Discoveries and Data: Exposures and Risk Factors


Combining our registry data with detailed air quality surveillance records, researchers are able to address for the first time whether air pollution causes birth defects.

UCLA scientists mapped addresses for children with and without birth defects to one of 30 air monitoring stations operated by the South Coast Air Quality Management District. Calculating from dates of birth, average levels of 4 pollutants were estimated for the first, second and third months of each gestation.


Higher levels of 2 pollutants during the second month of pregnancy increased risk for specific heart defects:

bullet-3795104 Carbon monoxide was linked to ventricular septal defects.
bullet-3795104 Ozone was linked to conotruncal heart defects, pulmonary artery/valve defects and aortic artery/valve defects.

There was a dose-response association: increasing exposure was linked to greater risk for heart defects. Those in the highest exposure brackets had about twice the risk of those with least exposure.

We found no clear effect for oral clefts, other heart defects or chromosome abnormalities.


An association between air pollution and birth defects is biologically plausible—major heart development occurs during the second month of gestation. Hypoxia (decreased oxygen) is associated with heart and other defects in animal studies. And smoking—which creates carbon monoxide and hypoxia—increases risk for oral clefts.

We can’t be sure if the pollutants studied here are the culprits, however. Carbon monoxide levels reflect automobile exhaust and may be a marker for some other component of tailpipe emissions.

The study’s exposure information was relatively incomplete—for example, it didn’t consider other exposure sources such as mothers’ smoking or commuting patterns. And, residence at birth may be different than in early pregnancy. That an effect was seen despite these limitations is intriguing and warrants further research.