Precipitation Levels May Be Associated With Autism

In the past 30 years, autism rates have increased from approximately one in 2,500 to one in 150 children, according to background information in the article. Some of the increase is likely due to more active monitoring and changes in diagnostic criteria. "Nevertheless, the possibility of a true increase in prevalence cannot be excluded," the authors write. "Despite the increase in prevalence and the resulting increased attention paid to the condition, knowledge about what causes autism is limited. It is understood that biological factors play an important role, but environmental triggers may also be important."

Michael Waldman, Ph.D., of Cornell University, Ithaca, N.Y., and colleagues obtained autism prevalence rates from state and county agencies for children born in California, Oregon and Washington between 1987 and 1999. Using daily precipitation reports from the National Climatic Data Center, they calculated average annual rainfall by county from 1987 through 2001—which spans the dates when the children were school-aged.

"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," the authors write. "Similarly, focusing on Oregon and California counties with a regional center, autism prevalence was higher for birth cohorts that experienced relatively heavy precipitation when they were younger than 3 years." This corresponds to the time at which autism symptoms usually appear and when any post-natal environmental factors would be present.

Several potential explanations exist for the positive association, the authors note. Precipitation may be associated with more indoor activities, such as television and video viewing, that affect behavioral and cognitive development. The increased amount of time spent indoors also may expose children to more harmful chemicals, such as those in cleaning products, or decrease their exposure to sunshine, which helps the body produce vitamin D. "Finally, there is also the possibility that precipitation itself is more directly involved," the authors write. "For example, there may be a chemical or chemicals in the upper atmosphere that are transported to the surface by precipitation."

Because there is no direct clinical evidence of an environmental trigger for autism that is associated with precipitation, the results are preliminary, the authors note. However, "further research focused on establishing whether such as trigger exists and on identifying it is warranted," they conclude.

This study was supported by unrestricted research grants from Cornell University.

Editorial: Findings Tentative but Worth Publishing

"As Waldman et al indicate, one can conceive that precipitation or its consequences (such as increased television watching, reduced vitamin D levels and enhanced exposure to indoor chemicals) might increase the incidence of autism," writes Noel S. Weiss, M.D., Dr.P.H., of the University of Washington, Seattle, in an accompanying editorial. "However, there are other possible explanations for the association with precipitation that they have observed."

"First, the criteria used to diagnose autism, and the completeness with which such diagnoses are identified by state agencies and regional centers, likely vary to a considerable extent across counties," Dr. Weiss continues. "Second, as is true in many cross-population comparisons, there may be unmeasured correlates of precipitation—beyond the consequences of precipitation—that bear on the occurrence of autism that themselves differ across counties."

"Of course, if a study's findings are no more than tentative ones—certainly, those of Waldman et al must be viewed as tentative—responsible authors will stress this," Dr. Weiss concludes. "In this instance, I believe that Waldman et al have indeed reported their results responsibly. They have made it clear that the message the public should take from their data regarding precipitation and autism is the same one suggested by an editorialist commenting on a recently observed modest association between prenatal exposure to cell phone use and behavior problems in childhood: 'No call for alarm, stay tuned'."