Some of the pollutants present in SHS remain principally in the gas phase and can be removed by ventilation, but a significant fraction adheres to indoor surfaces and persists for a longer time. Complex physicochemical transformations of those compounds take place after smoking (i.e., aging) that affect both short- and long-term exposure patterns of nonsmokers. Aging processes include chemical reactions of residual components of tobacco smoke deposited on indoor surfaces and may be influenced by pollutant transport between different indoor media (e.g., the deposition into deep reservoirs such as the gypsum core of wallboard panels). Physical and chemical transformations of tobacco smoke pollutants take place simultaneously over time scales that range from a few seconds to several weeks or months after their initial release during smoking. During an initial period of up to a few hours immediately after smoking, SHS and THS exposure coexist, with the latter becoming predominant once SHS is removed by ventilation.
Indoor sorption–desorption dynamics.
Indoor surface:volume ratios are often in the range of 1–10 m2
, which are much larger than typical outdoor ratios (Knutson et al. 1992
). Partitioning of volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) to surfaces is a key mediator of human exposure to indoor pollutants. Building materials and furnishings often operate as sinks, reservoirs, or sources for these chemicals. The affinity of a VOC to building products (such as carpet, gypsum board, upholstery, flooring material, and acoustic tiles) is inversely proportional to the vapor pressure of the compound and is affected by specific molecular interactions and competition with water vapor (Won et al. 2001
). Tobacco smoke contains both VOCs and SVOCs; the latter partition between aerosol particles and the gas phase according to Junge-Pankow model predictions (Liang and Pankow 1996
; Pankow et al. 1994
). Partitioning must include the indoor materials, as described by Weschler and Nazaroff (2008
). Nicotine is one of the major SVOCs released in large amounts during smoking (1–3 mg/cigarette) (Singer et al. 2003
). Other authors have reported higher amounts; the 1999 Massachusetts Benchmark Study reported nicotine levels in sidestream smoke ranging from 2.2 to 5.3 mg/cigarette, depending on cigarette brand (Borgerding et al. 2000
). Nicotine room-temperature vapor pressure (0.04 mmHg) is between three and four orders of magnitude lower than that of indoor VOCs such as toluene (22 mmHg) or benzene (100 mmHg).
Sorption and desorption have been monitored in realistic settings by carrying out experiments in real indoor environments or in room-sized environmental chambers. For example, Singer et al. (2002
) studied nicotine absorption dynamics in a room-sized 50-m3
chamber furnished with typical materials (wallboard, carpet, draperies, and furniture) that they exposed to tobacco smoke generated by machine-smoking. Exposure-relevant emission factors that account for sorptive uptake and re-emission have been determined for short-term (1 day) and long-term (1 month) periods for 26 gas-phase organic compounds present in tobacco smoke. Analytes included volatile aldehydes (formaldehyde, acrolein), aromatic hydrocarbons (benzene, toluene, naphthalene), nicotine, and tobacco-related amines (pyridine, 3-EP). The emission factor of each individual compound was influenced by sorption and re-emission from indoor surfaces and materials. For each analyte, sorptive losses (i.e., transfer from the gas phase to material surfaces) were found to be highest at the highest level of furnishing (i.e., when more effective surface area was available) and for lower room ventilation rates (i.e., higher pollutant residence times). Losses were more marked for the less-volatile chemicals, and they were particularly remarkable for nicotine. In a month-long cyclic smoking study, after an initial accumulation period of ~10 days, re-emission of accumulated nicotine from indoor surfaces became a source of gas-phase nicotine equal in strength to smoking (Singer et al. 2003
). In subsequent experiments using the same chamber (Singer et al. 2004
), pure chemicals were released by flash evaporation and allowed to partition between gas phase and indoor surfaces. Several tobacco smoke constituents (nicotine, ethenylpyridine, methyl naphthalenes, ortho-cresol) readily sorbed to chamber surfaces, with nicotine having the highest affinity for surfaces. Nicotine was almost completely removed from the gas phase and deposited on indoor surfaces, whereas most other chemicals showed more moderate partitioning behavior. The strong sorptive tendency of nicotine implies that indoor surfaces in environments where smoking is habitual can be loaded with large amounts of this alkaloid and other related THS components, creating a hidden reservoir of THS constituents that could be re-emitted long after the cessation of active smoking.
