In this observational study of children, urinary concentrations of three phthalate metabolites (MEP, MnBP, and MBzP) were associated positively with FeNO measured on the same day in separate adjusted models. This is the first population-based study to show an association between phthalate exposure and this measure of airway inflammation. All three associations remained largely unchanged after adjusting for seroatopy. When all four metabolites (MEP, MnBP, MBzP, MEHHP) were included in the same adjusted model, MEP and MBzP remained independent predictors of higher FeNO, whereas concentrations of MnBP, which are highly correlated with concentrations of MBzP, were no longer associated with FeNO. These results suggest that urinary biomarkers of exposure to two phthalates, believed to have substantial inhalational exposure, are associated with a measure of subclinical airway inflammation in children.
This observational study of children is the first to report a positive association between biomarkers of exposure to phthalates and FeNO
. The methodology for measuring FeNO
allows for noninvasive collection and measurement of NO produced by resident airway cells in response to inflammatory cytokines and mediators (37
). Although its diagnostic use in children remains debated (37
has been established as a biomarker of airway inflammation in response to air pollutants, with more than a decade of epidemiology studies (20
). In population-based studies, the majority of subjects with asthma have mild disease with infrequent exacerbations, making temporal associations between environmental exposures and exacerbations difficult to establish. Elevated FeNO
reflects eosinophilic airway inflammation in response to known asthma triggers and has been associated both with increased symptoms among subjects with asthma and with the development of asthma among children without asthma (20
). As such, it offers an objective biomarker to detect subclinical variations in airway inflammation reflecting increased risk for exacerbation. Our findings of associations with phthalates and FeNO
contrast with those from two small controlled chamber studies of adults exposed to polyvinyl chloride surfaces in which there was no increase in post- versus prechallenge FeNO
We hypothesized that children with report of wheeze in the last 12 months would be more susceptible to airway inflammation triggered by phthalate exposures than the nonwheezing children in our cohort. We observed a positive interaction between MBzP urinary concentrations and wheeze. There was also a positive interaction between urinary concentrations of MBzP and the probability of wheezing predicted from a latent class growth analysis of questionnaire data, suggesting that the MBzP and FeNO
association is stronger among a subset of children who wheeze. Although wheeze itself is episodic, the LCGA-based prediction was based on the pattern of wheezing at up to 15 time points between 3 months and 9 years of age and may offer a more stable indication of children with airway disease than a single questionnaire response. Additional advantages of the LCGA-based wheeze prediction are that it could be computed for all observations and that the output on the continuous probability scale may be more informative than a dichotomous classification. Direct comparison of our two wheezing measures is difficult, because the LCGA-derived probability of wheeze over 3 months was based on a different question than the ISAAC wheeze, and uncertainty estimates from the original LCGA model were not propagated into the LCGA-based probability of wheeze. The consistency of these observations using two different measures of wheeze lends support to these findings. Although corticosteroids may affect the strength of the correlation between FeNO
and wheeze (44
), the positive interaction remained after adjusting for or excluding children with reported use of corticosteroids (data not shown).
Urinary concentrations of MBzP recently have been shown to be associated with a nonspecific marker of systemic inflammation in a large population-based analysis. MBzP concentrations were associated with increased serum C-reactive protein in a dose-dependent fashion among 8,346 NHANES participants (45
). An interquartile range higher urinary MBzP concentration was associated with an average of 6.0% (95% CI, 1.7–10.8%) higher C-reactive protein, and this association held in analyses restricted to younger NHANES participants 6 to 12 or 13 to 19 years old. However, the mechanism through which phthalates are associated with systemic or airway inflammation remains unclear. Some phthalate metabolites, especially MEHP and MBzP, activate the ubiquitously expressed nuclear peroxisome proliferator–activated receptors (PPAR) α and γ (46
), which are ligand-activated transcription factors important in a variety of physiological processes, including airway inflammation and airway remodeling. For example, it has been shown that the expression of peroxisome proliferator–activated receptor γ is higher in the bronchial submucosa, airway epithelium, and smooth muscle of subjects with asthma than control subjects and is associated with decrements in lung function (47
Diurnal variations both in measures of urinary phthalates and FeNO
have been reported, allowing for the potential for confounding of an association between them. Among adults and children enrolled in NHANES, MEP was higher during midday collections, whereas MBzP was nonsignificantly higher during the evenings (7
). However, a panel study of adults found that 76% of the variance in urinary MEP was explained by between-person differences (48
). There are conflicting findings in the literature about whether FeNO
has circadian variation (49
), but at least one study in children reported higher FeNO
measured in the morning as compared with evening (50
). We recorded the time of day of FeNO
, but not urine collection. Nonetheless, FeNO
and urine were collected during a single office visit that typically lasted less than 2 hours. Therefore, the robustness of the findings after adjustment for time of day of FeNO
collection reduces the likelihood that these observations were due to confounding by the time of collection.
One strength of the cross-sectional study design is that it allows for the observation of effects that may be short-lived in response to exposures that vary over time. Studies of exposure to air pollutants, such as elemental carbon (22
), have demonstrated increased FeNO
hours after exposure, suggesting the importance of assessing the impact of pollutant exposures on FeNO
in short time windows. Associations between FeNO
and phthalate metabolites were evaluated cross-sectionally. However, it will be important in future studies to determine the relevance of chronic exposure to DEP and BBzP and elevated FeNO
to airway disease.
This study had several limitations. In an observational study, urinary concentrations of particular phthalate metabolites also may indicate exposure to other chemicals that share sources or chemical properties with the parent phthalates. For example, the mothers of the children in this analysis who reported the use of perfume or an index of uses of other personal care products during pregnancy had higher urinary concentrations of MEP, the main metabolite of DEP, than other pregnant women within the same cohort (51
). These participants also can be expected to have higher exposure to other chemicals in those products that could confound observed associations, such as artificial fragrances as seen in sampling of indoor air (52
). The substantially higher urinary concentrations of MEP in girls than boys warrants further investigation, as the burden of asthma and respiratory disease differentially increases for girls in adolescence, when exposure to DEP might be expected to remain high (53
). Furthermore, with phthalates, exposures occur to mixtures of correlated compounds. For example, in this study MnBP and MBzP were highly correlated (rho = 0.68), which may be due to shared sources of exposure and the fact that MnBP is a minor product of BBzP (6%) (54
). MBzP has been shown previously to be a more reproducible biomarker than MnBP in spot urine samples collected over 6 months from children in New York City (9
); its concentration may serve as a better marker of the mixture of related compounds than does the concentration of MnBP.
We report cross-sectional associations between children’s urinary concentrations of three phthalate metabolites and FeNO, a marker of airway inflammation. Concentrations of both MEP and MBzP remained associated with FeNO in a four-pollutant model. The association of concentrations of MBzP and FeNO was significantly stronger among children in this study with as compared with those without reported wheeze. The children with wheeze are presumed to have hyperreactive airways more susceptible to environmental exposures. These findings suggest a role for a ubiquitous exposure on airway inflammation in a susceptible population. Future studies can prospectively follow these children to observe whether associations persist through childhood and the long-term consequences to respiratory health of increased airway inflammation in children.