What Is Lung Cancer?

Lung cancer was rare before the beginning of the 20th century, but it is now the most common cause of death from cancer among both men and women in the United States. An estimated 160,000 Americans died of lung cancer in 2014.

Death rates are high because lung cancer is difficult to treat and usually is not detected until it has already spread. Most cases occur in people between the ages of 45 and 75 who have been exposed to cigarette smoke or other pollutants for many years. Overall, only about 16 percent of people with lung cancer are alive five years after diagnosis. Survival rates vary widely, however, by type of lung cancer (small cell versus non-small cell) and by stage at diagnosis.

Lung cancer may be primary or secondary. Primary lung cancer, which originates in the lungs, is grouped into two broad categories: small cell carcinoma (about 15 percent of cases) and non-small cell carcinoma (about 85 percent). Small cell carcinoma spreads especially quickly and is more difficult to treat; most people with this type die within a year of diagnosis. Primary lung cancer can spread to nearly any organ. Cancer that has spread to the lungs from other sites in the body is called secondary lung cancer. These types of tumors are usually incurable, in which case palliative treatments are used.

What causes lung cancer?

Smoking causes an estimated 87 to 90 percent of lung cancer cases. Smokers who quit (even after smoking for years) greatly reduce their risk. The risk, however, does remain.

Research suggests that cigarette smoking leads to lung cancer at least in part because the body converts benzo[a]pyrene diol epoxide (BPDE), a substance in cigarette smoke, into a potent carcinogen.

In addition, several comprehensive reports have concluded that passive smoking—inhaling the smoke from cigarettes smoked by others—also can cause lung cancer. Substantial evidence supports this conclusion. That includes these facts:

• Carcinogens are present in side-stream smoke emerging from the end of a burning cigarette.

• Cigarette smoke particles persist in the homes of smokers.

• Tobacco-smoke components can be detected in the body fluids of nonsmokers exposed to passive smoking.

In addition, nonsmokers have a 20 to 30 percent greater chance of developing lung cancer if they are exposed to secondhand smoke at home or work.

Taking into account the projected deaths from lung cancer and cardiovascular disease, some studies suggest that passive smoking is one of the leading causes of preventable deaths in this country.

Radon and asbestos

Exposure to toxic substances such as radon and asbestos also can lead to lung cancer. Radon is estimated to be the leading cause of lung cancer among nonsmokers and the second leading cause of the disease overall.

Radon is a colorless, odorless gas formed naturally during the decay of uranium found in rocks and soil. Radon in the soil can pollute the air of a home by entering through cracks or other openings, usually in the basement. The incidence of lung cancer caused by radon is highest among miners, who are often exposed to high levels. Most people who die of radon-associated cancer, according to the National Cancer Institute, are also smokers.

Chronic exposure to asbestos can cause lung cancer and mesothelioma—a cancer involving the pleura (membrane covering the lungs). The combination of asbestos exposure and cigarette smoking is especially dangerous. Air pollution, including traffic fumes and smokestack emissions, also may contribute to lung cancer.