment of an electronic-cigarette and reference cigarette smoke using the Ames assay in strains TA98 and TA100. Mutation Research/Genetic Toxicology and Environmental Mutagenesis. 2016;812:29–38. doi http://dx.doi.org/10.1016/j.mrgentox.2016.10.005. [PubMed]
194. Garcia-Arcos I, Geraghty P, Baumlin N, Campos M, Dabo AJ, Jundi B, et al. Chronic electronic cigarette exposure in mice induces features of COPD in a nicotine-dependent manner. Thorax . 2016 doi: 10.1136/thoraxjnl-2015-208039. [PMC free article] [PubMed] [Cross Ref]
195. Wu Q, Jiang D, Minor M, Chu HW. Electronic cigarette liquid increases inflammation and virus infection in primary human airway epithelial cells. PLoS One. 2014;9(9):e108342. doi: 10.1371/journal.pone.0108342. [PMC free article] [PubMed] [Cross Ref]
196. Lerner CA, Rutagarama P, Ahmad T, Sundar IK, Elder A, Rahman I. Electronic cigarette aerosols and copper nanoparticles induce mitochondrial stress and promote DNA fragmentation in lung fibroblasts. Biochem Biophys Res Commun. 2016;477(4):620–5. doi: 10.1016/j.bbrc.2016.06.109. [PMC free article] [PubMed] [Cross Ref]
197. Shen Y, Wolkowicz MJ, Kotova T, Fan L, Timko MP. Transcriptome sequencing reveals e-cigarette vapor and mainstream-smoke from tobacco cigarettes activate different gene expression profiles in human bronchial epithelial cells. Sci Rep. 2016;6:23984. doi: 10.1038/srep23984. [PMC free article] [PubMed] [Cross Ref]
198. Javed F, Kellesarian SV, Sundar IK, Romanos GE, Rahman I. Recent Updates on Electronic Cigarette Aerosol and Inhaled Nicotine Effects on Periodontal and Pulmonary Tissues. Oral Dis. 2017 doi: 10.1111/odi.12652. [PubMed] [Cross Ref]
199. Suber RL, Deskin R, Nikiforov I, Fouillet X, Coggins CR. Subchronic nose-only inhalation study of propylene glycol in Sprague-Dawley rats. Food Chem Toxicol. 1989;27(9):573–83. [PubMed]
200. Final report on the safety assessment of Ricinus Communis (Castor) Seed Oil, Hydrogenated Castor Oil, Glyceryl Ricinoleate, Glyceryl Ricinoleate SE, Ricinoleic Acid, Potassium Ricinoleate, Sodium Ricinoleate, Zinc Ricinoleate, Cetyl Ricinoleate, Ethyl Ricinoleate, Glycol Ricinoleate, Isopropyl Ricinoleate, Methyl Ricinoleate, and Octyldodecyl Ricinoleate. Int J Toxicol. 2007;26(Suppl 3):31–77. doi: 10.1080/10915810701663150. [PubMed] [Cross Ref]
201. Renne RA, Wehner AP, Greenspan BJ, DeFord HS, Ragan HA, Westerberg RB, et al. 2-week and 13-week inhalation studies of aerosolized glycerol in rats. Inhalation Toxicology. 1992;4:95–111.
