Rainfall Tentatively Associated With Autism In Children

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Main Category: Autism
Also Included In: Pediatrics / Children's Health;  Psychology / Psychiatry
Article Date: 04 Nov 2008 - 0:00 PST

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Populations of children who live in counties with higher levels of annual precipitation tend to have higher prevalence of autism, according to a report released on November 3, 2008 in Archives of Archives of Pediatrics & Adolescent Medicine, one of the JAMA/Archives journals. This association may indicate that autism may be influenced by an environmental trigger associated with precipitation.

In the past several decades. the prevalence of autism has increased, a trend with significance in public health and politics. According to the article, this increase has been from approximately one in 2,500 to one in 150 children. This increase has been partially attributed to differences in the way autism is detected. That is, over time, the criteria used to diagnose autism have changed, and monitoring of the disease has become more active. The authors indicate that these factors may not be solely responsible for the increase. "Nevertheless, the possibility of a true increase in prevalence cannot be excluded," they 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."

To investigate a potential link between the weather and autism, Michael Waldman, Ph.D., of Cornell University, Ithaca, N.Y., and colleagues, examined autism prevalence for children born in California, Oregon, and Washington from state and county agencies  between 1987 and 1999. These records were compared to the average annual rainfall in that county between 1987 and 2001, as calculated from daily precipitation reports from the National Climatic Data Center. This way, the authors were able to investigate the precipitation rates in the years when the children were school-aged, and link them to the appropriate number of afflicted children.

The authors interpret the results of their study carefully, focusing on the time between birth and three years. "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 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." Autism symptoms usually present in this period, and so might be more clearly linked to potential post-natal environmental factors.

There are many potential explanations for this positive association, according to the authors. For one, the precipitation may lead more children to perform indoor activities, such as television and audio viewing, that affect behavioral and cognitive development. By increasing the amount of time spend indoors, children may be exposed to more harmful chemicals such as those in cleaning supplies. Alternatively, the indoor time may lead to less exposure to sunshine, altering the levels of vitamin D in the child. The authors also do not rule out a direct link between precipitation itself and autism. "Finally, there is also the possibility that precipitation itself is more directly involved," they write. "For example, there may be a chemical or chemicals in the upper atmosphere that are transported to the surface by precipitation."

The authors note that these results are preliminary. There is no clinical evidence of an environmental trigger for autism that is associated with precipitation, but this study indicates that efforts should be made to investigate this association. They conclude: "Further research focused on establishing whether such as trigger exists and on identifying it is warranted."

In an accompanying editorial, Noel S. Weiss, M.D., Dr.P.H., of the University of Washington, Seattle, comments on the vague nature of the ecological study performed for the article. This includes several other alternate explanations for the differences in autism prevalence, which may not be linked to precipitation at all. "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," he writes. "However, there are other possible explanations for the association with precipitation that they have observed."

These include issues inherent in this sort of data collection. "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 writes. "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."

He concludes that appropriate wording of the conclusions of this study is necessary, without leaping to strong causal statements, for appropriate interpretation. "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 continues: "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'."

Autism Prevalence and Precipitation Rates in California, Oregon, and Washington Counties
Michael Waldman; Sean Nicholson; Nodir Adilov; John Williams
Arch Pediatr Adolesc Med. 2008;162(11):1026-1034.
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Precipitation and Autism: Do These Results Warrant Publication?
Noel S. Weiss
Arch Pediatr Adolesc Med. 2008;162(11):1095-1096.

Written by Anna Sophia McKenney
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