The crude and adjusted ORs (95%CI) to all pollutants are presented in Tables3 and4, respectively. Most of the difference between the crude and adjusted models could be attributed to the addition of the season of conception as well as the covariates of year of birth and religion. In general, we found insignificant associations between air pollution and congenital heart defects. However two exceptions were found for particulate matter. For continuous exposure to PM10 (10 μg/m3 increase)the adjusted OR for multiple