III.

Risk Factors

Smoking cigarettes is the single greatest cause of lung cancer. Smoking cigars or a pipe also increases the risk of lung cancer. Tobacco smoking causes as many as 90 percent of lung cancers in men and about 78 percent of those in women. Exposure to secondhand smoke—that is, inhaling the tobacco smoke of smokers—has occurred in most of the remaining cases. Substances in tobacco damage the cells in lungs, and over time the damaged cells can become cancerous. The great majority of lung cancer cases could be prevented and thousands of lives could be saved each year if people quit smoking.

Other environmental factors that have been linked to lung cancer include prolonged exposure to certain workplace substances, such as arsenic; asbestos; and high doses of ionizing radiation, for example, that emitted by uranium (see Radiation Effects, Biological). Long-term exposure to air pollution, which is largely composed of the remains of burned petroleum and coal, may also increase the risk of lung cancer. One constituent of indoor air pollution, naturally occurring radon gas, poses a special danger to cigarette smokers. This colorless, odorless gas silently seeps from the soil into the air in some regions of the world.

Not all heavy smokers develop lung cancer, however, suggesting that heredity may also play a role in the development of the disease. Some people seem to inherit an inability to break down certain types of cancer-causing chemicals, making them more susceptible to the disease. Exposure to tobacco smoke, for example, may cause key cancer-prevention genes to malfunction in these people. For instance, a tumor suppressor gene called FHIT prevents the growth and spread of cancerous tumors. Many people with lung cancer inherit a mutated form of the gene that fails to perform this job. The tumor suppressor gene p53 also prevents tumor growth, and researchers have learned that a mutated form of the p53 gene fails to block the growth of cancerous tumors. Cancer researchers have found that a genetic abnormality on chromosome six makes people more susceptible than others to developing lung cancer.

Some studies indicate that women are twice as likely as men to develop lung cancer. Researchers have found a gene called GRPR that causes abnormal cell growth in people exposed to cigarette smoke. The gene is more active in women than in men and it may explain why females are more susceptible to lung cancer.