Pregnancy and pollution: women living in areas with poor air quality have babies with lower birthweights.

Pregnant women exposed to moderate amounts of several common air pollutants have babies with lower birthweights than do women in areas with cleaner air, according to a new study.

Newborns with low birthweights face an increased risk of lifelong health problems. Previous studies searching for a link between air pollution and birthweight had yielded mixed results.

Now, in one of the largest studies of this kind, scientists at Yale University looked at records of 358,504 births in Massachusetts and Connecticut. The team found that four types of air pollution correlate with low birthweight. The culprits are carbon monoxide, nitrogen dioxide, and two classes of airborne particles: those smaller than 10 and smaller than 2.5 micrometers (designated [PM.sub.2.5]).

"Maternal exposure to air pollution may adversely affect risk of low birthweight, even in areas without high pollution levels," says Michelle L. Bell, lead scientist on the newly reported work. Air-pollution amounts were based on Environmental Protection Agency records for the 15 counties in which the women lived while pregnant. Only two counties--New Haven and Fairfield, Conn.--didn't meet EPA's air-quality standards, exceeding the standard for [PM.sub.2.5].

Carbon monoxide showed the largest effect. In one comparison, the scientists considered the average birthweights in counties at the 75 percent point in rank for a given pollutant and in counties at the 25 percent mark. For carbon monoxide, infants in those groups differed in birthweight by an average of 16.2 grams. The next-worst offender was [PM.sub.2.5], which showed a difference of 14.7 g, the scientists report online and in an upcoming Environmental Health Perspectives.

These differences in birthweight can increase the newborn's risk of complications such as gastrointestinal infections and respiratory problems in the first weeks of life, comments Srimathi Kannan of the University of Michigan in Ann Arbor.

However, only 4 percent of the babies in the study met the clinical standard for low birthweight--less than 2,500 g (about 5.5 pounds)--which is associated with life-threatening complications in infancy and heart disease in adulthood. A woman's risk of having a low-birthweight baby increased by no more than 5.4 percent when she lived in a county at the 75 percent mark for air pollutants rather than in a county at the 25 percent mark.

In arriving at these results, the researchers adjusted for many factors that can influence birthweight, such as prenatal care, gestational length, type of delivery, and the child's sex and birth order. They also considered the mother's race, education, marital status, age, and tobacco use, all of which have been shown to influence the weights of newborns.

The new study is "much more comprehensive in its investigation" than previous research, Kannan says, noting that the biological mechanisms linking these pollutants to reduced fetal growth are still poorly understood.