Levels of mercury currently regarded as safe for adults could impair brain function, according to a study in Brazil. The study has been dismissed as too small to be conclusive, but if it is right, mercury could be slightly reducing the mental performance of millions of people worldwide.
Low levels of mercury are already thought to damage the nervous systems of fetuses and babies. After a study in the Faroe Islands showed that children exposed to mercury in the womb have memory, attention and language problems at age seven, regulatory authorities in the US and UK advised pregnant and nursing mothers not to eat large predatory fish such as tuna, shark and king mackerel. Mercury and methyl mercury (a more toxic form generated by bacteria) are most concentrated in animals near the top of the food chain.
Now a study of villagers in Brazil suggests that adults may be at risk too. "Adults may be just as sensitive to mercury as children," claims Ellen Silbergeld at Johns Hopkins University in Baltimore, Maryland. Her team studied 52 men and 77 women living in fishing villages downstream of gold mines. Much of the mercury used to extract the gold ends up in rivers and in fish. "They act almost literally as a sponge," says Silbergeld.
The researchers tested the villagers' neurological abilities by asking them, for instance, to remember a story and thread beads onto a piece of string. The higher the levels of methyl mercury in the villagers' hair- a measure of recent exposure- the greater the deficits in memory and motor skills (Environmental Health: A Global Access Science Source, vol 2, paper 8).
Most worryingly, exposure levels were not particularly high. Hair concentrations in the villagers averaged 4 micrograms of mercury per gram of hair. This is just a tenth of the level considered dangerous for adults by the World Health Organization, and not much higher than that found in many countries. In the US and Japan, for instance, the average mercury concentration in hair is around 1 and 2 micrograms per gram respectively.
But some researchers are dismissive of the team's findings. "Small studies are of no help at all," says Gary Myers at the University of Rochester Medical Center in New York state, whose team recently added to the uncertainty by failing to find any adverse effects of methyl mercury exposure among children in the Seychelles. He says only long-term studies can rule out confounding factors such as other toxic chemicals.
But Silbergeld says her study is the first to apply sensitive neurological tests to adults, and believes the results justify larger studies. While the effect on each individual was small, she says, the overall impact on communities could be significant.
Author: James Randerson, New Scientist issue: 14th June 2003