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Women exposed to higher levels of air pollution during pregnancy are more likely to give birth to children who develop elevated blood pressure before age 10, according to a new study.

Past studies have linked high blood pressure in both children and adults with direct exposure to air pollution. The new research, published Monday in the ÃÛÑ¿´«Ã½ journal , set out to determine if indirect fetal exposure to air pollution might be associated with higher blood pressure during childhood.

The results found that women exposed to high levels of air pollution late in pregnancy might be at particular risk of having children with higher blood pressure.

“Our study indicates that the third trimester may be a particular window of susceptibility,” said the study’s senior author, Dr. Xiaobin Wang, a pediatrician and director of the Center on Early Life Origins of Disease at Johns Hopkins University Bloomberg School of Public Health in Baltimore.

The study looked at data taken between 1998 and 2012 from 1,293 pairs of mothers and children from the Boston Birth Cohort. Researchers examined outdoor air quality levels from the Environmental Protection Agency, taken near each woman’s home when they were pregnant. They then looked to see which children had higher systolic blood pressure – the top number in a blood pressure reading – during follow-up visits between the ages of 3 and 9.

Wang said children exposed to higher levels of air pollution in the womb during the third trimester were 61 percent more likely to have elevated blood pressure in childhood compared to those exposed to the lowest level. Based on their findings, the researchers estimate that about 2.4 million women of childbearing age in the U.S. could be affected.

“Future studies are needed to confirm our findings, but it is safe to say that women during pregnancy should try to avoid exposure to air pollution,” Wang said.

High blood pressure is a major risk factor for cardiovascular disease, contributing to the death of an estimated 7.5 million people worldwide each year.

The study findings lend additional support for at least maintaining, if not strengthening, air pollution standards set in 2012 under the U.S. federal law known as the Clean Air Act, said Noel Mueller, a coauthor of the study. “We need regulations to keep our air clean, not only for the health of our planet, but also for the health of our children,” said Mueller, an assistant professor of epidemiology at Johns Hopkins University Bloomberg School of Public Health.

Dr. Joel Kaufman, a professor of medicine, epidemiology and environmental health at the University of Washington’s School of Public Health, said the study “is another piece of evidence that air pollution exposures affect your health and could affect your baby’s health as well.”

Even so, Wang pointed out that because it’s an observational study, it does not prove a clear cause-and-effect. Kaufman, who was not involved in the research, also pointed out that the study didn’t use state-of-the-art measuring tools for assessing air pollutants.

“I don’t think the data from this study is enough to say you should move [from polluted areas] during pregnancy, but if you’re going to be exercising, which you’re still encouraged to do when you’re pregnant, you should think about avoiding heavily trafficked areas as a way to avoid more air pollution exposures,” Kaufman said.

According to a report issued this month from the World Health Organization, nine out of 10 people worldwide breathe highly polluted air, and 7 million die each year because of outdoor and indoor air pollution. The new study in Hypertension focused on fine particulate matter in outdoor air, which according to the EPA is made of hundreds of different chemicals and comes mostly from cars, power plants and industrial sources. Other sources include fires, chimneys, unpaved roads and construction sites.

“Cleaner air is associated with improved cardiovascular health for you and your children,” Kaufman said. “I think the takeaway message from this study is we need to focus on continuing efforts to reduce environmental pollution.”

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