Scientists in the U.S. say there appears to be a link between high levels of rainfall and the prevalence of autism - they say children living in high annual rainfall areas appear more likely to develop autism, which raises again the possibility of an environmental trigger for the disorder.
According to the research from Cornell University, precipitation may affect genetically vulnerable children and provide an environmental trigger for autism.
Over the last three decades autism rates have increased from approximately 1 in 2,500 to 1 in 150 children, and while some of the increase is possibly due to more active monitoring and changes in diagnostic criteria, the researchers say the possibility of a true increase in prevalence cannot be excluded.
They say despite the increase in prevalence and the increased attention paid to the condition, knowledge about what causes autism is limited and while it is understood that biological factors play an important role, environmental triggers may also be important.
Dr. Michael Waldman and colleagues obtained autism prevalence rates from state and county agencies for children born in California, Oregon and Washington between 1987 and 1999.
By using daily precipitation reports from the National Climatic Data Center, they were able to calculate the average annual rainfall by county from 1987 through 2001, which covers the dates when the children were school-aged.
They say autism prevalence rates for school-aged children in California, Oregon and Washington in 2005 were positively related to the amount of rainfall these counties received from 1987 through 2001 and in Oregon and California counties the same applied.
They say autism prevalence was higher in children that experienced relatively heavy rainfall when they were younger than 3 years which corresponds with the time when autism symptoms usually appear and when any post-natal environmental factors would be present.
The researchers say several potential explanations exist for the link - rainfall may be associated with more indoor activities, such as television and video viewing, that affect behavioral and cognitive development - the increased amount of time spent indoors also may expose children to more harmful chemicals, such as those in cleaning products, or decrease their exposure to sunshine, which helps the body produce vitamin D.
They also say there is also the possibility that rainfall itself is more directly involved in that there may be a chemical or chemicals in the upper atmosphere that are transported to the surface by rainfall.
The researchers say there is no direct clinical evidence of an environmental trigger for autism linked to rainfall and say their results, though preliminary, warrant further research to establish whether such a trigger exists.
This study was supported by unrestricted research grants from Cornell University.
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