Children who live in the U.S. Northwest's wettest counties are more likely to have autism, but it is unclear why, U.S. researchers reported on Tuesday.
Michael Waldman of Cornell University and colleagues were searching for an environmental link with autism, a condition characterized by learning and social disabilities.
They got autism rates from state and county agencies for children born in California, Oregon and Washington between 1987 and 1999 and plotted them against daily precipitation reports.
"Autism prevalence rates for school-aged children in California, Oregon and Washington in 2005 were positively related to the amount of precipitation these counties received from 1987 through 2001," they wrote in the Archives of Pediatrics & Adolescent Medicine.
Dr. Michael Fitzpatrick, a London physician who wrote "Defeating Autism: A Damaging Delusion", expressed doubt, noting that autism diagnoses are on the rise in all climates.
No one know what causes autism, whose symptoms range from severe social avoidance to repetitive behaviors and sometimes profound mental retardation.
The U.S. Centers for Disease Control and Prevention estimates that about one in every 150 children has autism or a related disorder such as Asperger's Syndrome. Rates in many countries have been rising, although that may be partly due to increased reporting and diagnosis of the condition.
Doctors agree there is a genetic component to autism. They also theorize that something in the environment and possibly conditions in the womb can trigger the condition.
The researchers said their study supports this idea.
Perhaps infants and toddlers are kept are kept indoors in front of the TV more in rainy climates, and that somehow causes brain changes, they said. Or perhaps they breathe in more harmful chemicals while indoors.
Vitamin D deficiency caused by insufficient time in the sun might also be a trigger, they said.
"Finally, there is also the possibility that precipitation itself is more directly involved," they wrote. Perhaps a chemical or chemicals in the upper atmosphere are transported to the surface through rain or snow.
"In recent years autism has been blamed on everything from discarded iPod batteries to mercury from Chinese power stations, from antenatal ultrasound scans to post-natal cord clamping, from diet to vaccines," Fitzpatrick said in a statement.
The U.S. Centers for Disease Control and Prevention has launched a long-term study to find the causes of autism and other childhood conditions.