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Autism Spectrum Disorders Health Center

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Rainfall, Autism May Be Linked

Children Living in High-Precipitation Areas More Likely to Have Autism, Study Shows

Precipitation-Autism Link: Study Details continued...

"Counties that received relatively large amounts of precipitation had a relatively high rate of autism," Nicholson tells WebMD. "Counties in Oregon and Washington west of the Cascades receive four times as much precipitation as counties east of the Cascades, and have an autism rate that is twice as high," he says.

Put another way: "If there were no rain, the autism rate would be a third lower according to our analysis," Nicholson says.

The amount of precipitation children were exposed to before 3 was also positively associated with autism rates, the researchers found. The age at diagnosis varies but is sometimes made as early as 18 months.

The team also looked at each county over time, taking into account different precipitation levels from different years, he says. This was done to rule out the effect of other factors, such as differences in the quality of the health care systems from one county to another.

The relationship between precipitation and autism held, he says.

Precipitation-Autism Link: Study Interpretations

The results do raise the possibility that an environmental trigger for autism may be linked to precipitation, perhaps affecting children who are genetically vulnerable, the researchers say.

"It could be the rain itself," Nicholson says. For instance, chemicals in the upper atmosphere may be transported to the surface by the rain, the researchers speculate.

Or, as Waldman suggests, the amount of time children spend indoors when they live in a heavy rainfall area might increase their exposure to potentially harmful chemicals, such as those from cleaning products. Being indoors could decrease their sunshine exposure, thus reducing vitamin D levels.

Precipitation-Autism Link: Caveats

The observation about precipitation and autism rates may not lead to any insights at all on the causes of the disorder, writes Noel Weiss, MD, DrPh, a professor of epidemiology at the University of Washington, Seattle, in an editorial accompanying the study.

"These results are really [meant] to alert other investigators to the hypothesis," he tells WebMD.

The findings, he adds, should not be taken as a reason to move to a drier climate or a reason to ban television viewing.

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