Can rain cause autism?

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There are a lot of different theories about what causes autism, but not much consensus of opinion. In recent years, vaccination has been mooted as a potential cause of this condition, but so have lots of other things including pre-natal ultrasounds, wireless technology and certain environmental toxins. This week another potential cause of autism was added to the list: rain. Research published in Archives of Pediatric and Adolescent Medicine has found that in the USA, autism is more common in parts of the country where they have the most rain [1]. The results were highly significant from a statistical point of view. And the potential impact appeared to be large. The authors of the study estimated that elimination of the ‘rainfall factor’ would cause autism prevalence to fall by 33-43 per cent (a lot).

Epidemiological studies of this nature are not good for proving cause and effect. In other words, we cannot tell from this study that rain causes autism, only that it’s associated with it. The authors of the study suggest a number of explanations for the association though. These include:

1. In rainy areas, kids are less likely to go outside and more likely to stay in watching TV. TV viewing in young children has, apparently, been linked to what the authors describe as psychopathological characteristics, including problems with language development and behaviours consistent with attention deficit hyperactivity disorder (ADHD).

2. Staying indoors might, through reduced sun exposure, lead to relatively low levels of vitamin D. Vitamin D is important for the production of the chemical calcitriol, which is important for brain development.

3. Staying indoors puts children at risk of increased exposure to household chemicals that might play a part in the development of autism.

Of course one other possibility is that rain is actually causing autism. Not the water part of rain, of course, but something that may come with it. The stand-out suspect here is the heavy metal mercury.

While the authors of the study do not mention mercury specifically, they cite a study in support of their theory that environmental toxins may cause autism which focused on the potential role mercury plays in autism. This study found a significant link between mercury released into the environment and rates of autism [2].

One person who has looked at the potential mercury/autism link more closely than perhaps anyone else is David Kirby, a journalist and author of Evidence of Harm. You can read Kirby’s take on the recent rainfall study here.

The authors of the rainfall study admit their study does not prove that the existence of an environmental trigger for autism, but say that their results are consistent with this idea. They add that further research into whether such a trigger exists is warranted. At the current time, we don’t know whether rain can cause autism or not. But the fact of the matter remains that through the delivery of mercury or some other toxin(s), it might.


1. Waldman M, et al. Autism prevalence and precipitation rates in california, Oregon, and washington counties. Arch Pediatr Adolesc Med. 2008;162(11):1026-34.

2. Palmer RF, et al. Environmental mercury release, special education rates, and autism disorder: an ecological study of Texas. Health Place. 2006;12(2):203-9.

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