202. Werley MS, McDonald P, Lilly P, Kirkpatrick D, Wallery J, Byron P, et al. Non-clinical safety and pharmacokinetic evaluations of propylene glycol aerosol in Sprague-Dawley rats and Beagle dogs. Toxicology. 2011;287(1–3):76–90. doi: 10.1016/j.tox.2011.05.015. [PubMed] [Cross Ref]
203. Lim HB, Kim SH. Inhallation of e-Cigarette Cartridge Solution Aggravates Allergen-induced Airway Inflammation and Hyper-responsiveness in Mice. Toxicological Research. 2014;30(1):13–8. doi: 10.5487/TR.2014.30.1.013. [PMC free article] [PubMed] [Cross Ref]
204. Lerner CA, Sundar IK, Yao H, Gerloff J, Ossip DJ, McIntosh S, et al. Vapors produced by electronic cigarettes and e-juices with flavorings induce toxicity, oxidative stress, and inflammatory response in lung epithelial cells and in mouse lung. PLoS One. 2015;10(2):e0116732. doi: 10.1371/journal.pone.0116732. [PMC free article] [PubMed] [Cross Ref]
205. Callahan-Lyon P. Electronic cigarettes: human health effects. Tobacco Control. 2014;23(suppl 2):i36–ii40. doi: 10.1136/tobaccocontrol-2013-051470. [PMC free article] [PubMed] [Cross Ref]
206. Hureaux J, Drouet M, Urban T. A case report of subacute bronchial toxicity induced by an electronic cigarette: Table 1. Thorax. 2014;69(6):596–7. doi: 10.1136/thoraxjnl-2013-204767. [PubMed] [Cross Ref]
207. Usuku K, Nishizawa M, Matsuki K, Tokunaga K, Takahashi K, Eiraku N, et al. Association of a particular amino acid sequence of the HLA-DR β1 chain with HTLV-I-associated myelopathy. European Journal of Immunology. 1990;20(7):1603–6. doi: 10.1002/eji.1830200729. [PubMed] [Cross Ref]
208. Orr KK, Asal NJ. Efficacy of Electronic Cigarettes for Smoking Cessation. Annals of Pharmacotherapy. 2014;48(11):1502–6. doi: 10.1177/1060028014547076. [PubMed] [Cross Ref]
209. Cravo AS, Bush J, Sharma G, Savioz R, Martin C, Craige S, et al. A randomised, parallel group study to evaluate the safety profile of an electronic vapour product over 12 weeks. Regul Toxicol Pharmacol. 2016;81(Suppl 1):S1–s14. doi: 10.1016/j.yrtph.2016.10.003. [PubMed] [Cross Ref]
210. Walele T, Sharma G, Savioz R, Martin C, Williams J. A randomised, crossover study on an electronic vapour product, a nicotine inhalator and a conventional cigarette. Part B: Safety and subjective effects. Regul Toxicol Pharmacol. 2016;74:193–9. doi: 10.1016/j.yrtph.2015.12.004. [PubMed] [Cross Ref]
211. Manzoli L, Flacco ME, Ferrante M, La Vecchia C, Siliquini R, Ricciardi W, et al. Cohort study of electronic cigarette use: effectiveness and safety at 24 months. Tob Control. 2016 doi: 10.1136/tobaccocontrol-2015-052822. [PMC free article] [PubMed] [Cross Ref]
212. Cibella F, Campagna D, Caponnetto P, Amaradio MD, Caruso M, Russo C, et al. Lung function and respiratory symptoms in a randomized smoking cessation trial of electronic cigarettes. Clinical science (London, England : 1979) 2016;130(21):1929–37. doi: 10.1042/cs20160268. [PubMed] [Cross Ref]
213. McConnell R, Barrington-Trimis JL, Wang K, Urman R, Hong H, Unger J, et al. Electronic-cigarette Use and Respiratory Symptoms in Adolescents. Am J Respir Crit Care Med. 2016 doi: 10.1164/rccm.201604-0804OC. [PubMed] [Cross Ref]
214. Polosa R, Morjaria JB, Caponnetto P, Caruso M, Campagna D, Amaradio MD, et al. Persisting long term benefits of smoking abstinence and reduction in asthmatic smokers who have switched to electronic cigarettes. Discov Med. 2016;21 [PubMed]
