AbstractObjectivesTo investigate the association between particulate matter with an aerodynamic diameter of less than 2.5 μm (PM2.5) exposure during each trimester of pregnancy and development of lower respiratory tract infections (LRTIs) during the first 3 years of life and whether GSTM1 gene polymorphisms modify these effects.MethodsThis study included 1,180 mother‐child pairs from the Cohort for Childhood Origin of Asthma and allergic diseases. The PM2.5 levels during pregnancy were estimated by residential address using land‐use regression models based on a national monitoring system. A diagnosis of LRTIs was based on a parental report of a physician's diagnosis. Real‐time polymerase chain reaction was used for GSTM1 genotyping.ResultsHigher PM2.5 exposure during the third trimester was associated with LRTIs at 1 year of age (aRR, 1.06; 95% CI, 1.00‐1.13). This result did not change after adjusting for PM2.5 exposures during the first and second trimesters (aRR, 1.06; 95% CI, 0.99‐1.13). This association was significant after adjusting for PM2.5 exposures during first year of age (aRR, 1.08; 95% CI, 1.02‐1.15) and exposures to NO2 and ozone at the third trimester (aRR, 1.07; 95% CI, 1.00‐1.16). In addition, PM2.5 exposure during the third trimester increased the risk of LRTIs at 1 year of age in cases with the GSTM1 null genotype (aRR, 1.26; 95% CI, 1.01‐1.57; P for interaction .20).ConclusionHigher PM2.5 exposure during the third trimester of pregnancy may increase the susceptibility to LRTIs at 1 year of age. This effect is modified by GSTM1 gene polymorphisms.