A summary of: Michael Waldman, PhD; Sean Nicholson, PhD; Nodir Adilov, PhD; John Williams, MD, MBA (2008). Autism Prevalence and Precipitation Rates in California, Oregon, and Washington Counties Arch Pediatr Adolesc Med, 162 (11), 1026-1034

The authors briefly reviewed epidemiological data suggesting that the highest autism rates in the USA are found on Northern and Western States, while the lowest autism rates are found in the deep south (e.g., Alabama). They concluded from these findings that an environmental trigger in these Northern and Western States (such as bad weather) may be a risk factor for the development of autism. To test this hypothesis, the authors examined precipitation rates in various counties within California, Oregon, and Washington and compared these rates to the prevalence of autism in said counties. The authors found that high rate of annual precipitation was associated with high rates of autism cases, even after controlling for variables such as county level income, population size, or access to specialized services. The authors argue that this association may be to a number of factors including 1) higher rates of television viewing in very young children (although no evidence was presented suggesting that young children in rainy counties watch more television than young children in less rainy counties), 2) vitamin D deficiencies due to less sun exposure (although no evidence was presenting suggesting that vitamin D is associated with autism, or that the rates of autism are higher in polar regions given the reduced sun exposure during winter months), 3) environmental triggers associated with playing indoors instead of outdoors, and 4) possible harmful chemicals transported by the rain.

Given the preliminary/speculative nature of this study, the manuscript was accompanied by a letter from Dr. Noel Weiss from the Department of Epidemiology of the University of Washington that, while arguing that these results may not advance our understanding of the causes of autism, lauds the editorial decision to accept this article for publication. Below I provide some excerpts of Dr. Waiss’ arguments:

First, Dr. Waiss provides a sensible interpretation of the findings:

…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. Possibly, the degree of completeness of reporting itself is associated with levels of precipitation. In Oregon and Washington, for example, could it be that state agencies in the western, rainy, relatively urbanized counties have enumerated a greater proportion of children with autism than their counterparts in the eastern, arid, relatively more rural counties?

However, in response to concerns regarding the potential misinterpretation and misuse of the findings by the public, Dr. Waiss states that:

The primary audience for the article of Waldman et al is not the practicing pediatrician, and certainly, it is not a member of the public at large. These individuals cannot take away any practical message from it. Rather, the primary target is an investigator interested in the causes of autism, someone who might be able to test one or more of the etiologic hypotheses that derive from the research of Waldman et al.

I do not agree with Dr. Waiss on this last point. It is no longer the case that most scientific research is read mostly by relevant scientists. The audience of scientific peer reviewed articles has expanded dramatically, mostly due to the internet and the new level of activism and involvement with research by relevant communities (e.g., autism ).

Finally, in regards to the findings, the study does not in any way show or suggest that rain causes autism. It only states that there is an association between rainy counties and autism rates. This may be due to a large number of factors, most of which (e.g., differences in how diagnostic data is collected between counties) have nothing to do with rain.


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5 Responses to Autism Rates and Precipitation (Rain)?

  1. Ivar T says:

    As an autistic I can tell that I hate sunny places.

  2. RAJ says:

    It’s always interesting to compare autism to other better understood conditions like leprosy. The genetic data pubished over many decades in leprosy mirror most of the genetic data published in autism over the same number of decades:


    One of the more mysterious questions regarding leposy is why it dissapeared in Europe roughly one hundred years ago While the genetic risk for infection after exposure to myobacterium laprae hasn’t changed (app 5% of the worldwide general poulation) much debate over its disappearance in Europe is related to significant changes in the environment due to modern sewer systems and clean water systems which may have led to the kind of environment that reduced myobacterium laprae perhaps to extinction in developed western society’s. Its impossible to prove but leprosy is now confined to underdeveloped countries with poor sanitation an poor drinking water along with a tropical climate that appears to be just the right assortment of environments that myobacterium laprae should thrive in.

  3. Anonymous says:

    It’s interesting for us. We had one child ( a girl, no ASD) who was born when we lived in West Washington State and a boy (ASD) born in California, but my wife split her pregancy between Washington State and California. A study looking at mid pregnancy and mid infancy transplants between wet/dry areas might shed some more insight into this and reduce things like tv as possible answers.

  4. AWW says:

    The possible connection of autism with wet weather does raise other questions. Do the communities in the states studied treat their drinking water differently in periods of heavy rainfall? Are differing amounts or additional chemicals used to treat drinking water then? Or, are other toxins getting into the water supply in increased amounts due to run-off after heavy rains? Were affected children breastfed or given formula and does this make a difference? How much water did the children drink?
    Does the increase in rainfall cause certain types of mold to flourish? Did affected children have access to areas in their homes that may have had an increase in mold spores, such as heating or ventilation systems, or were they crawling over floor spaces where adults may have tracked mold spores from basements or other contaminated areas? Why the connection?

  5. Sherrill Franklin says:

    Given the theory–and a compelling one at that–that low levels of the “sunshine vitamin,” (Vitamin D) may play a role in the development of autism, it would make sense that areas with very little sunshine have higher rates of autism. Vitamin D is essential for brain and nervous system functioning and development. Since the 1980s, we’ve been using sunscreen, we drink far less milk than in previous eras and we stay indoors in front of our computers–all practices that keep us out of the sun that has helped us synthesize Vitamin D. Continuing research in this area could prove interesting…

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