215. Polosa R, Morjaria JB, Caponnetto P, Prosperini U, Russo C, Pennisi A, et al. Evidence for harm reduction in COPD smokers who switch to electronic cigarettes. Respiratory Research. 2016;17(1):166. doi: 10.1186/s12931-016-0481-x. [PMC free article] [PubMed] [Cross Ref]
216. Goniewicz ML, Gawron M, Smith DM, Peng M, Jacob P, 3rd, Benowitz NL. Exposure to Nicotine and Selected Toxicants in Cigarette Smokers Who Switched to Electronic Cigarettes: A Longitudinal Within-Subjects Observational Study. Nicotine Tob Res. 2016 doi: 10.1093/ntr/ntw160. [PubMed] [Cross Ref]
217. McRobbie H, Phillips A, Goniewicz ML, Smith KM, Knight-West O, Przulj D, et al. Effects of Switching to Electronic Cigarettes with and without Concurrent Smoking on Exposure to Nicotine, Carbon Monoxide, and Acrolein. Cancer Prev Res (Phila) 2015;8(9):873–8. doi: 10.1158/1940-6207.capr-15-0058. [PubMed] [Cross Ref]
218. Pulvers K, Emami AS, Nollen NL, Romero DR, Strong DR, Benowitz NL, et al. Tobacco Consumption and Toxicant Exposure of Cigarette Smokers Using Electronic Cigarettes. Nicotine Tob Res. 2016 doi: 10.1093/ntr/ntw333. [PubMed] [Cross Ref]
219. O’Connell G, Graff DW, D’Ruiz CD. Reductions in biomarkers of exposure (BoE) to harmful or potentially harmful constituents (HPHCs) following partial or complete substitution of cigarettes with electronic cigarettes in adult smokers. Toxicology mechanisms and methods. 2016;26(6):443–54. doi: 10.1080/15376516.2016.1196282. [PMC free article] [PubMed] [Cross Ref]
220. D’Ruiz CD, Graff DW, Robinson E. Reductions in biomarkers of exposure, impacts on smoking urge and assessment of product use and tolerability in adult smokers following partial or complete substitution of cigarettes with electronic cigarettes. BMC Public Health. 2016;16:543. doi: 10.1186/s12889-016-3236-1. [PMC free article] [PubMed] [Cross Ref]
221. Campagna D, Cibella F, Caponnetto P, Amaradio MD, Caruso M, Morjaria JB, et al. Changes in breathomics from a 1-year randomized smoking cessation trial of electronic cigarettes. Eur J Clin Investig. 2016;46 doi: 10.1111/eci.12651. [PubMed] [Cross Ref]
222. Shahab L, Goniewicz ML, Blount BC, Brown J, McNeill A, Alwis KU, et al. Nicotine, Carcinogen, and Toxin Exposure in Long-Term E-Cigarette and Nicotine Replacement Therapy Users: A Cross-sectional Study. Ann Intern Med. 2017;166(6):390–400. doi: 10.7326/m16-1107. [PMC free article] [PubMed] [Cross Ref]
223. Jorenby DE, Smith SS, Fiore MC, Baker TB. Nicotine levels, withdrawal symptoms, and smoking reduction success in real world use: A comparison of cigarette smokers and dual users of both cigarettes and E-cigarettes. Drug Alcohol Depend. 2017;170:93–101. doi: 10.1016/j.drugalcdep.2016.10.041. [PubMed] [Cross Ref]
224. Marini S, Buonanno G, Stabile L, Avino P. A benchmark for numerical scheme validation of airborne particle exposure in street canyons. Environmental Science and Pollution Research. 2015;22(3):2051–63. doi: 10.1007/s11356-014-3491-6. [PubMed] [Cross Ref]
225. Vardavas CI, Anagnostopoulos N, Kougias M, Evangelopoulou V, Connolly GN, Behrakis PK. Short-term pulmonary effects of using an electronic cigarette: impact on respiratory flow resistance, impedance, and exhaled nitric oxide. Chest. 2012;141(6):1400–6. doi: 10.1378/chest.11-2443. [PubMed] [Cross Ref]
226. Ferrari M, Zanasi A, Nardi E, Morselli Labate AM, Ceriana P, Balestrino A, et al. Short-term effects of a nicotine-free e-cigarette compared to a traditional cigarette in smokers and non-smokers. BMC pulmonary medicine. 2015;15:120. doi: 10.1186/s12890-015-0106-z. [PMC free article] [PubMed] [Cross Ref]
