Science has long known about the precarious state of a developing baby, and the precautions expectant mothers must take to protect the baby's health. But a new nationwide study from the Harvard School of Public Health (HSPH) reveals the unprecedented concern of air pollution raising a child's risk of autism.

HSPH's study is the first to link air pollution and autism across the United States, and it illuminates a startling set of risk factors that the study's researchers urge pregnant women and families everywhere to consider when choosing their place of residence.

"Our findings raise concerns," said lead author Andrea Roberts, research associate in the HSPH Department of Social and Behavioral Sciences, in a news release, "since, depending on the pollutant, 20% to 60% of the women in our study lived in areas where risk of autism was elevated."

Two studies have already looked at the link between air pollution and autism, but in both cases the scope was much smaller. While the previous studies examined only three U.S. cities, HSPH scoured data nationwide.

Researchers used data from the Nurses' Health Study II, a long-term study based at Brigham and Women's Hospital involving 116,430 nurses that began in 1989. They used 325 women who had an autistic child and 20,000 women whose children were not autistic. Controlling for income, education, and smoking during pregnancy, the researchers used data from the Environmental Protection Agency to look at pollution levels at the time of birth, compared with which children had autism.

It is unclear whether the research controlled for severity of the disorder, as the current medical understanding of autism pegs it as a spectrum disorder — from barely communicating to high-functioning.

The results of the study confirmed the previously supposed link between the two factors. Women who lived in the 20 percent of locations with the highest levels of diesel particulates or mercury in the air were twice as likely to have a child with autism as those who lived in the 20 percent of areas with the lowest levels.

Other pollutants posed a likely threat as well. Women who lived in the 20 percent of locations with the highest levels of manganese, lead, methylene chloride, and other metals, were nearly 50 percent 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 Marc Weisskopf, senior study author and an associate professor of environmental and occupational epidemiology at HSPH, in the press statement.

So far the risks remain unexplained. Researchers don't know, for example, how each pollutant affects a child's risk individually. They admit further testing needs to be done to show how build-up can affect developing babies. The difference between boys and girls also requires further examination, HSPH found, as autism occurs more often in boys, and girls comprised only a small percentage of the study.

"A better understanding of this can help to develop interventions to reduce pregnant women's exposure to these pollutants," Weisskopf said in the statement.

Source: Roberts AL, Lyall K, Hart JE, et al. Perinatal air pollutant exposures and autism spectrum disorder in the children of Nurses' Health Study II participants. Environmental Health Perspectives. 2013.