MERCURY, which is known to damage the nervous system and disrupt mental development, can also cause infertility in men at levels well below those the World Health Organisation (WHO) says are safe.

Mike Dickman, biologist at the University of Hong Kong, and Clement Leung of the In Vitro Fertilisation Centre in Hong Kong, measured mercury levels in men with fertility problems. The duo discovered that hair of men with slow, deformed or sparse sperm contained 40 per cent more mercury on average than that of men who had normal sperm counts. At any given age, men with 3.7 parts per million (PPM) or more of mercury in their hair were twice as likely to be infertile as men with 2 PPM or less. The WHO's safety limit is 10 PPM.

Acute mercury poisoning is known to harm sperm formation in mice and monkeys. However, this is the first clear association between reduced fertility in men and environmental exposure to mercury. The results were published recently in The Science of the Total Environment (Vol 13, No 67).

The men got their mercury from seafood, says Dickman. Volcanoes and coal-fired power stations release the metal into the environment. It is converted to methyl mercury by marine bacteria and taken up by fish. "Men in Hong Kong who eat four or more seafood meals per week have significantly more mercury than those who eat less," he says.

The authors say the WHO's recommended safe level for mercury in food (0.5 PPM) is too high for places such as Hong Kong where people eat lots of seafood. In 1997, the US Environment Protection Agency estimated that at least 170,000 people in the US receive higher doses than what the WHO recommends.