227. Schober W, Szendrei K, Matzen W, Osiander-Fuchs H, Heitmann D, Schettgen T, et al. Use of electronic cigarettes (e-cigarettes) impairs indoor air quality and increases FeNO levels of e-cigarette consumers. Int J Hyg Environ Health. 2014;217(6):628–37. doi: 10.1016/j.ijheh.2013.11.003. [PubMed] [Cross Ref]
228. Harrell MB, Weaver SR, Loukas A, Creamer M, Marti CN, Jackson CD, et al. Flavored e-cigarette use: Characterizing youth, young adult, and adult users. Preventive medicine reports. 2017;5:33–40. doi: 10.1016/j.pmedr.2016.11.001. [PMC free article] [PubMed] [Cross Ref]
229. Bonhomme MG, Holder-Hayes E, Ambrose BK, Tworek C, Feirman SP, King BA, et al. Flavoured non-cigarette tobacco product use among US adults: 2013–2014. Tob Control. 2016;25(Suppl 2):i4–ii13. doi: 10.1136/tobaccocontrol-2016-053373. [PMC free article] [PubMed] [Cross Ref]
230. Hoffman AC, Salgado RV, Dresler C, Faller RW, Bartlett C. Flavour preferences in youth versus adults: a review. Tob Control. 2016;25(Suppl 2):i32–ii9. doi: 10.1136/tobaccocontrol-2016-053192. [PMC free article] [PubMed] [Cross Ref]
231. Allen JG, Flanigan SS, LeBlanc M, Vallarino J, MacNaughton P, Stewart JH, et al. Flavoring Chemicals in E-Cigarettes: Diacetyl, 2,3-Pentanedione, and Acetoin in a Sample of 51 Products, Including Fruit-, Candy-, and Cocktail-Flavored E-Cigarettes. Environ Health Perspect. 2016;124(6):733–9. doi: 10.1289/ehp.1510185. [PMC free article] [PubMed] [Cross Ref]
232. Farsalinos KE, Kistler KA, Gillman G, Voudris V. Evaluation of electronic cigarette liquids and aerosol for the presence of selected inhalation toxins. Nicotine Tob Res. 2015;17(2):168–74. doi: 10.1093/ntr/ntu176. [PMC free article] [PubMed] [Cross Ref]
233. Khlystov A, Samburova V. Flavoring Compounds Dominate Toxic Aldehyde Production during E-Cigarette Vaping. Environ Sci Technol. 2016;50(23):13080–5. doi: 10.1021/acs.est.6b05145. [PubMed] [Cross Ref]
234. Sherwood CL, Boitano S. Airway epithelial cell exposure to distinct e-cigarette liquid flavorings reveals toxicity thresholds and activation of CFTR by the chocolate flavoring 2,5-dimethypyrazine. Respir Res. 2016;17(1):57. doi: 10.1186/s12931-016-0369-9. [PMC free article] [PubMed] [Cross Ref]
235. Behar RZ, Davis B, Wang Y, Bahl V, Lin S, Talbot P. Identification of Toxicants in Cinnamon-Flavored Electronic Cigarette Refill Fluids. Toxicol In Vitro. 2013 doi: 10.1016/j.tiv.2013.10.006. [PubMed] [Cross Ref]
236. Bahl V, Lin S, Xu N, Davis B, Wang YH, Talbot P. Comparison of electronic cigarette refill fluid cytotoxicity using embryonic and adult models. Reproductive toxicology (Elmsford, NY) 2012;34(4):529–37. doi: 10.1016/j.reprotox.2012.08.001. [PubMed] [Cross Ref]
237. Werley MS, Kirkpatrick DJ, Oldham MJ, Jerome AM, Langston TB, Lilly PD, et al. Toxicological assessment of a prototype e-cigaret device and three flavor formulations: a 90-day inhalation study in rats. Inhalation Toxicology. 2016;28(1):22–38. doi: 10.3109/08958378.2015.1130758. [PMC free article] [PubMed] [Cross Ref]
238. Committee TPSA. Menthol Cigarettes and Public Health: Review of the Scientific Evidence and Recommendations. 2011 http://www.fda.gov/AdvisoryCommittees/CommitteesMeetingMaterials/TobaccoProductsScientificAdvisoryCommittee/ucm247605.htm.
