Autism is more prevalent among children who live in areas with higher annual rainfall levels, a new study suggests.
The research has found that, in California, Oregon and Washington, autism rates are higher than the state medians in counties that experience more than 43 centimetres of rain each year.
The study also found that autism was more prevalent among children who had lived in heavy rainfall areas before they were three years old, around the time autism is usually diagnosed.
The researchers speculate that several factors could explain the rain-autism connection, including:
- Rain may lead more children to stay indoors and watch television, which may affect their behavioural and cognitive development.
- Spending more time indoors may increase kids' exposure to indoor pollutants, such as harsh cleaning chemicals.
- Rain itself may carry toxic chemicals that interfere with children's development.
- Overcast days and spending more time indoors may lead to a vitamin D deficiency in children, which has been linked to a higher risk of developing a number of health problems.
Scientists have yet to establish indoor pollution or chemicals in rain as triggers of autism.
However the authors conclude that, "further research focused on establishing whether such a trigger exists and on identifying it is warranted."
The research was led by Cornell University professor Michael Waldman. The findings are published in the November issue of Archives of Pediatrics & Adolescent Medicine.
The cause or causes of autism still elude researchers. Autism symptoms can include communication and interaction difficulties, repetitive behaviours and learning impairments.
Autism rates have increased steadily over the last 30 years, from about one in 2,500 children to one in 150 children, according to information accompanying the study.
Some of the increase is due to better monitoring by doctors, as well as changes in diagnostic techniques.
However, the role biological and environmental factors play in incidence rates has yet to be determined.
In an editorial accompanying the study, Dr. Noel S. Weiss of the University of Washington, Seattle cautions that the findings are preliminary and do not "call for alarm" before further studies can be conducted.
