The lungs are part of the respiratory system. Their function is to supply oxygen to the blood while removing carbon dioxide.

Lung cancer may spread to the lymph nodes or other tissues in the chest (including the other lung). In many cases, lung cancer may also spread to other organs of the body, such as the bones, brain, or liver.

Lung cancer is the leading cause of cancer deaths in U.S. men and women.

Lung cancer can often be associated with known risk factors for the disease. Many risk factors can be changed, but not all can be avoided.

Tobacco: Studies show that smoking tobacco products in any form is the major cause of lung cancer. People who stop smoking and never start again lower their risk of developing lung cancer or of having lung cancer recur (come back). Many products, such as nicotine gum, nicotine sprays, nicotine inhalers, nicotine patches, or nicotine lozenges, as well as antidepressant drugs, may be helpful to people trying to quit smoking. Never smoking lowers the risk of dying from lung cancer.

Second-hand tobacco smoke also causes lung cancer. This is smoke that comes from a burning cigarette or other tobacco product, or smoke that is exhaled by smokers. People who inhale second-hand smoke are exposed to the same cancer-causing agents as smokers, although in weaker amounts. Inhaling second-hand smoke is called involuntary or passive smoking.

Environmental Causes: There are other causes of lung cancer in the environment, but their effect on lung cancer rates is small compared to the effect of cigarette smoking.

Cancer-causing agents that may be found indoors, especially in the workplace, include asbestos, radon, arsenic, chromium, nickel, tar, and soot. These substances can cause lung cancer in people who have never smoked, and combine with cigarette smoke to further increase lung cancer risk in smokers. Many countries are working to control these cancer-causing agents in the workplace.

Air pollution may also increase the risk of lung cancer. Studies show that lung cancer rates are higher in cities with higher levels of air pollution.

Beta Carotene: Studies show that heavy smokers who avoid taking beta carotene supplements may avoid further increasing their risk of lung cancer compared with smokers who do take beta carotene.

Diet and Physical Activity: Studies show that a diet rich in fruit, and possibly vegetables, may help lower the risk of lung cancer, while heavy alcohol drinking may increase the risk of lung cancer. In addition, studies show that people who are physically active may have a lower risk of lung cancer than those who are not, even after taking cigarette smoking into account.

Chemoprevention: Chemoprevention is the use of specific natural or man-made drugs to reverse, suppress, or prevent cancer growth. Chemoprevention is an area of active clinical research. It has not yet become standard therapy.