Children who live where there is heavy rainfall could be more likely to develop autism, according to media reports today. The Daily Telegraph said that researchers believe rain could be an “environmental trigger” for the condition, that makes “susceptible” children develop autism symptoms.

This study looked at annual rainfall in three US states and the number of children diagnosed with autism who lived in those areas in their first three years of life. It found there to be more children with autism living in wetter areas than in drier ones.

However this type of research can only demonstrate a link. It does not prove one way or another whether rainfall has any connection with the development of autism. There are many other factors that may determine why some children in these areas develop autism. The causes of autism remain unclear and are thought to include genetic, environmental and medical factors.

Where did the story come from?

Dr Michael Waldman and colleagues from Cornell University, Indiana University-Purdue University, and The Children’s Hospital of Philadelphia carried out this research. The study was funded by research grants from Cornell University. The study was published in the (peer-reviewed) medical journal: Archives of Pediatric Adolescent Medicine.

What kind of scientific study was this?

This was an ecological study in which the authors aimed to investigate the possibility of rainfall as an environmental trigger for autism among “genetically vulnerable” children. The researchers provide no information to explain why some children were identified as genetically vulnerable. The aim of the research was to contribute towards a better understanding of autism.

The research investigated three western US states: California, Oregon and Washington, where rainfall varies considerably from county to county. In 2005, the researchers used data from government agencies to look at the prevalence of autism in children aged six to 18 years.

The researchers used a “relative precipitation variable” –  the difference between annual rainfall levels in each county and the average rainfall for all counties in the sample. Average daily and annual rainfall for each county was also examined. In particular, they looked at the rainfall in three-year intervals when the groups of children were in their first three years of life, the time during which autism symptoms typically emerge.

The analyses focussed on whether autism was more common where it rained more. The researchers also accounted for factors such as income and ethnicity.

What were the results of the study?

The researchers found that within counties, there was more autism in school-aged children where average annual rainfall was higher. Autism rates were higher in the wetter counties of Oregon and Washington compared to those which experience less rainfall. In California, where there was less variability in rainfall across the counties there was a weaker relationship between rainfall and autism. Other demographic factors found to be associated with autism included a higher population (which increased autism prevalence) and a higher indigenous population (which decreased autism prevalence).

What interpretations did the researchers draw from these results?

The researchers conclude that their results are consistent with the existence of an environmental trigger for autism ‘among genetically vulnerable children’ – in this case, rain. They suggest that further studies are needed to focus on whether such a trigger for autism truly exists, and the possible reasons for it.

What does the NHS Knowledge Service make of this study?

This research found there to be a higher prevalence of autism among children living in areas that experienced higher rainfall than those living in drier areas. As the authors say, this finding supports the idea of an environmental trigger to autism.

However, the causes of autism remain unclear and possible causes include genetic, environmental and medical factors. This type of research can only demonstrate an association and only at a population level. It does not prove that levels of rainfall have any connection with the development of autism, or shed any light on how rain affects childhood development.

There are many other factors that may underlie this relationship. Although certain high-level demographic information has been accounted for, the study did not look at individual children, their socio-demographic background, home and family environments, education and peer groups, family or personal medical history, all of which may or may not have an effect on autism risk.

The study also only looked at three western states of the US, and further data from other geographic areas would support – or undermine – the theory that rainfall is an environmental trigger of autism.

If there is an association between rainfall and autism, it may be an indirect link. For example high rainfall causes children to stay indoors and watch more television and play more computer games. Or more rain may mean children are exposed to more chemicals in the environment. Diagnostic criteria for autism may also differ across counties.