Risk Factors

Many genetic and environmental factors increase the risk of developing cancer.

Family History and Genetic Factors: Some families have a significantly higher risk of developing certain cancers. Sometimes the increased risk is due to a single gene and sometimes it is due to several genes interacting together. Environmental factors--common to the family--may alter this genetic interaction and produce cancer.

An extra or abnormal chromosome may increase the risk of cancer. For example, people with Down syndrome, who have three instead of the usual two copies of chromosome 21, have a 12 to 20 times higher risk of developing acute leukemia.

See the table Most Common Cancers in Men and Women.

Age: Some cancers, such as Wilms' tumor, retinoblastoma, and neuroblastoma, occur almost exclusively in children. Why these cancers occur in the young is not well understood, but genetics is one factor. However, most cancers are more common in older people. In the United States, more than 60% of cancers occur in people older than 65; the risk of developing cancer doubles every 5 years after age 25. The increased cancer rate is probably due to a combination of increased and prolonged exposure to carcinogens and weakening of the body's immune system.

Environmental Factors: Numerous environmental factors increase the risk of developing cancer.

Pollution in the air, whether from industrial waste or cigarette smoke, can increase the cancer risk. Many chemicals are known to cause cancer and many others are suspected of doing so. For example, asbestos exposure may cause lung cancer and mesothelioma (cancer of the pleura), especially in smokers. The time between exposure to the chemicals and development of the cancer may be many years.

Smoking cigarettes produces carcinogens that substantially increase the risk of developing cancers of the lung, mouth, larynx, kidney, and bladder.

Exposure to radiation is a risk factor in the development of cancer. Extended exposure to ultraviolet radiation, primarily from sunlight, causes skin cancer. Ionizing radiation is particularly carcinogenic. Exposure to the radioactive gas radon, which is released from soil, increases the risk of lung cancer. Normally, radon disperses rapidly into the atmosphere and produces no harm. However, when a building is placed on soil with a high radon content, radon can accumulate within the building, sometimes producing sufficiently high levels in the air to cause harm. Radon is breathed into the lungs, where it may eventually cause lung cancer. If an exposed person also smokes, the risk of lung cancer is further increased.

Geography: The risk of cancer varies according to where people live, although the reasons for the geographic differences are often complex and poorly understood. This geographic variation in cancer risk is probably multifactorial: a combination of genetics, diet, and environment.

For example, the risk of colon and breast cancers is low in Japan, yet in Japanese people who immigrate to the United States, the risk increases and eventually equals that of the rest of the American population. In contrast, the Japanese have extremely high rates of stomach cancer. When these people immigrate to the United States and eat a Western diet, the risk declines to that of the United States, although the decline may not be evident until the next generation.

Diet: Substances consumed in the diet can increase the risk of cancer. For instance, a diet high in fat has been linked to an increased risk of colon, breast, and possibly prostate cancer. People who drink large amounts of alcohol are at much higher risk of developing esophageal cancer. A diet high in smoked and pickled foods or in barbecued meats increases the risk of developing stomach cancer.

Viral Infections: Several viruses are known to cause cancer in humans, and several others are suspected of causing cancer. The papillomavirus (which causes genital warts) is one cause of cervical cancer in women. Hepatitis B virus can cause liver cancer. Some human retroviruses cause lymphomas and other cancers of the blood system.

Some viruses produce cancer in certain countries but not in others. For instance, the Epstein-Barr virus causes Burkitt's lymphoma (a type of cancer) in Africa and cancers of the nose and pharynx in China.

Inflammatory Diseases: Inflammatory diseases often produce an increased risk of cancer. Such diseases include ulcerative colitis (which can result in colon cancer). Infection with certain parasites can produce inflammation that may result in cancer. For example, infection with the parasite Schistosoma (Bilharzia) may cause bladder cancer by chronically irritating the bladder.