Urban air pollution linked to birth defects. (EH Update).

Exposure to two common air pollutants may increase the chance that a pregnant woman will give birth to a child with certain heart defects, according to a study reported in the American Journal of Epidemiology (January 1, 2002). The study provides the first compelling evidence that air pollution may play a role in causing some birth defects.

Researchers from the UCLA School of Public Health and the California Birth Defects Monitoring Program analyzed information collected by the California Birth Defects Monitoring Program on more than 9,000 babies born from 1987 to 1993 in Los Angeles, Orange, San Bernardino, and Riverside counties. Using measurements made regularly at 30 locations by the South Coast Air Quality Management District, which manages air quality in the four-county region, researchers compared air quality near the homes of children born with birth defects to air quality in the neighborhoods of children born healthy.

Pregnant women who were exposed to increased levels of ozone and carbon monoxide faced an elevated risk of having a child with conotruncal heart defects, pulmonary-artery/valve defects, and aortic-artery/valve defects. This group of heart defects occurs 1.76 times per 1,000 births, with about 935 cases in California each year. Many of these babies face open-heart surgery before they reach one year of age.

For women living in the areas with the highest levels of carbon monoxide and ozone, the risk tripled when compared with the risk for women who lived in areas of the air basin with the cleanest air. Among women who lived in areas with moderately higher pollution levels, the risk of birth defects doubled.

"We're not sure carbon monoxide is the culprit, because it could be just a marker for something else in tailpipe exhaust," said Gary Shaw of the California Birth Defects Monitoring Program and a co-author of the study. "The fact that certain heart defects are turning up in the second month of pregnancy when hearts are being formed suggests something serious may be happening. The dose-response aspect of this study certainly strengthens the findings and underscores the need for additional research. Unlike other health factors like diet or lifestyle, a pregnant woman has almost no control over the quality of air she breathes."

Researchers did not find a link between birth defects and exposure to nitrogen dioxide and larger-sized particulate matter--other air pollutants that are commonly found in the South Coast Air Basin and other urban regions. The monitoring network for particulate matter is, however, less extensive that that for other pollutants, and no monitoring is done for very small particulates, which are often found alongside carbon monoxide. The study also found no correlation between exposure to air pollution and other common birth defects such as cleft palates.

The study suggests that despite a significant decrease in urban air pollution nationally, there may be pollution problems that are not yet understood. "There has been a big reduction in the levels of criteria air pollutants like ozone and carbon monoxide over the years," Ritz said. "But there still may be air toxics and fine particles or other secondary pollutants that occur alongside carbon monoxide and ozone, but that we don't measure routinely or know about, and those things may pose health risks we don't yet understand."

More research is needed, to address some limitations of the study:

* Researchers were only able to estimate mothers' exposures to routinely measured air pollutants.

* They relied on air pollution concentrations collected at the air quality monitoring station nearest each mother's home, which could be as far as 10 miles away.

* They were unable to evaluate other potential risk factors, including maternal smoking, occupational exposures, vitamin supplement use, diet, and obesity.

Also, Ritz said, future studies should examine whether it is the routinely measured pollutants or other potentially harmful substances in air pollution that are responsible for the birth defects.

Researchers at the Southern California Particle Center are working to identify the chemical components of the microscopic soot that is contained in vehicle tailpipe emissions.