September 18 (HealthDay News) -- A study of Texas residents suggests that tiny metallic bits of air pollution could account for some cases of lung cancer.

The researchers aren't sure exactly how dangerous the particles are, nor do they fully understand their potential relationship to tobacco smoke.

Still, "It's disturbing that there might be something in the environment causing the problem," said study author Dr. Yvonne Coyle, an associate professor of internal medicine at the University of Texas Southwestern Medical Center at Dallas. "It could be these metals, and we need to look at that further."

According to Coyle, 10 percent to 15 percent of lung cancer cases occur among nonsmokers. One possible explanation: Inhalation of air pollution, especially fine particulate matter -- bits of metal that are too small to be seen with the naked eye but can still enter the lungs.

Mining, smelting and petroleum production all produce this type of pollution, Coyle said, as can motor vehicle exhaust.

But while air pollution has been directly linked to respiratory disorders and heart disease, its role in lung cancer is still under debate.

In the new study, Coyle and her colleagues tried to determine if exposure to metallic bits of air pollution was associated with higher levels of lung cancer. To find the answer, they compared lung cancer rates in 254 Texas counties from 1995-2000 to federal reports that companies filed when they released pollution between 1988 and 2000.

The researchers found an "association" between various types of lung cancer and releases of zinc, chromium and copper. When the study results were adjusted to take into account the effects of factors such as gender and race, zinc was still linked to lung cancer.

The findings of the federally funded study appear in the September issue of the Journal of Thoracic Oncology.

The study doesn't say how much more likely it is for people to develop lung cancer if they're exposed to higher levels of the pollutants.

Also, the role of smoking is unclear because the county-by-county statistics didn't reveal whether the individual lung cancer patients smoked. However, Coyle said smoking levels were consistent across the counties studied.

More research needs to be done to confirm the results and "determine who is at the greatest risk, given this exposure," Coyle said.

Dr. Michael Thun, head of epidemiologic research for the American Cancer Society, suggested that the value of the study is limited because it didn't take into account smoking by the lung cancer patients.

"It's clear that smoking is such a powerful cause of lung cancer that it's very difficult in wealthy countries to identify any separate contributions from air pollution," Thun said. "It's extremely hard to measure, and this study doesn't solve that problem."

SOURCES: Yvonne Coyle, M.D., associate professor, internal medicine, University of Texas Southwestern Medical Center at Dallas; Michael Thun, M.D., head, epidemiologic research, American Cancer Society, Atlanta; September 2006 Journal of Thoracic Oncology