Autism is more common in rainy, coastal areas of the Pacific Northwest than in drier, inland parts of three states in the region, according to new research that suggests a possible link between the brain disorder and precipitation.

Autism was twice as common in the damp counties west of the Cascade Mountains than in those east of the range, which get four times less rain, the study in Archives of Pediatric and Adolescent Medicine shows. Kids in counties in California where rainfall was heavier also had higher rates of the disorder than children whose first three years of life were spent in drier weather.

Autism causes impaired social interactions, delayed speech, and repetitive movements or behaviors. For unknown reasons, autism prevalence has surged over the past 30 years from an estimated one in 2,500 to one in 150 U.S. children, according to the Centers for Disease Control and Prevention (CDC). It's not clear why those rates are more elevated in damp areas — including states not in the study such as New Jersey, Massachusetts, Maine and Minnesota — but bad weather that keeps genetically vulnerable kids indoors could play a role, the study authors write.

"Rates vary a lot from state to state — it doesn't seem to be random," says study-co author Michael Waldman, the Charles H. Dyson professor of management and a professor of economics at Johnson Graduate School of Management at Cornell University in Ithaca, N.Y. "There's been a big move toward thinking environmental triggers may be important, but we don’t know where to look. It [the study] provides a potentially important clue."

Children in rainy states may watch more TV, which has been linked to problems with language development and attention-deficit hyperactivity disorder. Because they spend less time in the sun, those kids may be deficient in vitamin D, which is needed for the brain-building steroid calcitriol, the scientists suggest.

It's also possible that chemicals in some household cleaners trigger autism, and children who spend more time indoors have greater exposure to them. In addition, the study says, the rain itself carries chemicals that may affect certain children.

California, Oregon and Washington State all house leading academic medical centers. The study controlled for the presence of those centers near the coast as well as for greater wealth and parental education that might lead urban children to be more readily diagnosed, Waldman says. His team is now analyzing autism rates in other bad-weather states.

Image by iStockphoto/Marcin Pawinski