No assumptions about vaccines can be made on the basis of the air pollution study. "Mercury in the air is a different type than in vaccines," Windham said.
The new study examined elemental mercury, which is released into the air from coal-burning power plants, chlorine factories and gold mines. It spreads globally and builds up in food chains, particularly in oceans. Levels of mercury are increasing in many parts of the world, largely from power plants in China and India.
The researchers had not expected to be able to discern a relationship between autism and the air pollution data.
The five metals and solvents are common industrial pollutants, but air is only one source of exposure, because they also contaminate water and food.
Some experts say that if there is a link between mercury and autism, it most likely comes from fish consumption, the main route of mercury exposure. A 20-year, ongoing study in Denmark's Faroe Islands has shown that children have slightly reduced intelligence when mothers consumed excessive mercury in seafood.
The largest limitation or uncertainty in the Bay Area study is that the pollution data did not come from measurements of compounds to which the mothers were actually exposed. Instead, they were based on estimates calculated by the U.S. Environmental Protection Agency using computer modeling of industrial emissions.
Windham said that "there could be other explanations" for the link they found. For example, it could be that women who live in the worst-polluted areas also smoke more or eat more contaminated seafood. The scientists did not track down the mothers to compare lifestyles.
Researchers at Johns Hopkins University's School of Public Health are conducting a similar study in the Baltimore area to see if they replicate the findings.
