The findings, published in the Proceedings of the National Academy of Sciences, suggest that steel mill workers and people living near those mills should be checked for damage to their health, said the researchers, at McMaster University in Hamilton, Ontario.

"Our findings suggest that there is an urgent need to investigate the genetic consequences associated with exposure to chemical pollution through the inhalation of urban and industrial air," they wrote in their report.

Christopher Somers, James Quinn and colleagues did an earlier study that showed gulls living near a steel mill on Lake Ontario had genetic mutations.

In this study, they raised two groups of mice - one half a mile (1 km) downwind of the mill and one about 20 miles (30 km) away.

The mice made to breathe the polluted air had 1.5 times to twice as many mutations in their DNA as the mice breathing fresh country air, Somers and colleagues reported.

They did not look to see if any important genes were affected - such a study would have been too expensive and would have taken too many mice, Quinn said in an interview. But it is accepted scientific practice to look at especially mutation-prone areas of DNA for a tendency to mutation.

DNA mutates at a regular rate - it's the process that drives evolution. But extra mutations can lead to trouble, and genetic mutation is what causes cancer and other diseases.

Virtually all the extra mutations were inherited from the father mice, the researchers said. While this does not mean that females are not susceptible, it does suggest that steel workers, who are mostly male, have an extra risk, they said.

Steel mills have been around for a long time, but it is not clear whether people who live near them have a higher genetic mutation rate. "There has been work showing elevated cancer rates for steel mill workers," Quinn said.

"For people living near steel mills it's a little bit tougher to show. Some might work in the steel mills, some might smoke, some might drink a lot."

Somers said the study, ironically, was undertaken to show that efforts to clean up Hamilton harbor had worked.

"This had been one of most polluted places, if not the most polluted place, in Canada," Somers, a graduate student studying under Quinn, said. "There has been a concerted effort to clean up Hamilton harbor and reduce air emissions."

The experiment had been aimed at showing these had helped. "We haven't really seen that," he said.

Integrated steel mills, which produce new steel, use huge amounts of coal and emit hundreds of compounds, many of which could affect DNA, Somers said.

"Our hypothesis is that it is a group of substances called polycyclic aromatic hydrocarbons, or PAHs. These result from incomplete fossil fuel combustion," Somers said.