A US study finds unborn and very young children living in traffic-congested places more than double their chances of autism - but some experts say the research ignores a number of other factors.
The findings from scientists in California, published in the journal of Archives of General Psychiatry, investigated the possible link between autism rates and traffic pollution.
The authors claim that unborn children and those up to the age of one more than doubled their chances of developing the disorder living in traffic-congested areas, and that children in neighbourhoods with the highest air pollution levels were three times more at risk than those living in areas least exposed.
The components of these particles could be hazardous to the brain - Doctor Heather Volk
Lead scientist Doctor Heather Volk, from Keck School of Medicine at the University of Southern California said: "This work has broad potential public health implications.
"We've known for a long time that air pollution may affect the brain. From studies conducted in the lab we know that we can breathe in tiny particles and they can produce inflammation.
"Particles have varied composition and there are many chemicals that can bind them. The components of these particles could be hazardous to the brain."
But British scientists in the field of autism have criticised the findings, saying bias in this study cannot be ruled out as a number of factors have not been taken into account, such as lifestyle, nutrition and family history. While the paper does show an association between estimated air pollution and autism, it does not show that air pollution causes autism.
Professor Emily Simonoff, professor of child and adolescent psychiatry at the Institute of Psychiatry, King's College London, said the results should be interpreted with caution: "This is potentially an important finding and it is therefore essential to consider the strengths and limitations of the study.
"At present pregnant women should continue to look after their health during pregnancy but should not be unduly concerned."
How the study works
The study looked at data on 245 children without the condition and 279 affected by autism. Air pollution records from the US Environmental Protection Agency were used to estimate exposure to nitrogen dioxide and small sooty particles produced from vehicle exhausts.
Doctor Volk and her team assessed a range of factors, including how far away people lived from roads, traffic levels, meteorological factors such as wind direction and information from air quality monitors.
Until further research is carried out we will not know definitely if the association is there. Sophia Xiang Sun
Sophia Xiang Sun, of the Autism Research Centre at the University of Cambridge, said: "Although traffic-related air pollution might be one of the contributing factors to the development of autism, other factors cannot be ruled out.
"These factors include second-hand smoking during pregnancy, medical conditions related to pregnancy, indoor air pollution, especially if the family has a history of mental disorders as autism is highly genetic.
"Further research is needed to investigate the potential association between traffic-related air pollution and autism, ideally a prospective study that monitors traffic-related air pollution with the control of indoor air pollution and smoking.
"Until further research is carried out we will not know definitely if the association is there and, if it is there, how direct and to what degree."
Correlational, not causal?
Autism is a lifelong disability that affects how a person communicates with and relates to other people. It is a spectrum condition which means that while all people with autism share certain difficulties the condition affects them in different ways.
The exact cause of autism is still unknown but past research has suggested that a combination of genetic and environmental factors may account for changes in brain development. What precise role the environment plays however is still being investigated.
Professor Uta Frith, emeritus professor of cognitive development at UCL, said: "It seems to me very unlikely that the association is causal, rather than correlational. The recent paper adds detailed measures of air pollution but does not get us any further since it does not present a convincing mechanism by which pollutants could affect the developing brain to result in autism.
"Rather than taking the results at face value I would like to know what it implies to live near a highway. It could imply all sorts of disadvantages, any of which might be associated with increased risk of autism - and with increased risk of other disorders as well."