Texas researchers have found a possible link between autism and mercury in the air and water.
Studying individual school districts in Texas, the epidemiologists found that those districts with the highest levels of mercury in the environment also had the highest rates of special education students and autism diagnoses.
The study does not prove that mercury causes autism, cautioned the lead author, Raymond F. Palmer of the University of Texas Health Science Center in San Antonio, but it provides a “provocative” clue that should be further investigated.
“Mercury is a known neurotoxin,” said Dr. Isaac Pessah of UC Davis’ MIND Institute, who was not involved in the study. “It’s rather intriguing that the correlation is so positive,” meaning that there was a strong, direct relationship between mercury and autism levels. “It makes one worry.”
California has the highest environmental burden of mercury of any state in the country, and it also has what appears to be the highest rate of autism as well -- although some critics attribute this perceived high rate to enhanced surveillance associated with the state’s special education program.
Autism is a severe developmental disorder in which children seem isolated from the world around them. There is a broad spectrum of symptoms, but the disorder is marked by poor language skills and an inability to handle social relations.
The incidence of autism has grown dramatically over the last two decades, from about one in every 2,000 children to as high as one in every 166. Researchers have been hard-pressed to explain the increase, but many believe mercury to be the culprit.
The purported link between autism and mercury has been a subject of intense debate. In the past it has centered primarily on the mercury-containing preservative thimerosal, which was once widely used in vaccines.
Many parents have argued that thimerosal causes autism because their children seemed to develop the neurological disorder shortly after they received childhood vaccinations.
That link has been largely discredited, and researchers are beginning to look at the potential effects of the metal from other sources.
Mercury is routinely released from power plants burning fossil fuels, and it spreads widely in air and water. Much of the fish consumed in some regions is contaminated with mercury. In California, gold mining was a big mercury source, and there are many mercury hot spots near mines and downstream, such as in Clear Lake.
In the new study, Palmer and his colleagues used Environmental Protection Agency data about the release of mercury in 2001 in Texas’ 254 counties and correlated that with the number of special education cases and autism diagnoses in the 1,200 school districts. Texas is fourth in the amount of mercury released into the environment annually, trailing California, Oregon and West Virginia.
The study, which will appear in the journal Health & Place, found that for every 1,000 pounds of mercury released into the environment, there was a 43% increase in special education services and a 61% increase in the autism rate.
The exception to the rule was Brewster County, which had a high autism rate but did not report significant mercury levels to the EPA. When Palmer investigated, however, he found that the county had been home to one of the largest mercury mines in the nation.
“Perhaps [the mercury] just stays in the environment forever. We don’t know,” Palmer said.
More work will be required to determine whether mercury is the agent that causes the disorder. Palmer is expanding his studies to look for historical correlations -- attempting to determine, for example, if increases in the rate of autism over time can be associated with increases in mercury release.
Dr. Irva Hertz-Picciatto and her colleagues at the MIND Institute, meanwhile, have begun a potentially more definitive study in which they are measuring the levels of mercury and other toxic metals, such as cadmium and lead, in children with autism to see if they are higher than in healthy children. Results will be available in a couple of years.