They found that male nonsmokers were about 25 percent more likely to die from lung cancer than women nonsmokers even though they developed the disease at similar rates.

Men who had never smoked had a 1.1 percent risk of dying from lung cancer, compared to 0.8 percent for the women, the study found. This compares to about 22 percent among men who smoke and 12 percent of women who smoke.

The study in North America, Europe and Asia was the largest ever done on nonsmokers and lung cancer.

"Lung cancer is a significant public health and medical problem even beyond the overwhelming disease burden caused by tobacco smoking," Dr. Michael Thun of the American Cancer Society, who led the study, said in a telephone interview.

Thun said it is unclear why male nonsmokers are more likely to die from lung cancer than women.

"That's an interesting and unresolved question," he said.

In this study, a person was considered a lifelong nonsmoker if he or she reported never smoking, regardless of exposure to secondhand smoke, Thun's team reported in the Public Library of Science journal PLoS Medicine."

The study also indicated that death rates among lifelong nonsmokers have remained stable over the past several decades.

Factors other than smoking are estimated to account for 10 to 15 percent of lung cancer deaths. In the United States, for example, that translates to between 16,000 and 24,000 of the more than 161,000 lung cancer deaths forecast this year.

Lung cancer is the leading cause of cancer death in men and the second leading cause of cancer death among women worldwide, according to the American Cancer Society, with about 975,000 men and 376,000 women forecast to die annually.

Risk factors unrelated to smoking include workplace or environmental exposures to radon and asbestos, certain metals such as chromium, cadmium and arsenic, some chemicals, radiation, air pollution, coal smoke or indoor emissions from burning certain fuels, the American Cancer Society said.