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.
Ozone, or O3, is a gas made up of three oxygen atoms. Although a natural layer of ozone in the stratosphere helps protect life on Earth from the sun's rays, ozone at ground level is harmful to health. It is created through interactions among tailpipe exhaust, gasoline vapors, industrial emissions, chemical solvents and natural sources and is worsened by sunlight and heat.
The study findings echo results from the few, smaller studies examining the relationship between ozone and birth weight. Animal studies support the role of O3 in reduced birth weight: in these models, pregnant rats were particularly vulnerable to lung inflammation from O3. Researchers suspect that inflammation from O3 may prompt the release of certain chemicals into the bloodstream, which may harm the placenta.
Carbon monoxide, meanwhile, is an odorless gas that primarily comes from vehicle exhaust. In high concentrations, the gas can harm healthy people; and in lower concentrations, it can hurt those with heart disease and can affect the nervous system.
The gas reduces hemoglobin's ability to carry oxygen where it is needed in the body; that may hurt the delivery of oxygen to a fetus. However, more research is needed to understand the roles of ozone and carbon monoxide in fetal development.
"Fetal growth and birth weight are strongly linked to morbidity and mortality during childhood and adulthood," Gilliland says, "so it's clear that air quality is important to everyone's healthy development."