In a major breakthrough, Canadian scientists have shown that DNA in mice is damaged if the animals breathe downwind from steel mills.

This could make the rodents and their progeny more susceptible to cancer and other diseases.

The research was conducted in Hamilton, the centre of Canada's steel industry, and suggests that thousands of people who work or live around the plants or similar facilities elsewhere in the world may face genetic damage from air pollution.

The researchers suspect that the mutations, found at twice the expected rate, were caused by exposure to polycyclic aromatic hydrocarbons, a family of compounds emitted from coke ovens during steelmaking. The biggest mutation was on mice sperm.

"You don't see this kind of doubling in nature," said James Quinn, a biologist at Hamilton's McMaster University and one of the researchers. "You see it in places like Hamilton Harbour."

The research is the first to show that genetic mutations can be caused by exposure to pollutants other than radiation in outside air.

The study was conducted by a team from McMaster University and Health Canada. A paper outlining its findings was published yesterday in the Proceedings of the National Academy of Sciences.

The researchers conclude that "there is an urgent need to investigate the genetic consequences associated with exposure to chem-ical pollution through the inhalation of urban and industrial air."

The finding shows that chemical substances in the air can affect DNA, just as radiation, for instance in nuclear fallout, can do.

In the Canadian tests, the researchers found that the offspring of mice placed a kilometre from Hamilton's steel mills had as many as twice the number of genetic mutations as did the offspring of mice raised at a llama farm 30 kilometres from the city.

The mice near the steel mills had litters of about 20 per cent fewer pups than the rural mice.

The experiment was conducted in the fall of 1999, when two groups of mice were simultaneously placed near the steel mill and on the farm. Both groups were kept in identical, electrically heated sheds for 70 days and fed the same brand of commercial mouse chow and bottled water. The proximity of the steel mills was the only difference between the groups.

Mr. Quinn had conducted research that found high levels of mutations in herring gulls nesting near steel mills in Hamilton, Sault Ste. Marie, Detroit and Gary, Ind. But it was not clear whether this result was caused by air pollution or contaminated fish the birds ate.

The mouse study indicates that air pollution is clearly the culprit, the researchers said.

Polycyclic aromatic hydrocarbons, or PAHs, are viewed as the likely cause because they are industrial chemicals known to cause mutations and are found in Hamilton air at concentrations more than 50 times above those at the rural test site.

There are about 100 varieties of PAHs; they are found in cigarette smoke, grilled food, mothballs and vehicle exhaust.

The finding could shed light on why coke ovens are such a dangerous place to work.

At Stelco Inc. in Hamilton, a union health-and-safety official said coke-oven workers receive extra pension credits, allowing earlier retirement because of occupational dangers.