Women exposed to high levels of air pollution may be at an increased risk of having a child with autism, a new study suggests.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

US scientists analysed data from a long-term study of women that began in 1989. From this, they focused on 325 women who had gone on to have a child with autism and 22,000 women who had a child without the condition.

The scientists used data on air pollution provided by the US Environmental Protection Agency to estimate the women's exposure to various pollutants while pregnant. They also looked at potential links between autism and levels of pollution at the time and place of birth.

The study found that women who lived in the 20% of locations with the highest levels of mercury or diesel particulates in the air were twice as likely to go on to have a child with autism compared to women who lived in the 20% of areas with the lowest levels.

Sources of mercury pollution include incinerators and cement plants, while diesel particulates are found in the exhaust fumes of various vehicles, including cars, trucks and buses.

Meanwhile, women who lived in the 20% of locations with other types of air pollution, such as lead and methylene chloride, were around 50% more likely to have a child with autism compared to women who lived in the 20% of places with the lowest levels.

Lead pollution used to come largely from cars, but is now produced in places such as incinerators and metal processing plants. Methylene chloride is used, among other things, as a solvent in paint strippers and as a propellant in aerosols for products such as insect sprays.

"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. A better understanding of this can help to develop interventions to reduce pregnant women's exposure to these pollutants," the team from the Harvard School of Public Health said.

Details of these findings are published in the journal, Environmental Health Perspectives.