Early-life exposure to PM 2.5 and risk of acute asthma clinical encounters among children in Massachusetts: a case-crossover analysis.

Background: Associations between ambient particulate matter < 2.5 μm (PM _2.5 ) and asthma morbidity have been suggested in previous epidemiologic studies but results are inconsistent for areas with lower PM _2.5 levels. We estimated the associations between early-life short-term PM _2.5 exposure and the risk of asthma or wheeze clinical encounters among Massachusetts children in the innovative Pregnancy to Early Life Longitudinal (PELL) cohort data linkage system.
Methods: We used a semi-bidirectional case-crossover study design with short-term exposure lags for asthma exacerbation using data from the PELL system. Cases included children up to 9 years of age who had a hospitalization, observational stay, or emergency department visit for asthma or wheeze between January 2001 and September 2009 (n = 33,387). Daily PM _2.5 concentrations were estimated at a 4-km resolution using satellite remote sensing, land use, and meteorological data. We applied conditional logistic regression models to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CI). We also stratified by potential effect modifiers.
Results: The median PM _2.5 concentration among participants was 7.8 μg/m ³ with an interquartile range of 5.9 μg/m ³ . Overall, associations between PM _2.5 exposure and asthma clinical encounters among children at lags 0, 1 and 2 were close to the null value of OR = 1.0. Evidence of effect modification was observed by birthweight for lags 0, 1 and 2 (p < 0.05), and season of clinical encounter for lags 0 and 1 (p < 0.05). Children with low birthweight (LBW) (< 2500 g) had increased odds of having an asthma clinical encounter due to higher PM _2.5 exposure for lag 1 (OR: 1.08 per interquartile range (IQR) increase in PM _2.5 ; 95% CI: 1.01, 1.15).
Conclusion: Asthma or wheeze exacerbations among LBW children were associated with short-term increases in PM _2.5 concentrations at low levels in Massachusetts.