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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 course 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."

More information

For more about autism, visit the U.S. National Institutes of Health.

SOURCES: Michael Waldman, Ph.D., professor, economics, Johnson Graduate School of Management, Cornell University, Ithaca, N.Y.; Noel S. Weiss, M.D., Dr.P.H., professor, epidemiology, University of Washington, Seattle; Richard Lathe, Ph.D., Pieta Research, Edinburgh, U.K., and author, Autism, Brain and Environment; November 2008, Archives of Pediatri
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