• Birth defects
• ambient air pollution
• epidemiology
• exposure monitoring
• particulates

Congenital anomalies, a leading cause of fetal loss, contribute significantly to preterm birth and childhood and adult morbidity. Serious structural defects are present in 3%–8% of newborns worldwide and, while some can readily be attributed to chromosomal or syndromic disorders or known teratogenic or fetotoxic agents, a recent commentary maintained that the causes for most anomalies remain a mystery.1 Maternal smoking during pregnancy has long been associated with birth defects2 and numerous biological pathways have been identified whereby particulate air pollutants might impact on the placenta and fetus (reviewed in Kannan et al3) providing biological rationale for the assessment of air pollution's influence on fetal development. Although quantitatively lower than exposure from maternal smoking, exposure to ambient air pollutants affects large populations and, importantly, is not modifiable by the individual and is thus of great public health and policy relevance.

The study by Dolk et al4 in this issue (see page 223) is the latest in the small set of publications investigating air pollution's impact on birth defects,5–10 a subset of the larger body of literature focussing on air pollution's effect on the more common adverse birth outcomes (ie, preterm birth and low birth weight) which are considered more general indicators of abnormal fetal development.

The California5 and Texas6 studies reported positive associations between some specific cardiac defects and several but different pollutants measured at the air monitoring station closest to the mother's home during weeks 3–8 of gestation. …