Childhood asthma development and early life exposure to traffic-related air pollution: effect modification by indoor biological agents.

INTRODUCTION Early-life exposure to traffic-related air pollution (TRAP) is associated with childhood asthma, but whether indoor biological co-exposures modify the association is unclear. We examined associations between early-life TRAP exposure and asthma-related outcomes and assessed interactions with indoor biological co-exposures.

Methods

We used data from the CHILD Study, a Canadian prospective birth cohort. Residential ambient nitrogen dioxide (NO2) concentrations during the first three months post-birth was used as a proxy for TRAP exposure, while endotoxin, dog and cat allergens were measured in house dust collected at three months. We assessed TRAP associations with asthma at age 5 and recurrent wheeze between ages 2 and 5 (n=3077) and effect modification (n= 1006) by indoor co-exposures.

Results

NO2 exposure was associated with increased odds of asthma development (OR = 1.36, 95% CI: 1.00-1.84; borderline significant) and recurrent wheeze (OR = 1.34, 95% CI: 1.10-1.64) per 10 ppb increase, adjusted for sex, household income, maternal history of asthma and time away from home. These associations were sensitive to the method used to account for study site, with similar effect estimates but loss of statistical significance after simple site adjustment, while remaining significant in mixed-effects models. Significant interaction was observed between NO2 and endotoxin, with stronger associations in the high-endotoxin (> median 73.6 EU/mg) group for asthma (OR = 3.25, 95% CI: 1.29-8.15). Given the differences in baseline risks between strata, this interaction should be interpreted with caution. No significant interactions were observed for pet allergen levels and pet ownership at 3 months after birth.

Conclusions

Early life TRAP exposure was associated with increased risk of asthma development and recurrent wheeze later in life, and endotoxin co-exposures may modify these associations. Future studies are needed to confirm these findings.