Cigarette smoking is the most important cause of lung cancer. Research as far back as the 1950s clearly established this relationship.

• Cigarette smoke contains more than 4,000 chemicals, many of which have been identified as causing cancer.
• A person who smokes more than one pack of cigarettes per day has a risk of developing lung cancer 20-25 times greater than someone who has never smoked.
• Once a person quits smoking, his or her risk for lung cancer gradually decreases. About 15 years after quitting, the risk for lung cancer decreases to the level of someone who never smoked.
• Cigar and pipe smoking increases the risk of lung cancer but not as much as smoking cigarettes.

About 90% of lung cancers arise due to tobacco use. The risk of developing lung cancer is related to the following factors:

• the number of cigarettes smoked,
• the age at which a person started smoking, and
• how long a person has smoked (or had smoked before quitting).

• Passive smoking, or secondhand smoke, presents another risk for lung cancer. An estimated 3,000 lung cancer deaths occur each year in the U.S. that are attributable to passive smoking.
• Air pollution from motor vehicles, factories, and other sources probably increase the risk for lung cancer, and many experts believe that prolonged exposure to polluted air is similar to prolonged exposure to passive smoking in terms of risk for developing lung cancer.
• Asbestos exposure increases the risk of lung cancer by nine times. A combination of asbestos exposure and cigarette smoking raises the risk to as much as 50 times. Another cancer known as mesothelioma (a type of cancer of the pleura or of the lining of the abdominal cavity called the peritoneum) is also strongly associated with exposure to asbestos.
• Lung diseases, such as tuberculosis (TB) and chronic obstructive pulmonary disease (COPD), also create a risk for lung cancer. A person with COPD has a four to six times greater risk of lung cancer even when the effect of cigarette smoking is excluded.
• Radon exposure poses another risk.
• Radon is a by-product of naturally occurring radium, which is a product of uranium.
• Radon is present in indoor and outdoor air.
• The risk for lung cancer increases with significant long-term exposure to radon, although no one knows the exact risk. An estimated 12% of lung cancer deaths are attributable to radon gas, or 15,000 to 22,000 lung cancer-related deaths annually in the U.S. Radon gas is the second leading cause of lung cancer in the U.S. As with asbestos exposure, smoking greatly increases the risk of lung cancer with radon exposure.
• Certain occupations where exposure to arsenic, chromium, nickel, aromatic hydrocarbons, and ethers occurs may increase the risk of lung cancer.