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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.    Electronic supplementary material  The online version of this article (10.1186/s12940-018-0361-6) contains supplementary material, which is available to authorized users.

Early-life exposure to PM2.5 and risk of acute asthma clinical encounters among children in Massachusetts: a case-crossover analysis