Scientists do not fully understand the causes of cancer, but studies show that some people are more likely to develop the disease than others. The incidence of cancer varies enormously among different regions. The highest death rate from all cancers in males is 272 per 100,000 men in Hungary while the lowest death rate of 80 men per 100,000 is found in Mauritius, an island off the coast of eastern Africa. For women the highest cancer rate is 140 per 100,000 women in Denmark compared to only 63 per 100,000 women in Azerbaijan in western Asia. The figures for the United States are 156 per 100,000 men and 108 per 100,000 women. For particular cancers, the difference between countries may be as high as 40-fold. Differences also occur within populations. Cancer rates vary between sexes, races, and socioeconomic groups, for example.
Scientists called epidemiologists study particular populations to identify why cancer rates vary (see Epidemiology). One method they use is to compare behavior and characteristics such as the gender, age, diet, or race of cancer patients to those of healthy people. Population studies provide useful information about risk factors that increase the likelihood of developing cancer.


One of the greatest risk factors for cancer is prolonged or repeated exposure to carcinogens—chemical, biological, or physical agents that cause the cellular damage that leads to cancer. The details of how carcinogens cause cancer remain unclear. One theory is that exposure to carcinogens, when combined with the effects of aging, causes an increase in chemicals in the body called free radicals. An excessive number of free radicals causes damage by taking negatively charged particles called electrons from key cellular components of the body, such as DNA. This may make genes more vulnerable to the mutating effects of carcinogens.

Tobacco Smoke

Smoking causes up to 30 percent of cancer deaths in the United States and Canada, making tobacco smoke the most lethal carcinogen in North America. Smoking is associated with cancer in the lungs, esophagus, respiratory tract, bladder, pancreas, and probably cancers of the stomach, liver, and kidneys. The risk of cancer increases depending on the number of cigarettes smoked per day, the cigarette’s tar content, and how many years a person smokes. Starting to smoke while young significantly increases the risk of developing cancer.
Each year in the United States, about 3,000 nonsmoking adults die of lung cancer caused by exposure to the smoke of others’ cigarettes, called secondhand smoke or environmental tobacco smoke. Nonsmoking spouses of smokers are 30 percent more likely to develop lung cancer than those married to nonsmokers. Breathing secondhand smoke also increases the risk of cancer in the children of smokers and in nonsmokers who work in smoky places, such as restaurants and bars.
Cigars, pipes, and smokeless tobacco have also been implicated in increased risk for cancer. Cigars contain most of the same cancer-producing chemicals as cigarettes, and people who smoke cigars have a 30 percent higher risk of developing cancer than nonsmokers. Oral cancers occur more frequently in people who use smokeless tobacco, or snuff. Snuff users, for example, are 50 times more likely to develop cancers of the cheek or gum than nonusers.

Hamburger and Fries
A diet high in fatty foods, such as what some consider to be the quintessential American meal—a hamburger and fries—can contribute to many cancers.
Owen Franken/Corbis

Diet accounts for about another 30 percent of cancer deaths in the North America. Saturated fats from red meats, such as hamburger or steak, and other animal products are linked with several cancers. High salt intake increases the risk of stomach cancer. Adult obesity increases the risk for cancer of the uterus in women and also appears to increase the risk for cancers in the breast, colon, kidney, and gallbladder. Alcohol consumption increases the risk of cancer of the esophagus and stomach, especially when combined with smoking.
Some carcinogens are living organisms. Certain viruses, bacteria, and parasites account for about 15 percent of all cancer deaths in the United States. Cancer-causing viruses include the human papilloma virus (HPV), a sexually transmitted virus responsible for 70 to 80 percent of all cases of cancer of the cervix. Hepatitis B and C viruses cause almost 80 percent of all liver cancer in the world. Epstein-Barr virus can also be carcinogenic, causing cancer of the lymphatic system. Human immunodeficiency virus (HIV) or a type of herpesvirus can lead to rare cancers of the lymphatic and circulatory systems. Helicobacter pylori, a bacterium associated with stomach ulcers, likely causes cancer of the stomach.
In developing countries, parasitic organisms are major carcinogens. In parts of Africa, China, and southern Asia, infestation with the liver fluke Clonorchis sinensis causes a form of liver cancer. In North Africa, infection with the parasite Schistosoma haematobium causes cancer of the bladder.


Exposure to electromagnetic radiation, invisible, high-energy light waves such as sunlight and X rays, accounts for about 2 percent of all cancer deaths (see Radiation Effects, Biological). Most cancer deaths from radiation are from skin cancer, which is triggered by too much sun exposure. Sunlight that reaches the Earth’s surface contains two kinds of ultraviolet (UV) radiation. UV-A and UV-B both contribute to sunburn and skin cancer as well as to conditions such as premature wrinkling of the skin. Depletion of the ozone layer, which absorbs ultraviolet radiation in the upper atmosphere, will continue to increase skin damage and skin cancer rates in the future.
Radon, a colorless, odorless, radioactive gas, seeps from the Earth in some regions of the United States. Breathing the gas over a long period has been linked to a small number of lung cancer cases. Providing adequate air circulation in a building reduces exposure to radon. Infrequently, radiation exposure associated with medical treatments, such as therapeutic radiology, leads to cancer. This type of exposure is responsible for about 1 percent of all U.S. cancer deaths.

