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 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 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.