MONDAY, Nov. 3, 2008 (HealthDay News) -- Children who live in areas of the United States that get a lot of precipitation appear to have a higher risk of developing autism, a new study suggests.
Because these children may spend more time indoors or because rain brings chemicals in the atmosphere to the ground, they might be exposed to environmental triggers that can trigger a genetic predisposition to autism, the researchers say.
"There seems to be a strong association between precipitation and autism diagnosis rates," said lead researcher Michael Waldman, a professor of economics at the Johnson Graduate School of Management at Cornell University.
Waldman, whose son was diagnosed with autism spectrum disorder, isn't saying that rain causes the condition. "Our finding strongly suggests that there is some factor which is positively correlated with precipitation, which is serving as a trigger for autism," he said.
One possible explanation for this correlation is vitamin D deficiency, Waldman said. "There is a fair amount of research that vitamin D deficiency in young children causes problems. As children aren't outside as much, they aren't getting enough vitamin D, and that's serving as a trigger for autism," he said.
Another possibility is children are spending too much time watching TV or videos, Waldman said. "There are various papers showing associations between early childhood television viewing and various problems concerning cognitive outcomes, sleep problems, behavior problems, etc.," he noted.
A third possibility is exposure to chemicals in the home which trigger autism, Waldman said. In addition, there may be a chemical or chemicals in the upper atmosphere that are transported to the surface by precipitation.
There is debate about whether autism is caused by genetics alone or genetics and the environment, Waldman said. "Our results are inconsistent with it being just genetic."
The report was published in the November issue of the Archives of Pediatrics & Adolescent Medicine.
For the study, Waldman's team looked at the prevalence of autism among children in California, Oregon and Washington. They also used data from the National Climatic Data Center to calculate the average annual rainfall by county in these states.
The researchers found among school-aged children in these states that the prevalence of autism rose as the amount of precipitation increased. In fact, the prevalence in autism increased up to 30 percent in the rainiest counties.
Over the past three decades, the number of children diagnosed with any form of autism has increased from one in 2,500 children to one in 150 children. Some of the increase is most likely due to better diagnosis and the changing definition of autism, which now encompasses a variety of conditions called autism spectrum disorder.
Waldman's group, however, insists that a real increase in the numbers of autistic children cannot be ruled out.
Richard Lathe, an autism expert from Pieta Research in Edinburgh, Scotland, thinks that Waldman might be on to something.
"Nevertheless, one must be vigilant, because statistical correlations do not necessarily imply causality," Lathe said. "This caveat aside, the authors demonstrate, with better than 99 percent certainty, that the correlation is not by chance."
Lathe thinks the most likely explanation for the association between autism and rain is that rain carries chemicals in the atmosphere to the ground.
"This explanation is plausible," Lathe said. "Emissions from manufacturing industries, power plants, and from domestic waste incineration generally rise to the troposphere to be diluted into the large volume of the atmosphere. Precipitation can dump this load back on the land, to be absorbed by plants and animals in the food chain," he said.
One possibility that needs to be addressed further is that exposure need not be in the child, Lathe said. "There has been a suggestion that maternal exposure to environmental toxins might contribute to autism in children," he said.
These results are not definitive evidence in favor of the hypothesis that autism has an environmental trigger, but the results are consistent with the hypothesis, Lathe said. "For the future, one feels it will be essential to study levels of toxins in soil, crop and food samples from the different counties investigated in the Waldman study. A positive correlation would greatly reinforce the environmental hypothesis," he said.
Dr. Noel S. Weiss, a professor of epidemiology at the University of Washington in Seattle, and author of an accompanying journal editorial, thinks the results of the study need to be taken with a grain of salt.
"This is a coarse analysis," Weiss said. "There are difficulties, because autism is not that unequivocally defined, and the criteria for diagnosis can vary from place to place and over time. The message is really to other scientists who might examine this relationship."
Weiss isn't convinced that the association between autism and precipitation is real. "It could be, but I don't think so," he said. "But it's probably worth looking into."
For more about autism, visit the U.S. National Institutes of Health.