Dr Irva Hertz-Picciotto, chief of the division of environmental and occupational health at the University of California, Davis, has been trying to establish how much of the dramatic, eight-fold rise in autism in California can be attributed to the broadening of the diagnostic criteria for autism, the increased diagnosis of autism at younger ages, the fact that today's parents are vastly more aware of autism than they were a decade ago and how much is due to environmental factors.

‘The awareness thing is very hard to quantify,’ she says, ‘but at some point, as more and more parents became aware of autism, the increase should have levelled off. Instead we see a continued increase.’

Dr Hertz-Picciotto notes that the lion's share of autism funding goes to genetic studies; she argues that it's high time more effort was put into looking for environmental factors that cause autism in genetically susceptible individuals.

‘A lot has changed in the environment over the last 15 years and these changes include things like medications people take and assisted reproduction technology as well as what is in soaps, shampoos and toothpaste.’

Autism expert Dr Michael Cuccaro of the University of Miami agrees that it is time to consider environmental factors. ‘There are environmental risk factors that give rise to a wide range of developmental conditions, and there's no reason to think autism isn't one of them. Environmental studies are already under way and research organisations are eager to fund them but the difficulties go far beyond funding. We only have 20,000 to 25,000 genes but we have a hundred thousand environmental exposures. How do you control for that?’ he says. ‘And your genes stay the same, while environmental exposures may have come and gone and this makes these studies very difficult to do.’

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The Hertz-Picciotto study appears in the January issue of Epidemiology.

First published in February 2009

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