Researchers have discovered several causes of lung cancer -- most are related to the use of tobacco.

• Cigarettes. Smoking cigarettes causes lung cancer. Harmful substances, called carcinogens, in tobacco damage the cells in the lungs. Over time, the damaged cells may become cancerous. The likelihood that a smoker will develop lung cancer is affected by the age at which smoking began, how long the person has smoked, the number of cigarettes smoked per day, and how deeply the smoker inhales. Stopping smoking greatly reduces a person's risk for developing lung cancer.

• Cigars and Pipes. Cigar and pipe smokers have a higher risk of lung cancer than nonsmokers. The number of years a person smokes, the number of pipes or cigars smoked per day, and how deeply the person inhales all affect the risk of developing lung cancer. Even cigar and pipe smokers who do not inhale are at increased risk for lung, mouth, and other types of cancer.

• Environmental Tobacco Smoke. The chance of developing lung cancer is increased by exposure to environmental tobacco smoke (ETS) -- the smoke in the air when someone else smokes. Exposure to ETS, or secondhand smoke, is called involuntary or passive smoking.

• Radon. Radon is an invisible, odorless, and tasteless radioactive gas that occurs naturally in soil and rocks. It can cause damage to the lungs that may lead to lung cancer. People who work in mines may be exposed to radon and, in some parts of the country, radon is found in houses. Smoking increases the risk of lung cancer even more for those already at risk because of exposure to radon. A kit available at most hardware stores allows homeowners to measure radon levels in their homes. The home radon test is relatively easy to use and inexpensive. Once a radon problem is corrected, the hazard is gone for good.

• Asbestos. Asbestos is the name of a group of minerals that occur naturally as fibers and are used in certain industries. Asbestos fibers tend to break easily into particles that can float in the air and stick to clothes. When the particles are inhaled, they can lodge in the lungs, damaging cells and increasing the risk for lung cancer. Studies have shown that workers who have been exposed to large amounts of asbestos have a risk of developing lung cancer that is 3 to 4 times greater than that for workers who have not been exposed to asbestos. This exposure has been observed in such industries as shipbuilding, asbestos mining and manufacturing, insulation work, and brake repair. The risk of lung cancer is even higher among asbestos workers who also smoke. Asbestos workers should use the protective equipment provided by their employers and follow recommended work practices and safety procedures.

• Pollution. Researchers have found a link between lung cancer and exposure to certain air pollutants, such as by-products of the combustion of diesel and other fossil fuels. However, this relationship has not been clearly defined, and more research is being done.

• Lung Diseases. Certain lung diseases, such as tuberculosis (TB), increase a person's chance of developing lung cancer. Lung cancer tends to develop in areas of the lung that are scarred from TB.

• Personal History. A person who has had lung cancer once is more likely to develop a second lung cancer compared with a person who has never had lung cancer. Quitting smoking after lung cancer is diagnosed may prevent the development of a second lung cancer.

Researchers continue to study the causes of lung cancer and to search for ways to prevent it. We already know that the best way to prevent lung cancer is to quit (or never start) smoking. The sooner a person quits smoking the better. Even if you have been smoking for many years, it's never too late to benefit from quitting.

The best way to prevent lung cancer is to quit, or never start, smoking.