Spectroscopic evidence suggests that amines adsorb predominantly in a protonated state in the presence of moisture (Destaillats et al. 2007
; Ongwandee et al. 2007
). Sorptive interactions of nicotine and other tobacco alkaloids are strongly influenced by the presence of other common airborne acids and bases, such as carbon dioxide (CO2
) and ammonia (NH3
), respectively, that are often present at high concentrations indoors. In bench-scale studies, the sorptive capacity of common materials such as carpet and wallboard toward trimethylamine, a model amine, increased in the presence of CO2
and decreased in the presence of NH3
as a consequence of the enhancing protonation capacity of CO2
(acid) and the competition with NH3
(base) (Ongwandee et al. 2005
; Ongwandee and Morrison 2008
Indoor chemical transformations.
Reactions driven by oxygenated and nitrogenated atmospheric species are the source of indoor secondary pollutants of potential toxicological relevance (Morrison 2008
). A recent study identified the formation of carcinogenic tobacco-specific nitrosamines (TSNAs) from the reaction of adsorbed nicotine with nitrous acid (HONO) (Sleiman et al. 2010b
). HONO is typically produced indoors by combustion sources and heterogeneous conversion of atmospheric nitrogen oxides. Nicotine adsorbed to a model surface showed high reactivity toward HONO, leading to the formation of three TSNAs: 1-(N
-nitrosamino)-1-(3-pyridinyl)-4-butanal (NNA), 4-(methylnitrosamino)-1-(3-pyridinyl)-1-butanone (NNK), and N
-nitroso nornicotine (NNN). The structures of these compounds as well as their mechanisms of formation are shown in (adapted from Sleiman et al. 2010b
). NNA, which is not present in freshly emitted tobacco smoke, was the predominant TSNA. Because of their low vapor pressures, these TSNAs are likely associated with indoor surfaces and dust. In addition to TSNAs, nitrosation of nicotine generated low levels of N
-nitrosopyrrolidine (a carcinogenic volatile nitrosamine) and various other multifunctional by-products.
Figure 1 Physical-chemical processes of nicotine reactions with nitrous acid on indoor surfaces. (A) Illustration of surface-mediated nitrosation of nicotine. (B) Proposed mechanism for the formation of TSNAs. Adapted from Sleiman et al. (2010b). Abbreviations: (more ...)
Ozone and related atmospheric oxidants [hydroxyl radical and hydrogen peroxide (H2
)] may generate oxidized products by reaction with some of the tobacco smoke components that remain sorbed to indoor surfaces. Thus, some of the respiratory symptoms associated with tobacco smoke may originate not from directly emitted air pollutants, but from volatile by-products that have low thresholds for eye, skin, and upper respiratory tract irritation (Destaillats et al. 2006
; Singer et al. 2006
The atmospheric lifetime of ozone is long enough to allow for its transport to the indoor environment, where it reacts at rates often higher than typical ventilation removal rates, leading to typical indoor/outdoor ratios between 0.2 and 0.7 (Weschler 2000
). Typical indoor ozone levels in most settings are ≤ 20 ppb by volume (ppbV). However, much higher ozone levels may be generated using devices marketed as air purifiers and often used to remove tobacco odors (Boeniger 1995
; Hubbard et al. 2005
The reaction of ozone with VOCs emitted during smoking was studied in a room-sized chamber (Shaughnesy et al. 2001
). Ozone reacted rapidly with unsaturated VOCs such as isoprene, pyrrole, and styrene but was relatively inert toward aromatic hydrocarbons. The main by-products were volatile aldehydes, which included formaldehyde, acetaldehyde, and benzaldehyde. Although amine ozonation is typically slow in the gas phase, sorption of nicotine to indoor surfaces can extend its indoor residence time and make it more available for ozonation (Petrick et al. 2010
). The reactivity of nicotine sorbed to model surfaces toward ozone was evaluated in laboratory experiments; formaldehyde, N
-methyl formamide, myosmine, ethyl pyridyl ketone, nicotinaldehyde, and cotinine were formed and were re-emitted into the gas phase (Destaillats et al. 2006
). Ozone reactions with nicotine or with SHS also formed ultrafine particles, as shown in , in which several multifunctional oxidized species with high asthma hazard index (Jarvis et al. 2005
) values were identified. illustrates the molecular structures of the identified nicotine oxidation by-products and their pathways of formation.
Mass spectrum and size distribution of secondary organic aerosol generated during nicotine reaction with ozone. Adapted from Sleiman et al. (2010a) with permission from Elsevier. dN/dlogP is the normalized particle number per size range.
Reaction products and proposed pathways (shown by arrows) for nicotine reactions with ozone. Reprinted from Sleiman et al. (2010a) with permission from Elsevier. m/zis the mass to charge ratio used to interpret mass spectral data.