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Médiascience International
A new report shows that children's brains can be affected by low doses of mercury while they are in the womb. It can be brought about by their mothers eating mercury-contaminated fish and whale meat.
by Pete White
Fish is a valuable part of a balanced diet. But there is anxiety that the amount of mercury in some fish is so high that it could be dangerous. The American Food and Drug Administration, for example, put out a warning in September 1997 that predatory fish such as swordfish and sharks may contain methylmercury levels in excess of its one part per million (ppm) recommended limit. The FDA's advice is that the fish is safe to eat, as long as the quantity does not exceed more than one portion a week. Other countries have stricter limits for allowable mercury contaminations and some experts believe that it may be time to tighten the limits ever further.
Symptoms of mercury poisoning in adults include numbness and tingling sensations around the lips, fingers and toes if the exposure is mild. At higher levels, people have difficulty walking or speaking and tremors or jerky movements indicate high levels of exposure. It can even lead to death. Methylmercury poisoning first became an issue in the 1950s, when more than 1,000 people living around Minimata Bay, Japan, died and thousands of others were disabled after eating contaminated seafood. The most severe cases occurred in children who had been exposed to the toxicant before birth. Local industry was releasing mercury into the environment and fish caught locally were found to have up to 40ppm methylmercury.
Half of the 10,000 tonnes of mercury released into the atmosphere each year comes from the burning of fossil fuels such as coal as well as the incineration of household and industrial wastes. This mercury soon finds its way into rivers and oceans where some of it is taken up by fish in the form of methylmercury. The methylmercury levels in most fish range from less than 0.01ppm to 0.5ppm. But large predator fish such as sharks and swordfish can have concentrations of over 1ppm, while in some marine mammals it can exceed 8ppm.
There is little information about the levels of methylmercury that might damage the foetus and infant. It is, however, generally agreed that before, and soon after, birth babies are likely to be particularly vulnerable. In addition, the mercury concentration is up to 40 per cent higher in the blood of a foetus than in that of the mother.
Poisoned whale meat
The task of a recent European Commission-funded research project was to see whether low levels of exposure to methylmercury could affect unborn babies. Professor Philippe Grandjean, of the biology unit at Odense University in Denmark, chose to study people living in the Faroe Islands because they ate varying amounts of pilot whale meat, a source of methylmercury, while sharing similar lifestyles. The Faroe Islands also has a well-developed health care system, which makes long-term follow-up studies possible.
In one of the largest and most intensive studies ever in this field, members of the research team collected samples of maternal hair and blood from the babies' umbilical cords at birth. Some of the mercury in a person's diet ends up incorporated in their hair. Therefore, by analysing how much mercury was in these samples, they could discover the baby's level of exposure during pregnancy.
Then, when the children reached seven years of age, they were asked to undergo a series of 20 different, detailed neuropsychological tests. These tests assessed how rapidly their brains were developing. The researchers collected a second set of hair and blood samples so that they could see how much mercury the children had been exposed to since birth. Over 900 children took part in the study.
Children are the victims
The data clearly showed mild neurological deficits in children whose mothers had eaten food containing increased amounts of methylmercury while they were pregnant. The greater the level of exposure, the greater the risk that slight delays in the development of attention span, memory, language and other brain functions may occur. "A doubling in mercury exposure may cause a developmental delay of approximately two months for several functions," explained Grandjean.
It was also possible to detect delayed development in children who had been exposed to doses of methylmercury below the currently-recommended safe levels, corresponding to a hair-mercury concentration of 10ppm.
Grandjean stresses that their data does not suggest that eating fish is dangerous, but that contaminated food should be avoided and pregnant women, or women intending to become pregnant, need to be especially careful.
A ban on the eating of fish from some Swedish lakes and rivers
"Although most European and American diets do not include whale meat, the study is relevant to general concern about mercury pollution because the developmental delays appeared at relatively low levels of maternal mercury intake," Grandjean said.
He points out that in some parts of Europe freshwater fish is also contaminated. Sweden, for instance, has banned the eating of fish from several lakes and rivers and throughout the continent pike and eel may have high concentrations.
Some Mediterranean fishing communities have high levels of methylmercury exposure, especially the Italian coast of the Tyrrhenian Sea. In the Azores, very high concentrations (up to 26ppm) have been found in a tasty fish called white sea bream.
Madeira is also a hot spot, with black scabbard (called espada in Portuguese) containing more than 0.5ppm. Some species of tuna also contain more than 0.5ppm.
"While the average concentration of methylmercury in European fish may be low, we need to locate the groups that may be at risk," added Grandjean.
He proposes a four-point cautionary plan:
Monitoring of mercury in fish should be improved;
More detailed advice should be developed and given to pregnant women;
Major primary sources of mercury release into the environment should be identified;
Mercury pollution should be prevented.
This article was supplied by the French science journalism agency, Médiascience International. For more information on this and other articles by Médiascience International,
write to: mediasc@pophost.eunet.be
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