WASHINGTON (AP) -- Exposure to air pollution from steel mills may cause genetic mutations that are passed by fathers to their offspring, according to a study in mice.

Ecology scientists at McMaster University in Hamilton, Ontario, Canada, said pairs of mice exposed for about 70 days to air pollution downwind from a steel mill produced young that carried up to twice the number of genetic mutation found in animals that lived in clean air.

Christopher M. Somers and James S. Quinn, two of the co-authors of a study appearing in the Proceedings of the National Academy of Sciences, said the new research supports earlier findings that suggested that genetic mutations among seagulls exposed to steel mill air pollution.

A toxicology expert questioned the study's methods and conclusions.

In the study, Somers, Quinn and their colleagues used two groups of 20 mice, half male and half female. One group was placed in a shed six-tenths of a mile downwind from two steel mills in Hamilton Harbor near the western shore of Lake Ontario. The other group was housed in a shed in a rural area 18 miles away.

After 10 weeks of exposure, both groups were returned to the laboratory. Breeding pairs were established randomly within each of the groups.

The animals exposed to the steel mill air pollution had a breeding success rate of 85 percent, compared with the 95 percent success rate among the rural mice. Mean litter size for the steel mill group was 7.9 pups per couple, versus 9.6 pups for the rural couples.

After profiling the DNA of the pups, researchers found that the offspring of mice exposed to the steel mill air pollution had up to twice the number of abnormal DNA sequences as the pups from the rural couples.

Somers said an analysis showed the genetic changes had been passed to the pups from their fathers.

None of the specific genetic mutations detected would affect development or appearance of the mice, said Somers. However, he said genetic mutations have been linked to cancer and other diseases.

Quinn said the mutations may be caused by polycyclic aromatic hydrocarbons, or PAHs, known to be present in some stack gases coming from steel mills. Steel mills burn coal or other fuels to refine ore or rough metal into steel. He said the PAHs are known to cause mutations and can enter the body by attaching to small airborne particles that are inhaled.

The authors said the study suggests that humans and wildlife exposed to airborne particles "may be at risk of developing germline (inheritable) mutations."

But "that is a big stretch" not supported by the data, said Coreen A. Robbins, a consultant for GlobalTox, a toxicology and industrial hygiene company based in Redmond, Wash.

She said confirming studies need to be done before steel mill air pollution can be conclusively linked to genetic mutations.

Robbins also questioned the methodology used, noting there was no effort to specifically chemically analyze the air breathed by the animals.

"It looks like from the study that we have no idea what these animals were exposed to," she said.

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