Everyone knows smoking is a bad idea, but those who can't give it up may get a little protection from exercise, a study suggests.

In a study of older women, researchers found that a physically active smoker had a 35 percent lower risk of lung cancer than a sedentary smoker.

Even so, one expert called that reduction trivial because smoking itself is so risky. And Dr. Kathryn Schmitz, the study's lead author, stressed that exercising does not give women a free pass to smoke.

"The most important thing that smokers can do to reduce the risk of lung cancer is quit smoking," said Schmitz, an assistant professor at the University of Pennsylvania's Center for Clinical Epidemiology and Biostatistics.

Those who quit smoking are 10 to 11 times less likely to develop lung cancer than those who smoke, she said.

The research, published in this month's issue of Cancer Epidemiology Biomarkers & Prevention, was based on information from the Iowa Women's Health Study. That project began in 1986 to follow nearly 42,000 older women. The women filled out health questionnaires over the years, including information about their smoking status and physical activity.

By the end of 2002, the data included 36,410 participants, and 777 had lung cancer.

Of those, 125 were non-smokers, 177 were former smokers, and 475 were current smokers.

Schmitz, who was then at the University of Minnesota's School of Public Health, worked with four colleagues to analyze the data.

Among smokers, the greatest number of cancer cases, 324, came from women who currently smoked and weren't very active. Among physically active smokers, there were 151 cases of lung cancer.

The greatest benefits went to those who had quit smoking and also exercised, with just 82 cancer cases compared to 95 in sedentary former smokers.

Among the exercisers, the lowest risk of lung cancer was found in those who had moderate workouts more than four times a week, or vigorous workouts two or more times a week.

It is still unclear why physical activity might have a preventive effect on lung cancer. Studies over the years have produced conflicting results on that subject. Researchers say it could be that improved pulmonary function reduces both the concentration of carcinogenic particles and the extent to which they are deposited in the lungs.

Also, being more physically active could make smokers more aware of the damage they have caused their lungs — leading them to smoke less or quit, Schmitz said.

Dr. Norman Edelman, chief medical officer for the American Lung Association, echoed Schmitz's comments that the report should not give physically active female smokers "a false sense of security."

"We don't want people to get the wrong message," Edelman said. "A regular smoker has a risk of lung cancer 10 times that of a nonsmoker, and 35 percent reduction in that risk is trivial."

He noted the study does not address the effect of exercise on other smoking-related health problems, such as emphysema and heart disease. He also said that because the active women were less likely to be overweight, it was unclear if the lower lung cancer rate was a result of their exercising or their weight. Some cancers are more common in the obese, Edelman said.