Studies suggest that children have poor performance on neurological tests--particularly tests of fine motor skills, attention, language, drawing and verbal memory--if their mothers consumed mercury-contaminated fish or seafood during pregnancy. The immune system and heart could also be harmed. Scientists are uncertain whether there is a precise time during pregnancy when the fetus' brain is most sensitive to injury.
Most of the findings come from studies of children in the Faroe Islands of Denmark, where people eat large amounts of fish and whale meat highly contaminated with mercury.
About 40 tons of mercury a year are emitted into the air by coal-burning power plants in the United States. It then falls into lakes, rivers and oceans and is converted to methylmercury, the most harmful form for human beings.
Mercury is found in fish worldwide because it is transported long distances in the air. Forty U.S. states have issued mercury advisories for fish.
California has no coal-burning power plants, but large amounts of mercury are still washing into San Francisco Bay from old, abandoned gold mines.
"It's a problem most everywhere, but there can be regional concentrations that make the problem even worse," Bender said.
Mercury emissions from power plants have declined because of equipment designed to control smog and sulfur. Further reductions would take technology that could cost the nation's utilities billions of dollars a year.
"If we're causing a public health risk by releasing mercury, then we are going to address that," Bailey said, but "it has to make sense from an environmental and business standpoint."