The Environmental Protection Agency believes that about 630,000 of the roughly 4 million babies born annually in the United States -- twice as many as previously thought -- may be exposed to dangerous levels of mercury in the womb, according to an analysis released Thursday.
The primary source of newborns' exposure to mercury is the fish and shellfish their mothers eat. Mercury in children can impair motor functions, learning capacity, vision and memory, and can cause a variety of other symptoms related to neurological damage.
The EPA's analysis reflects a new understanding among scientists in the U.S. and Japan that umbilical cord blood has higher mercury concentrations than a mother's blood, said Kate Kathryn Mahaffey, the author of the analysis and a division director in the EPA's toxics and pesticides office.
The new information comes as the Food and Drug Administration is redrafting its guidelines on how much and what kind of seafood women of childbearing years can eat without putting their babies at risk.
At the same time, the EPA is taking public comment on its proposal to limit power plant emissions of mercury into the air. The emissions are believed to work their way into ground water and then through the food chain, reaching the fish sold in supermarkets.
The EPA based its estimates on data from the Centers for Disease Control and Prevention, which last year found that 8% of women of childbearing age had mercury blood levels higher than 5.8 parts per billion, the level the EPA then considered safe.
Given the new finding that umbilical cord blood has higher concentrations of mercury, the EPA believes that the safe level for mercury in mothers' blood is 3.5 parts per billion. About 15% of women of childbearing age had blood levels that high, according to the CDC study.
The EPA formerly thought there was a 1-1 ratio between mercury concentrations in blood and the blood that reached the fetus. But it now believes that umbilical cord blood has 1.7 times more mercury, on average.
Mahaffey stressed that the science was evolving and that the estimates could change. But she urged expectant mothers and women who planned to become pregnant to choose fish that have lower mercury levels and higher levels of Omega-3 fatty acids, which are beneficial to developing fetuses.
Swordfish, shark, tilefish and king mackerel are all high in mercury but relatively low in Omega-3 fatty acids. Sockeye salmon and herring have low mercury levels but are high in Omega-3 fatty acids.
"It's important for the public to recognize the nutritional value of fish, and it's important that we work hard to keep the food supply as low in contaminants as we can," Mahaffey said.
Environmental and public health groups said the report should serve as a wake-up call to the Bush administration.
"This heightens the urgency for FDA to give women adequate advice on what fish are safe to eat, and it ups the burden on the administration to cut mercury pollution from coal-fired power plants," said Jane Houlihan, vice president of the Environmental Working Group, a research and advocacy organization.
"The problem is twice as serious as previously believed."
Houlihan said the FDA should particularly warn women not to eat albacore, or white meat tuna, which is high in mercury but is not on the FDA's list of fish to avoid.
Last week, the EPA announced its plan to reduce mercury emissions from power plants by 70% over 15 years.
The agency's own Children's Health Protection Advisory Committee called the proposal inadequate.