Researchers have found evidence that high levels of mercury exposure while a child is still in the womb can cause lasting, irreversible brain damage.

Mercury in the children's diets also seemed to cause damage, which the researchers say might suggest a revision of current fish consumption guidelines is in order.

Other experts, however, do not believe this is the final word. "I don't think it's definitive," says Dr. Gary Myers, a professor of neurology and pediatrics at the University of Rochester Medical Center.

This paper and another, which reports heart function changes related to mercury, appear in the February issue of the Journal of Pediatrics.

In high enough quantities, methylmercury, which can contaminate both salt water and freshwater fish, is known to have adverse effects on the nervous system.

National and state guidelines on fish consumption differ but focus on protecting pregnant women and women of childbearing age. Some advisories also extend to children.

The current researchers looked at brain function in 14-year-old children in the Faroe Islands, located in the North Atlantic Ocean between Norway and Iceland. The diet in the Faroes is high in mercury-containing fish, especially whale meat.

"In the Faroes, when the mothers were pregnant, the exposure was about five to 10 times higher than the U.S. average exposure, but it has declined substantially," says study author Dr. Philippe Grandjean, an adjunct professor of environmental health at Harvard School of Public Health in Boston.

The children had been exposed to mercury both prenatally and after they were born.

More than 1,000 mothers and their children participated in the study, providing cord blood samples taken at birth and hair samples from their children, taken at ages 7 and 14.

In most cases, the mercury level in the mothers' hair at the time of birth exceeded one microgram per gram, which is the exposure limit recommended by the U.S. Environmental Protection Agency. Children had lower levels.

The researchers measured brain function by putting electrodes on the children's skull (much like an EKG), and stimulating the brain with different sounds. One ear got a sharper "rat-a-tat-tat" sound, while the other got a dull hissing, Grandjean explains.

The electronic system connected to the electrodes was capable of picking up tiny signals from inside the brain.

"It's a simple method of looking into how the nerve cells fire information to each other using electrical signals," Grandjean says. "That's where we saw that there were delays the higher the mercury exposure."

The researchers saw the same effects in the children when they were 7 and 14. "This indicates to us that if a brain is exposed to mercury during development in the mother's womb, then the effect that we see during childhood is going to be a lasting effect," he says. "We're seeing that a brain that is affected during development will not recover, and I think this sort of puts the mercury question into perspective. If this was just a passing effect, it's something entirely different. But here we're talking about something that will stay with that child for a lifetime, and that will affect the child's education, academic success, financial success and whole equality of life."

These particular delays are associated with deficits in memory, language, attention, and motor speed, among other things, Grandjean says.

The researchers also found that current dietary levels of mercury were also having an effect on the children. "We're now talking about effects that actually occur within the range of exposure in the United States," Grandjean says. "It indicates to me that the vulnerability of the brain during development is not just a matter of gestation. This is something that extends into childhood and probably into adolescence."

Other experts are not convinced that current levels of mercury pose such grave dangers.

"Exposure to mercury in the Faroes is associated with consumption of whale meat, and whales are not part of most people's seafood diet, and they happen to have a variety of other toxins present in them," says Myers, the University of Rochester neurologist. "It's very hard to extrapolate from a diet that contains sea mammals to one that's a pure fish diet. Some people feel that that's legitimate, but it seems a bit of a stretch to myself."

In addition, Myers adds, some of the children studied did eat whale meat after they were born.

"The next logical step would be to do the same thing with people who only consume fish," Myers says. "The exposure here in the United States is from fish, so if you want to know what happens to people who eat fish, the logical way to approach that is to look at them."

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