239. Hoffman AC. The health effects of menthol cigarettes as compared to non-menthol cigarettes. Tob Induc Dis. 2011;9(Suppl 1):S7. doi: 10.1186/1617-9625-9-s1-s7. [PMC free article] [PubMed] [Cross Ref]
240. Gerloff J, Sundar IK, Freter R, Sekera ER, Friedman AE, Robinson R, et al. Inflammatory Response and Barrier Dysfunction by Different e-Cigarette Flavoring Chemicals Identified by Gas Chromatography-Mass Spectrometry in e-Liquids and e-Vapors on Human Lung Epithelial Cells and Fibroblasts. Applied in vitro toxicology. 2017;3(1):28–40. doi: 10.1089/aivt.2016.0030. [PMC free article] [PubMed] [Cross Ref]
241. Cardinale A, Nastrucci C, Cesario A, Russo P. Nicotine: specific role in angiogenesis, proliferation and apoptosis. Crit Rev Toxicol. 2012;42(1):68–89. doi: 10.3109/10408444.2011.623150. [PubMed] [Cross Ref]
242. Shields PG. Long-term Nicotine Replacement Therapy: Cancer Risk in Context. Cancer Prev Res (Phila) 2011;4(11):1719–23. [PubMed]
243. Sorensen LT, Toft B, Rygaard J, Ladelund S, Teisner B, Gottrup F. Smoking attenuates wound inflammation and proliferation while smoking cessation restores inflammation but not proliferation. Wound repair and regeneration : official publication of the Wound Healing Society [and] the European Tissue Repair Society. 2010;18(2):186–92. doi: 10.1111/j.1524-475X.2010.00569.x. [PubMed] [Cross Ref]
244. Li Q, Zhou X, Kolosov VP, Perelman JM. Nicotine suppresses inflammatory factors in HBE16 airway epithelial cells after exposure to cigarette smoke extract and lipopolysaccharide. Transl Res. 2010;156(6):326–34. doi: 10.1016/j.trsl.2010.09.001. [PubMed] [Cross Ref]
245. Tsoyi K, Jang HJ, Kim JW, Chang HK, Lee YS, Pae HO, et al. Stimulation of alpha7 nicotinic acetylcholine receptor by nicotine attenuates inflammatory response in macrophages and improves survival in experimental model of sepsis through heme oxygenase-1 induction. Antioxid Redox Signal. 2011;14(11):2057–70. doi: 10.1089/ars.2010.3555. [PubMed] [Cross Ref]
246. Goncalves RB, Coletta RD, Silverio KG, Benevides L, Casati MZ, da Silva JS, et al. Impact of smoking on inflammation: overview of molecular mechanisms. Inflamm Res. 2011;60(5):409–24. doi: 10.1007/s00011-011-0308-7. [PubMed] [Cross Ref]
247. Lunney PC, Leong RW. Review article: Ulcerative colitis, smoking and nicotine therapy. Aliment Pharmacol Ther. 2012;36(11–12):997–1008. doi: 10.1111/apt.12086. [PubMed] [Cross Ref]
248. Mabley J, Gordon S, Pacher P. Nicotine exerts an anti-inflammatory effect in a murine model of acute lung injury. Inflammation. 2011;34(4):231–7. doi: 10.1007/s10753-010-9228-x. [PMC free article] [PubMed] [Cross Ref]
249. Comer DM, Elborn JS, Ennis M. Inflammatory and cytotoxic effects of acrolein, nicotine, acetylaldehyde and cigarette smoke extract on human nasal epithelial cells. BMC pulmonary medicine. 2014;14:32. doi: 10.1186/1471-2466-14-32. [PMC free article] [PubMed] [Cross Ref]
