The report was published in the November issue of the Archives of Pediatrics & Adolescent Medicine.
For the study, Waldman's team looked at the prevalence of autism among children in California, Oregon and Washington. They also used data from the National Climatic Data Center to calculate the average annual rainfall by county in these states.
The researchers found among school-aged children in these states that the prevalence of autism rose as the amount of precipitation increased. In fact, the prevalence in autism increased up to 30 percent in the rainiest counties.
Over the past three decades, the number of children diagnosed with any form of autism has increased from one in 2,500 children to one in 150 children. Some of the increase is most likely due to better diagnosis and the changing definition of autism, which now encompasses a variety of conditions called autism spectrum disorder.
Waldman's group, however, insists that a real increase in the numbers of autistic children cannot be ruled out.
Richard Lathe, an autism expert from Pieta Research in Edinburgh, Scotland, thinks that Waldman might be on to something.
"Nevertheless, one must be vigilant, because statistical correlations do not necessarily imply causality," Lathe said. "This caveat aside, the authors demonstrate, with better than 99 percent certainty, that the correlation is not by chance."
Lathe thinks the most likely explanation for the association between autism and rain is that rain carries chemicals in the atmosphere to the ground.
"This explanation is plausible," Lathe said. "Emissions from manufacturing industries, power plants, and from domestic waste incineration generally rise to the troposphere to be diluted into the large volume of the atmosphere. Precipitation can dump this load back on the land, to be abso
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