Air pollution may raise preterm birth risk

NEW YORK (Reuters Health) - Exposure to air pollution later in pregnancy may raise a woman's risk of delivering her baby too soon, a new study suggests.

The findings, reported in the journal Epidemiology, do not prove that air pollution, per se, triggers preterm labor in some women. But they do provide "limited support" for that theory, the researchers say.

Some previous studies have linked air pollution exposure in both the first and third trimester of pregnancy to an increased risk of preterm delivery.

In the current study, researchers looked at the relationship between air pollution levels and the risk of preterm delivery among more than 476,000 women who gave birth in the Atlanta metropolitan area between 1994 and 2004.

The researchers tracked daily pollution levels using data from various air-quality monitors in five counties.

They found that while there were no clear connections between preterm birth and most of the air pollutants they studied, three particular air pollutants were related to a higher risk.

Specifically, the daily rate of preterm births inched upward when levels of fine particulate matter were elevated over the previous week. The increase was seen among women who lived within 4 miles of an air-quality monitoring site.

Car exhaust is the prime source of fine particulate matter, but it is also produced by power plants and certain other industrial sources. The particles are small enough to be inhaled deeply into the lungs.

The strongest evidence was for a role of nitrogen dioxide (NO2), a marker of motor vehicle exhaust, noted lead researcher Dr. Lyndsey A. Darrow, of Emory University in Atlanta.

NO2 was linked to an increase in the daily preterm-birth rate when levels were elevated in the previous six weeks.

In theory, Darrow told Reuters Health in an email, exposure to air pollution later in pregnancy could activate an inflammatory response in the body that, in turn, might trigger early labor. She noted that there is increasing evidence that inflammation is involved in preterm birth.

Still, the role of air pollution in premature birth remains unclear.

"The scientific evidence is mixed as to whether or not urban air pollution has an effect on fetal development," Darrow said.

She suggested that pregnant women who want to "err on the side of caution" try to avoid air pollution sources like high-traffic areas.

SOURCE: Epidemiology, September 2009.