of seafood - the ones who are at the greatest risk of toxicity.

"Individuals with high methylmercury exposures from frequent fish consumption might have little or no margin of safety," the NAS said in a summary of its report that was released Tuesday. "The population at highest risk is the children of women who consumed large amounts of fish and seafood during pregnancy."

The full report was to be released to Congress later Tuesday by the NAS, which advises the federal government on scientific matters.

The NAS researchers looked at current Environmental Protection Agency references, and said that some populations may be more in harm's way than previously thought when it comes to suffering the damaging effects of methylmercury.

Scientific evidence confirms that low doses of methylmercury can be devastating to the developing fetus and can cause irreversible damage to the developing nervous system. Babies born with acute exposures have had severe disabilities, including mental retardation, seizure disorders, cerebral palsy, blindness and deafness. There may be more subtle effects as well, such as a lowered IQ, abnormal muscle tone and a loss of motor function.

Heavy metals such as mercury and their toxic effects are finally garnering more public attention. Last July, the FDA caught drug companies by surprise when they ordered them to stop using mercury as a preservative and antibacterial in infant vaccination preparations. Previously, the manufacturers had used thimerosal -- which is 49-percent ethylmercury by weight and is nearly identical molecularly to methylmercury -- in the vaccines.

Senators Patrick Leahy, D-Vt., and Tom Harkin, D-Iowa, had been concerned that the FDA was being cavalier in not enforcing the current EPA allowable levels. The FDA is supposed to take legal action and remove a product from the market if they find more than the allowable levels of mercury in fish.

"This report validates the soundness of the EPA's scientific methods and standards as well as its commitment to the public's health," Leahy said Tuesday in a statement. "After several long years of delay and waiting for the last word in scientific evidence, we now have the last word."

Humans ingest methylmercury primarily by eating fish and other seafood. It is then rapidly absorbed through the gastrointestinal tract and easily penetrates the placental barrier as well as the blood-brain barrier.

Fetuses, with their developing brains and nervous systems, are particularly vulnerable to the poison, the report said, and some may be receiving exposures at levels that could cause adverse effects. The committee estimated that some 60,000 children may be born in the United States every year with neurological problems that can lead to poor school performance due to their in-utero methylmercury exposure.

"Trends in methylmercury exposure, including regional differences, should be analyzed, as should subpopulations whose diets are high in fish and seafood," said Robert Goyer, chair of the committee that authored the report.

The EPA's current reference dose for methylmercury is 0.1 parts per million (1 ppm), which would most likely be found in an average serving of predator fish, such as swordfish, tuna, shark, sea bass or halibut. Mercury is also found in some shellfish, such as crab.

The EPA has recommended that pregnant and nursing women eat fish only once per month. The agency estimated that if a woman ate only 3 ounces of tuna per day, that would put her fetus at risk for neurological problems.

Mercury occurs naturally in the environment, but it is also spewed out into the air by power plants burning fossil fuels such as oil, coal and gas; medical waste incineration is also a large contributor to mercury in the environment. It is carried in the air and deposited into the soil and eventually ends up in surface groundwater, and later the ocean. The mercuric ions are converted to the readily absorbed methylmercury by bacteria in the aquatic sediments.

Once mercury enters the food chain, it is accumulated in the fat cells of fish. The large fish tend to have larger amounts because they eat the smaller contaminated fish.

The FDA has set an "action level" at 1 ppm but some groups have complained that they do not enforce this level. The Mercury Policy Project released a report last month in anticipation of Tuesday's NAS report that blasted the government's enforcement of current action levels.

"The scant, but existing, FDA data show that significant quantities of seafood exceeding the action level are sold throughout the United States. FDA's data indicates that 36 percent of swordfish, 33 percent of shark and nearly 4 percent of the large tuna sampled exceeded the FDA's 1 ppm action level for methylmercury," the report said.

The NAS report said that more studies were needed on the impact of methylmercury and illnesses experienced later in life -- such as hypertension and cardiovascular disease.

They recommended that regional differences in exposures be looked at, especially those with high consumption of fish; they also advised that dental amalgams and the mercury exposure from fillings be monitored more closely.

Leahy said: "There are no more excuses. It is time to cut mercury pollution and to protect Americans' health. For years, Congress and the special interests have kicked the ball down the field; this report brings us to the goal line."

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