The finding may have nothing to do with the rainfall or snow itself, they say, but rather factors associated with the precipitation, such as the need to stay indoors more.
"Our results suggest there is an environmental trigger associated with precipitation," says Michael Waldman, PhD, the study's lead author and the director of the Institute for the Advancement of Economics at Cornell University in Ithaca, N.Y.
"I strongly believe it's not the precipitation itself," he tells WebMD. "My sense is, if truly there is an environmental trigger, my guess is it is one of the factors related to indoor activity." On that list: chemical exposure to indoor substances such as cleaning products, TV viewing, and vitamin D deficiency from too little sunlight.
Autism: The Back Story
In the past 30 years, the rates of autism have increased from about one in 2,500 children to one in 150. Although some of the increase is attributed to a broadened definition of autism, not all can be linked to that, experts say. In the past few years, more researchers have been exploring the possibility that the disorder involves an interaction of genetics and environmental factors, Waldman says.
Autism spectrum disorders refer to autism and a broader range of developmental disorders marked by social impairment, language or communication difficulties, and unusual behavior, such as discomfort with human contact.
Precipitation-Autism Link: Study Details
Two years ago, Waldman and his colleagues had zeroed in on the effects of television viewing as a risk factor for autism, says Sean Nicholson, PhD, associate professor of policy analysis and management at Cornell and a co-author of the study, published in the November issue of Archives of Pediatrics & Adolescent Medicine.
"And the more we worked on it, the more we said, 'Let's take a step back,'" Nicholson says.
They decided to look at precipitation, selecting to focus on California, Washington, and Oregon. They obtained autism prevalence rates in 2005 for children born in those three states between 1987 and 1999 and calculated average annual precipitation by county from 1987 to 2001. They also computed the autism rates in relation to the average annual precipitation in the counties when the children were younger than age 3.
"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 age 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.
The results are not definitive evidence of a precipitation-autism link, as Waldman, Nicholson, and their colleagues acknowledge in the report.
"But the results are consistent with the hypothesis, and, therefore, further research focused on establishing whether such a trigger exists and on identifying it is warranted," they conclude.
By Kathleen Doheny
Reviewed by Louise Chang
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