Mercury in Air Pollution: A Link to Autism?

Can mercury (search) in air pollution cause autism (search)? Nobody knows. But a new study suggests it's worth a look.

Mercury is one heck of a toxic substance. A fraction of a teaspoon can render all the fish in a 20-acre lake unsafe to eat.

A recent Institute of Medicine study found no link between the tiny amounts of mercury in childhood vaccines and autism. But the jury is still out on whether prenatal exposure to mercury might affect brain development.

Could environmental exposure to mercury affect autism rates (search)? Raymond F. Palmer, PhD, and colleagues at the University of Texas Health Science Center explored the issue. They looked at EPA figures on industrial mercury release — most of which comes from coal-fired power plants — in 254 Texas counties. They compared these figures to autism rates from the counties' 1,184 school districts, taking into account other factors linked to autism.

Mercury Dumping, More Autism

"We found that for every 1,000 pounds of mercury released by industry, there was a 17 percent increase in autism," Palmer tells WebMD. "This is one of the most highly toxic [nerve-damaging substances] on earth. So when we talk about 50 pounds, 1,000 pounds, 2,000 pounds released into the environment, this is a tremendous amount of potential exposure. But we do not know how these exposures get into the body."

The findings are scheduled to appear in the journal Health & Place.

No Proof Mercury Pollution Causes Autism

Palmer is quick to point out that this kind of study does not prove mercury pollution causes autism.

"We show a significant relationship between mercury release into the environment and autism. But that does not prove causation," Palmer says.

Palmer's team is planning a long-term study to see whether actual fetal mercury exposure can be linked to autism.

"The ultimate goal of this work is to help us think about what we want to let out in the environment," Palmer says. "Is there an acceptable risk for mercury exposure? We don't know. We would like to affect political action, but we need harder data to do that. This study is just a first step."

Does Mercury Trigger Autism?

Philip W. Davidson, PhD, is studying the effects of mercury exposure from eating mercury-contaminated fish on child development. He's professor of pediatrics and chief of the Strong Center for Developmental Disabilities at the University of Rochester School of Medicine and Dentistry in New York.

"The question is, if a fetus is exposed to mercury, does that raise the threshold for autism? Nobody knows," Davidson tells WebMD. "As far as I know, no study has isolated symptoms of autism in individuals exposed to methyl mercury [the most dangerous form of mercury]. … We think autism comes from prenatal sources. Does mercury trigger autism? Who knows? We don't yet have any evidence."

Women, Fish, and Mercury

Davidson and colleagues are studying mercury exposure in pregnant women and their offspring in Seychelles, where the diet contains large amounts of fish. He warns, however, that the study of mercury is very complex. Studies must take into account the form of mercury, the dose of mercury, and the time of life a person is exposed to mercury.

So far, he says, the data suggest that fetal exposure to mercury is more likely to be harmful than mercury exposure during childhood.

To protect developing babies from high levels of potentially brain-damaging mercury, the government issued guidelines in March 2004 to warn women who are pregnant, nursing, or even considering having children to eat no more than two servings of fish each week.

By Daniel J. DeNoon, reviewed by Michael W. Smith, MD

SOURCES: Palmer, R.F. Health & Place, published online Feb. 17, 2005. Raymond F. Palmer,PhD, associate professor, University of Texas Health Science Center, San Antonio.Philip W. Davidson, PhD, professor of pediatrics and chief, Strong Center for Developmental Disabilities, University of Rochester School of Medicine and Dentistry, Rochester, N.Y.