250. Lam DC, Luo SY, Fu KH, Lui MM, Chan KH, Wistuba II, et al. Nicotinic acetylcholine receptor expression in human airway correlates with lung function. Am J Physiol Lung Cell Mol Physiol. 2016;310(3):L232–9. doi: 10.1152/ajplung.00101.2015. [PubMed] [Cross Ref]
251. Vukelic M, Qing X, Redecha P, Koo G, Salmon JE. Cholinergic receptors modulate immune complex-induced inflammation in vitro and in vivo. J Immunol. 2013;191(4):1800–7. doi: 10.4049/jimmunol.1203467. [PubMed] [Cross Ref]
252. Baez-Pagan CA, Delgado-Velez M, Lasalde-Dominicci JA. Activation of the Macrophage alpha7 Nicotinic Acetylcholine Receptor and Control of Inflammation. Journal of neuroimmune pharmacology : the official journal of the Society on NeuroImmune Pharmacology. 2015;10(3):468–76. doi: 10.1007/s11481-015-9601-5. [PMC free article] [PubMed] [Cross Ref]
253. Zhou MS, Chadipiralla K, Mendez AJ, Jaimes EA, Silverstein RL, Webster K, et al. Nicotine potentiates proatherogenic effects of oxLDL by stimulating and upregulating macrophage CD36 signaling. Am J Physiol Heart Circ Physiol. 2013;305(4):H563–74. doi: 10.1152/ajpheart.00042.2013. [PubMed] [Cross Ref]
254. Wang Y, Zhang F, Yang W, Xue S. Nicotine induces pro-inflammatory response in aortic vascular smooth muscle cells through a NFkappaB/osteopontin amplification loop-dependent pathway. Inflammation. 2012;35(1):342–9. doi: 10.1007/s10753-011-9324-6. [PubMed] [Cross Ref]
255. Yang X, Zhao C, Gao Z, Su X. A novel regulator of lung inflammation and immunity: pulmonary parasympathetic inflammatory reflex. Qjm. 2014;107(10):789–92. doi: 10.1093/qjmed/hcu005. [PubMed] [Cross Ref]
256. Nizri E, Brenner T. Modulation of inflammatory pathways by the immune cholinergic system. Amino Acids. 2013;45(1):73–85. doi: 10.1007/s00726-011-1192-8. [PubMed] [Cross Ref]
257. Treinin M, Papke RL, Nizri E, Ben-David Y, Mizrachi T, Brenner T. Role of the alpha7 Nicotinic Acetylcholine Receptor and RIC-3 in the Cholinergic Anti-inflammatory Pathway. Central nervous system agents in medicinal chemistry. 2016 [PubMed]
258. Filippini P, Cesario A, Fini M, Locatelli F, Rutella S. The Yin and Yang of non-neuronal alpha7-nicotinic receptors in inflammation and autoimmunity. Current drug targets. 2012;13(5):644–55. [PubMed]
259. Hayashi S, Hamada T, Zaidi SF, Oshiro M, Lee J, Yamamoto T, et al. Nicotine suppresses acute colitis and colonic tumorigenesis associated with chronic colitis in mice. American journal of physiology Gastrointestinal and liver physiology. 2014;307(10):G968–78. doi: 10.1152/ajpgi.00346.2013. [PubMed] [Cross Ref]
260. El Dib R, Suzumura EA, Akl EA, Gomaa H, Agarwal A, Chang Y, et al. Electronic nicotine delivery systems and/or electronic non-nicotine delivery systems for tobacco smoking cessation or reduction: a systematic review and meta-analysis. BMJ Open. 2017;7(2):e012680. doi: 10.1136/bmjopen-2016-012680. [PMC free article] [PubMed] [Cross Ref]
261. Kalkhoran S, Glantz SA. E-cigarettes and smoking cessation in real-world and clinical settings: a systematic review and meta-analysis. Lancet Respir Med. 2016;4(2):116–28. doi: 10.1016/s2213-2600(15)00521-4. [PMC free article] [PubMed] [Cross Ref]
