You eat right, exercise and don't smoke, but you're drawn to the milieu of smoky nightclubs. Is the secondhand smoke you breathe harmful?

If so, how much smoke does it take to cause disease — and if you quit going to clubs, does your risk drop?

No one knows the answers to those questions. But there's wide agreement among medical experts that secondhand smoke contains carcinogens and other toxins that can cause lung cancer and heart disease.

"Based on our understanding of how cancer is caused, any exposure causes some risk," said Terry Pechacek, a scientist with the federal Centers for Disease Control and Prevention's (CDC) Office of Smoking and Health. "Are we talking about 1 in a million chances, 1 in 10 million chances? The risk can be very small."

Tobacco smoke that enters the body can change any cell it touches, Pechacek said. "Once a cell has been converted by exposure — a single cell of the trillions of cells in the body — then you're playing roulette. Will the body systems kill that (carcinogen) off before it becomes a life-threatening tumor? There's a whole lot of uncertainty."

A recent Minnesota study shows how potent environmental tobacco smoke can be. Eleven nonsmoking volunteers who spent four hours in a smoky casino had high levels of two tobacco-related cancer agents in their urine, said Kristin Anderson, an epidemiology professor at the University of Minnesota and lead author of the study.

Could one such exposure in a smoky bar damage body cells? The answer, said Pechacek, probably is yes. "Would that mean that it would turn into a life-threatening cancer? Unlikely, but we're dealing in a probabilities game there. No exposure is always the safest route."

Cancer risk is cumulative. "The longer the exposure, the higher the risk, but we're unable to say at what level it makes a critical difference," Anderson said. "On the other hand, we also can't say what level is safe."

Anderson's study is significant because it shows that people exposed to tobacco smoke in a public setting — not in a lab or other controlled environment — "are actually taking up and absorbing carcinogens in the smoke, which is a pretty powerful statement," said Stephen Babb, a consultant to the CDC on smoking and health.

Forty years after the first surgeon general's report documented the health risks of smoking, there's little disagreement that cigarette smoke can cause lung cancer and heart disease in nonsmokers. Nearly every agency with an interest in the topic supports that premise.

the U.S. surgeon general, the U.S. Environmental Protection Agency, the National Academy of Sciences, the National Institutes of Health and the Occupational Safety and Health Administration.

The CDC estimates that 3,000 nonsmokers die of lung cancer caused by secondhand smoke each year. An additional 35,000 die of heart disease, also from secondhand smoke, the CDC says; other estimates have been as high as 62,000.

The knowledge that secondhand smoke can cause lung cancer, heart diseases and other conditions has prompted states, cities, even countries to ban smoking in bars, restaurants, other public places and workplaces.

In Minnesota, 38 communities have proposed smoking bans but only three cities, Moose Lake, Cloquet and Duluth, and one county, Olmsted, have approved them. A statewide smoking ban died in the Minnesota Legislature recently. It would have banned smoking in workplaces not covered by existing bans, including bars and restaurants.