NEW YORK - Children whose mothers were exposed to high levels of fine particulate pollution in late pregnancy have up to twice the risk of developing autism as children of mothers breathing cleaner air, scientists at Harvard School of Public Health reported on Thursday.
The greater the exposure to fine particulates emitted by fires, vehicles, and industrial smokestacks the greater the risk, found the study, published online in Environmental Health Perspectives.
Earlier research also found an autism-pollution connection, including a 2010 study that found the risk of autism doubled if a mother, during her third trimester, lived near a freeway, a proxy for exposure to particulates.
But this is the first to examine the link across the United States, and "provides additional support" to a possible link, said Heather Volk of the University of Southern California Children's Hospital, who led earlier studies.
United States diagnoses of autism soared to one in 68 children in 2010 (the most recent data) from one in 150 in 2000, government scientists reported in March. Experts are divided on how much of the increase reflects greater awareness and how much truly greater incidence.
Although the disorder has a strong genetic basis, the increasing incidence has spurred scientists to investigate environmental causes, too, since genes do not change quickly enough to explain the rise.
The Harvard study included children of the 116,430 women in the Nurses' Health Study II, which began in 1989. The researchers collected data on where the women lived while pregnant and levels of particulate pollution.
They then compared the prenatal histories of 245 children with autism spectrum disorder to 1,522 normally-developing children, all born from 1990 to 2002.
There was no association between autism and fine particulate pollution before or early in pregnancy, or after the child was born. But high levels of exposure during the third trimester doubled the risk of autism.
Evidence that a mother-to-be's exposure to air pollution affects her child's risk of autism "is becoming quite strong," said Harvard epidemiologist Marc Weisskopf, who led the study, suggesting a way to reduce the risk.
It is not clear how tiny particles might cause autism, but they are covered with myriad contaminants and penetrate cells, which can disrupt brain development.
Last year the Environmental Protection Agency, citing the link to asthma, lung cancer and cardiovascular disease, tightened air quality standards for fine particulate pollution. States have until 2020 to meet the new standards.