New study links urban air pollution and birth defects for the first time


New study links urban air pollution and birth defects for the first time —


Exposure to 2 pollutants during the 2nd month of pregnancy increased risk for specific heart defects. Carbon monoxide—found in automobile exhaust and industrial emissions—was linked to ventricular septal defects. Ozone—a by-product when other pollutants react in sunny conditions—was linked to conotruncal heart defects, pulmonary artery/valve defects and aortic artery/valve defects.

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


The association of birth defects with air pollution is biologically plausible— the second month of gestation is a period of major heart development in humans. Hypoxia (decreased oxygen) is associated with heart and other defects in animal studies. While data in humans are sparse, researchers from the California Birth Defects Monitoring Program and other researchers have found links between mothers’ smoking and other types of birth defects. (Smoking creates carbon monoxide and causes hypoxic conditions.)

We don’t know if the pollutants studied here are the causes in the birth defects observed however. Carbon monoxide levels reflect automobile exhaust, and may be a marker for some other component of tailpipe emissions.

The study’s investigators caution that more study is needed. Exposure information was not ideal—for example, it did not consider other exposure sources such as mother’s smoking or commuting patterns. And, the residence at the time of birth may not be the same as that in early pregnancy.

Given these considerations, however, the fact that an effect was seen is intriguing and warrants further study.


Researchers estimated mothers’ exposure to air pollutants in early pregnancy, comparing children with birth defects to those without. Cases with oral clefts, various types of heart defects and/or chromosome abnormalities were ascertained by the California Birth Defects Monitoring Program among 734,000 births from 1987-1993 in Los Angeles, Orange, San Bernardino and Riverside counties. Mothers’ residence at delivery was mapped to one of 30 ambient air monitoring tracking stations operated by the Southern California Air Quality Management District. Calculating from dates of birth, the average levels of 4 pollutants—carbon monoxide, ozone, nitrogen dioxide and particulate matter—was estimated for the 1st, 2nd and 3rd months of each gestation.