For the second time this year, a scientific paper has linked air pollution and autism. The latest study, carried out by researchers at Harvard School of Public Health, suggests exposure to air particulates during pregnancy -- especially the third trimester -- may increase a woman's child developing autism.

"Our data add additional important support to the hypothesis that maternal exposure to air pollution contributes to the risk of autism spectrum disorders," lead study author Marc Weisskopf, associate professor of environmental and occupational epidemiology at Harvard, said in a press release. "The specificity of our findings for the pregnancy period, and third trimester in particular, rules out many other possible explanations for these findings."

Earlier this year, a University of Pittsburgh study found a correlation between exposure levels of chromium and styrene -- two air-based toxins known to be linked to endocrine system disruption and neurodevelopmental problems -- and autism rates.

The new Harvard study is the first to focus on a link between fine particulate matter or soot, tiny bits of dust, carbon and other chemicals, and autism diagnoses. Both studies compared autism rates with localized pollution data supplied by the Environmental Protection Agency. The EPA monitors fine particulate matter (PM2.5), particles 2.5 microns in diameter or smaller, at sites all over the United States.

In an effort to tease out a relationship between pollution and autism, researchers at Harvard compared localized pollution levels with 245 children with autism spectrum disorder and a control group of 1,522 children without autism. Scientists charted exposure levels before, during and after pregnancy, and found that while exposure levels before and after pregnancy showed no correlation, the children of mothers with high exposure during pregnancy were up to two times more likely to develop autism. The higher the exposure levels, the greater the risk.

"The evidence base for a role for maternal exposure to air pollution increasing the risk of autism spectrum disorders is becoming quite strong," Weisskopf said. "This not only gives us important insight as we continue to pursue the origins of autism spectrum disorders, but as a modifiable exposure, opens the door to thinking about possible preventative measures."

As always, researchers acknowledge the need for more research. Even when accounting for the immense variety of independent variables, establishing causation is exceedingly difficult. Still, researchers say the correlation is specific enough to begin considering specific recommendations for avoiding exposure during pregnancy.

"If it was this study by itself, I wouldn't take much notice. But it's now the fifth that has come to the same conclusion," researcher Frank Kelly told the BBC. Kelly, who was not involved in the study, is the director of the environmental research group at King's College London.

"It is biologically plausible, the placenta is there to ensure the fetus has optimal supply of nutrients, but if chemicals are entering the mother's body then the fetus will have access to those, too," Kelly added. "Women should be made aware of the potential links so they don't get excessive exposure."

The latest study was published in the journal Environmental Health Perspectives.