Babies born to women exposed to high ozone levels during pregnancy are at heightened risk for being significantly underweight, according to researchers at the Keck School of Medicine of the University of Southern California.
Women who breathe air heavily polluted with ozone are at particular risk for having babies afflicted with intra uterine growth retardation-which means babies only fall within the 15th percentile of their expected size. The findings were published early online on the Web site of Environmental Health Perspectives, the journal of the National Institute for Environmental Health Sciences (NIEHS).
"These findings add further evidence that our ozone standards are not protecting the most vulnerable members of the population," says Frank D. Gilliland, M.D., Ph.D., professor of preventive medicine at the Keck School and the study's senior author.
Gilliland and his colleagues examined birth records from 3,901 children who were born in California between 1975 and 1987 and participated in the Children's Health Study. Researchers with the USC-led Children's Health Study have monitored levels of major pollutants in a dozen Southern California communities since 1993, while following the respiratory health of more than 6,000 students in those communities.
The researchers gathered data such as the children's gestational age and birth weight, as well as their mothers' zip code of residence at birth. Then they determined levels of ozone, carbon monoxide and other pollutants in the air in each zip code of residence during each mother's pregnancy. Researchers only considered full-term births for the study and controlled for factors such as mothers' smoking habits.
They found that each increase of 12 parts per billion (ppb) of average daily ozone levels over a mother's entire pregnancy was associated with a drop of 47.2 grams (g)-about a tenth of a pound-in a baby's birth weight. And the association was even stronger for ozone exposure over the second and third trimesters, Gilliland says.
In addition, for each 17 ppb increase in average daily ozone levels during a mother's third trimester of pregnancy, the risk of intra uterine growth retardation increased by 20 percent, the scientists report.
The effects were strongest when total average daily ozone exposure rose above 30 ppb. Ozone levels varied from less than 20 ppb in cleaner areas to above 40 ppb in more polluted areas of Southern California.
Carbon monoxide levels affected birth weight as well. They found that each increase of 1.4 parts per million of carbon monoxide concentration during the first trimester was associated with 21.7 g (about .05 pound) decrease in birth weight and a 20 percent increase in risk of intra uterine growth retardation.