Environmental and Occupational Chemicals

Air Pollution in Cubatão, Brazil
A thick layer of fumes hovers over the city of Cubatão in São Paulo state, Brazil. Pollution in the air, water, and soil accounts for about 2 percent of all American cancer deaths, and may be responsible for a higher percentage of cancer deaths in countries with less stringent pollution control laws. Lung cancer rates tend to be higher in urban and industrial centers, where air pollution is a constant problem

Air pollution, water pollution, and pollutants in the soil account for about 2 percent of all cancer deaths in the country, particularly due to lung and bladder cancer. Lung cancer rates are generally higher in cities, where increased industry and automobile traffic produce air pollution. Some people encounter carcinogenic chemicals in their working environment. Occupational carcinogens account for about 5 percent of all U.S. cancer deaths and include such industrial chemicals as benzene, asbestos, vinyl chloride, aniline dyes, arsenic, and certain petroleum products (see Occupational and Environmental Disease).

Hereditary Factors

Evidence suggests that heredity plays a role in developing cancer. Some gene mutations associated with cancer are inherited. For example, inheritance of the mutated tumor suppressor genes BRCA1 or BRCA2 greatly increases the risk of breast cancer in young women. About 50 to 60 percent of women with inherited BRCA1 or BRCA2 mutations will develop breast cancer by the age of 70. Inherited mutations in the genes MSH2, MLH1, PMS1, and PMS2, all of which repair DNA, are especially prevalent in a rare form of hereditary colon cancer.
Scientists suspect that many other hereditary factors contribute to cancer. In addition to inherited mutations, other genetic variations, particularly those influencing how the body responds to carcinogens, may create a greater susceptibility to cancer. The identities of the majority of these genetic variations are not yet known.

Steroid Hormones

Medical research suggests that cancers of the reproductive organs may be affected by naturally occurring steroid hormones produced by the endocrine system. These hormones stimulate reproductive organ cells to divide and grow. In women, relatively high or long exposure to the female sex hormone estrogen seems to increase the risk of breast and uterine cancers. Thus, early age at first menstruation, late age at menopause, having children after age 30, and never having children, all of which affect the duration of estrogen exposure in the body, increase the risk for these cancers. Some evidence also suggests that estrogen replacement therapy (ERT), in which women take estrogen to offset the unpleasant effects of menopause, may also increase the risk of some cancers of the reproductive organs. The risk appears to go down significantly, however, when estrogen and another female sex hormone, progesterone, are taken together. At one time studies showed a link between birth control pills and cancer. However, these studies examined early forms of birth control pills, which contained high levels of estrogen. Today’s birth control pills contain progesterone, as well as lower levels of estrogen, and carry very little risk of cancer. Male sex hormones, particularly testosterone, also appear to play a role in cancers of the male reproductive organs, but this role is not yet well understood.

Population Demographics

Population studies show that a person’s age, race, and gender affect the probability that he or she will develop cancer. Most cancers occur in adults middle-aged or older. The risk of cancer increases as individuals age because genetic mutations accumulate slowly over many years, and the older a person is, the more likely that he or she will have accumulated the collection of mutations necessary to turn an otherwise healthy cell into a cancerous cell. Women aged 20 to 29, for example, account for just 0.3 percent of all cases of breast cancer, but women over age 50 account for more than 75 percent of breast cancer cases. Cancer of the prostate gland shows similar age discrimination. According to the American Cancer Society (ACS) and the National Cancer Institute of Canada (NCIC), more than 75 percent of all prostate cancers are diagnosed in men who are over the age of 65.
Statistics show that men are more likely to develop cancer than women. In the United States, half of all men will develop cancer at some point in their lifetimes. About one-third of all American women will develop cancer. Cancer statistics for Canada are similar. Stomach cancer is about twice as common in men than in women, as are certain types of kidney cancer. However, the reasons for the discrepancy between the sexes are unknown.
Some cancers are more prevalent in particular races than others. In the United States, for example, bladder cancer is twice as common in white people than it is in black people. White women are slightly more likely to develop breast cancer than are black women, but black women are more likely to die of the disease. Asian, Hispanic, and Native American women have the lowest breast cancer risk. On the whole, African Americans, especially men, are more likely to develop cancer—and more likely to die from it—than members of any other group in the United States. Reasons for the discrepancies between races are still not entirely clear, but many epidemiologists trace them to differences in diet and exercise, unequal access to medical care, and exposure to carcinogens.

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