27 November 2012 Last updated at 03:14 GMT
Autism: Traffic pollution linked, study suggests
By James Gallagher
Health and science reporter, BBC News
The possibility that autism is linked to traffic pollution has been raised by researchers in California.
Their study of more than 500 children said those exposed to high levels of pollution were three times more likely to have autism than children who grew up with cleaner air.
However, other researchers said traffic was a "very unlikely" and unconvincing explanation for autism.
The findings were presented in the Archives of General Psychiatry journal.
What is autism?
Autism and Asperger's syndrome are part of a range of disorders that can cause difficulties with communication and social skills
The conditions can lead to isolation and emotional problems for those living with them
Conditions can vary from very mild, where the person can function as well as anyone else, to so severe they cannot take part in normal society
The conditions are collectively known as autistic spectrum disorders and affect more than 580,000 people in the UK
Source: BBC Health
Data from the US Environmental Protection Agency were used to work out levels of pollution for addresses in California.
This was used to compare exposure to pollution, in the womb and during the first year of life, in 279 children with autism and 245 without.
The researchers from the University of Southern California said children in homes exposed to the most pollution "were three times as likely to have autism compared with children residing in homes with the lowest levels of exposure".
They have previously shown a link between autism and living close to major roads.
They warn that there could be "large" implications because air pollution is "common and may have lasting neurological effects".
However, other researchers questioned how pollution could alter the brain's development and lead to autism.
Uta Frith, a professor of cognitive development at University College London, said: "It seems to me very unlikely that the association is causal."
She said the study did 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".
One of the challenges with this style of study is that it is difficult to account for every aspect of life which might affect the probability of developing autism, such as family history.
It means the study cannot say that autism is caused by traffic pollution, merely that there could be a link between the two.
Sophia Xiang Sun, from the University of Cambridge's autism research centre, argued that cutting pollution would be a good idea anyway.
"We know that traffic-related air pollution can contribute to many other diseases and conditions, and it is biologically plausible it also has a role in pathways of autism.
"However, whether or not the potential association between autism and traffic-related air pollution exists, reduction of traffic-related air pollution would be good for public health."
Traffic-Related Air Pollution, Particulate Matter, and Autism
Heather E. Volk, PhD, MPH; Fred Lurmann; Bryan Penfold; Irva Hertz-Picciotto, PhD; Rob McConnell, MD
Arch Gen Psychiatry. 2012;():1-7. doi:10.1001/jamapsychiatry.2013.266.
Published online November 2012
Context Autism is a heterogeneous disorder with genetic and environmental factors likely contributing to its origins. Examination of hazardous pollutants has suggested the importance of air toxics in the etiology of autism, yet little research has examined its association with local levels of air pollution using residence-specific exposure assignments.
Objective To examine the relationship between traffic-related air pollution, air quality, and autism.
Design This population-based case-control study includes data obtained from children with autism and control children with typical development who were enrolled in the Childhood Autism Risks from Genetics and the Environment study in California. The mother's address from the birth certificate and addresses reported from a residential history questionnaire were used to estimate exposure for each trimester of pregnancy and first year of life. Traffic-related air pollution was assigned to each location using a line-source air-quality dispersion model. Regional air pollutant measures were based on the Environmental Protection Agency's Air Quality System data. Logistic regression models compared estimated and measured pollutant levels for children with autism and for control children with typical development.
Setting Case-control study from California.
Participants A total of 279 children with autism and a total of 245 control children with typical development.
Main Outcome Measures Crude and multivariable adjusted odds ratios (AORs) for autism.
Results Children with autism were more likely to live at residences that had the highest quartile of exposure to traffic-related air pollution, during gestation (AOR, 1.98 [95% CI, 1.20-3.31]) and during the first year of life (AOR, 3.10 [95% CI, 1.76-5.57]), compared with control children. Regional exposure measures of nitrogen dioxide and particulate matter less than 2.5 and 10 μm in diameter (PM2.5 and PM10) were also associated with autism during gestation (exposure to nitrogen dioxide: AOR, 1.81 [95% CI, 1.37-3.09]; exposure to PM2.5: AOR, 2.08 [95% CI, 1.93-2.25]; exposure to PM10: AOR, 2.17 [95% CI, 1.49-3.16) and during the first year of life (exposure to nitrogen dioxide: AOR, 2.06 [95% CI, 1.37-3.09]; exposure to PM2.5: AOR, 2.12 [95% CI, 1.45-3.10]; exposure to PM10: AOR, 2.14 [95% CI, 1.46-3.12]). All regional pollutant estimates were scaled to twice the standard deviation of the distribution for all pregnancy estimates.
Conclusions Exposure to traffic-related air pollution, nitrogen dioxide, PM2.5, and PM10 during pregnancy and during the first year of life was associated with autism. Further epidemiological and toxicological examinations of likely biological pathways will help determine whether these associations are causal.
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