262. Kim V, Oros M, Durra H, Kelsen S, Aksoy M, Cornwell WD, et al. Chronic bronchitis and current smoking are associated with more goblet cells in moderate to severe COPD and smokers without airflow obstruction. PLoS One. 2015;10(2):e0116108. doi: 10.1371/journal.pone.0116108. [PMC free article] [PubMed] [Cross Ref]
263. Mascaux C, Laes JF, Anthoine G, Haller A, Ninane V, Burny A, et al. Evolution of microRNA expression during human bronchial squamous carcinogenesis. Eur Respir J. 2009;33(2):352–9. [PubMed]
264. Takizawa H, Tanaka M, Takami K, Ohtoshi T, Ito K, Satoh M, et al. Increased expression of inflammatory mediators in small-airway epithelium from tobacco smokers. Am J Physiol Lung Cell Mol Physiol. 2000;278(5):L906–13. [PubMed]
265. Mancini NM, Bene MC, Gerard H, Chabot F, Faure G, Polu JM, et al. Early effects of short-time cigarette smoking on the human lung: a study of bronchoalveolar lavage fluids. Lung. 1993;171(5):277–91. [PubMed]
266. Mascaux C, Laes JF, Anthoine G, Haller A, Ninane V, Burny A, et al. Evolution of microRNA expression during human bronchial squamous carcinogenesis. European Respiratory Journal. 2008;33(2):352–9. doi: 10.1183/09031936.00084108. [PubMed] [Cross Ref]
267. Groningen Leiden Universities Corticosteroids in Obstructive Lung Disease study g. Kunz LIZ, Lapperre TS, Snoeck-Stroband JB, Budulac SE, Timens W, et al. Smoking status and anti-inflammatory macrophages in bronchoalveolar lavage and induced sputum in COPD. Respiratory Research. 2011;12(1) doi: 10.1186/1465-9921-12-34. [PMC free article] [PubMed] [Cross Ref]
268. Wen Y, Reid DW, Zhang D, Ward C, Wood-Baker R, Walters EH. Assessment of airway inflammation using sputum, BAL, and endobronchial biopsies in current and ex-smokers with established COPD. Int J Chron Obstruct Pulmon Dis. 2010;5:327–34. doi: 10.2147/copd.s11343. [PMC free article] [PubMed] [Cross Ref]
269. Klech H, Hutter C. Side-effects and safety of BAL. Eur Respir J. 1990;3(8):939–40. 61-9. [PubMed]
270. Stratton K, Shetty P, Wallace R, Bondurant S. Clearing the smoke: the science base for tobacco harm reduction--executive summary. Tob Control. 2001;10:189–95. [PMC free article] [PubMed]
271. Shields PG. Tobacco smoking, harm reduction, and biomarkers. J Natl Cancer Inst. 2002;94(19):1435–44. [PubMed]
272. Levy DT, Cummings KM, Villanti AC, Niaura R, Abrams DB, Fong GT, et al. A framework for evaluating the public health impact of e-cigarettes and other vaporized nicotine products. Addiction. 2016 doi: 10.1111/add.13394. [PubMed] [Cross Ref]
273. Chen J, Bullen C, Dirks K. A Comparative Health Risk Assessment of Electronic Cigarettes and Conventional Cigarettes. Int J Environ Res Public Health. 2017;14(4) doi: 10.3390/ijerph14040382. [PMC free article] [PubMed] [Cross Ref]
274. Baumung C, Rehm J, Franke H, Lachenmeier DW. Comparative risk assessment of tobacco smoke constituents using the margin of exposure approach: the neglected contribution of nicotine. Sci Rep. 2016;6:35577. doi: 10.1038/srep35577. [PMC free article] [PubMed] [Cross Ref]