It's a logical conclusion: Lung cancer is caused by cigarette smoking.

But it's not the only cause.

Yes, smoking is the greatest risk factor for lung cancer. In fact, it contributes to 80% to 90% of U.S. lung cancer deaths. But even if you've never smoked or if you quit a long time ago, you may still be at risk of lung cancer.

Other factors that can increase your risk include:

• Exposure to secondhand smoke. Even if you don't smoke, your risk of lung cancer increases if you're exposed to smoke from others.
• Exposure to radon gas. Radon is produced by the natural breakdown of uranium in soil, rock and water. Unsafe levels of radon can be found in buildings, including some homes.
• Exposure to asbestos and other carcinogens. Workplace exposure to asbestos and other substances known to cause cancer — including arsenic, chromium and nickel — also can increase your risk of lung cancer.
• Family history of lung cancer. If you have a parent, sibling or child who has had lung cancer, your own risk of the disease is increased.
• Radiation therapy. Cancer survivors who had radiation therapy to their chest area are at an increased risk of lung cancer.
• Exposure to air pollution. Try to stay inside with the windows closed on days when outdoor air quality is not healthy. The air quality report is available where you get your weather news.
• Beta carotene supplements. Studies suggest that people who smoke and also take high-dose beta carotene supplements may increase their risk of lung cancer.

Fortunately, most of these risk factors are avoidable. Talk to your health care team about how you can help keep your lungs healthy for life.