(TIME.com) -- Pollutants in the air are known to affect brain development, but the first national study of in utero exposure and autism rates raises new concerns.
Researchers at the Harvard School of Public Health (HSPH) say that early-life exposure to pollution, including diesel particulates, mercury and lead, could contribute to a higher risk of autism disorders.
They came to that conclusion after analyzing data from a nationwide sample of 116,430 nurses participating in the Nurses' Health Study II, an on-going survey that began in 1989. Among the volunteers, 325 had children with autism, and most of them lived in areas with higher levels of pollutants than those who didn't have children affected by the developmental disorder.
Last year, a study of over 500 kids found that those with autism were two to three times more likely than other kids to have been exposed to car exhaust, smog, and other air pollutants early on.
But those studies involved mothers and children in limited geographic areas; in the current study, published online in the the journal Environmental Health Perspectives, the scientists surveyed pollution exposure and autism rates across the entire U.S.
They compared autism rates to levels of pollutants measured by the U.S. Environmental Protection Agency during the women's pregnancies.
Expectant mothers who lived in the 20% of locations around the country with the highest pollution levels in the form of diesel particulates or mercury were two times as likely to have a child with autism compared to those who lived in the 20% of locations with the lowest levels of pollution. Women who lived in the 20% of areas with the highest levels of other pollutants like lead, manganese, methylene chloride and other metals were nearly 50% more likely to have a child with autism.
"Our results suggest that new studies should begin the process of measuring metals and other pollutants in the blood of pregnant women or newborn children to provide stronger evidence that specific pollutants increase risk of autism," said senior study author Marc Weisskopf, an associate professor of environmental and occupational epidemiology at HSPH, in a statement. "A better understanding of this can help to develop interventions to reduce pregnant women's exposure to these pollutants."
Documenting the effect that prenatal exposure can have on children's development could help to untangle some of the conflicting evidence about how pollutants may contribute to autism and other disorders such as cancer, hyperactivity and obesity.
The connection between air pollution and weight gain was quite dramatic; researchers measured polycyclic aromatic hydrocarbons (PAHs) found in cigarette smoke and car exhaust and found that kids born to mothers with the highest PAH levels during their third trimester had a 79% greater risk of becoming obese.
By the time the kids turned 7, their risk was more than 2.25 times higher, most likely because the chemicals can disrupt hormones that regulate growth and development.
It's still not clear how each of the pollutants may be hampering normal childhood development, but toxic buildup could result from blood vessels that contract or harden prematurely in an effort to protect tissues from excess exposure to the chemicals. That idea is supported by some studies in adults that have linked exposure to air pollution with hardening of the arteries and a higher risk of heart disease.
While it's no surprise that exposure, even in utero, to potentially harmful chemicals found in the air can adversely affect children's brains and bodies, studies like Weisskopf's that reveal these correlations are the first step toward figuring out which pollutants are especially harmful and which agents are most closely tied to certain diseases.
That in turn could lead to smarter ways of measuring these agents in expectant mothers' blood and possibly intervening with treatments to reduce or even prevent some of these conditions.
This article was originally published on TIME.com.