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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