Children living in regions with higher levels of annual rainfall may be more predisposed to autism, a study released on Monday suggests. However, it’s not the water itself that leads to autism, researchers believe, but the indoor activities children do on rainy days.

Autism is a spectrum of different disorders ranging in severity and in symptoms from the mild Asperger’s syndrome to more severe autism. The disease, characterized by poor social interactions, impaired communication and repetitive behavior, affects as many as 1 in every 150 kids in the U.S., according the Centers for Disease Control and Prevention. Studies have shown that males are four times more likely to have autism than females. Previous statistics showed there was only one child out of 2,500 suffering from autism. The increase, however, has been attributed mainly to improvements in the way doctors are able to diagnose the disease.

In the past few years, more researchers have been exploring the possibility that the disorder involves an interaction of genetics and environmental factors. The new study focuses on environmental factors leading to autism.

“I strongly believe it’s not the precipitation itself. My sense is, if truly there is an environmental trigger, my guess is it is one of the factors related to indoor activity,” 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.

These indoor activities involve chemical exposure to indoor substances such as cleaning products, TV viewing, and vitamin D deficiency from too little sunlight, all of them affecting cognitive development.

For the study, Dr. Waldman and his colleagues analyzed autism prevalence rates from state and county agencies for children born in California, Oregon and Washington from 1987 to 1999. They also calculated average annual rainfall by county from 1987 through 2001.

The researchers found that in areas of California, Oregon and Washington that experienced high levels of rain and snowfall from 1987 to 2001, autism rates among school-related children rose when measured in 2005. Those children diagnosed with autism would have been under 3 during the periods of high precipitation, the period during which autism is generally diagnosed.

“Counties that received relatively large amounts of precipitation had a relatively high rate of autism,” says Sean Nicholson, PhD, associate professor of policy analysis and management at Cornell and a co-author of the study. He added that counties in Oregon and Washington west of the Cascades, where the amount of precipitation is four times higher than counties east of the Cascades, register an autism rate that is twice as high.

However, the researchers stressed that the association between autism and rain has not been clinically proved, and that possible explanations need further testing.

The study was published in the Archives of Pediatrics and Adolescent Medicine, a journal belonging to the American Medical Association.