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.
| 1. 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,
several thousand 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. For this reason smoking has been
banned in many 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.
2. Diet: Diet can also contribute to cancer. Saturated fats from red meats,
such as hamburger or steak, and high-fat dairy 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.
Exposure to electromagnetic radiation,
invisible, high-energy light waves such as sunlight and X rays,
accounts for a small percentage of cancer deaths. 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.
| 4. |
Environmental
and Occupational Chemicals |
Air pollution, water pollution, and pollutants in the
soil contribute particularly 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 include such industrial chemicals
as benzene, asbestos, vinyl chloride,
aniline dyes, arsenic, and certain petroleum products.
Evidence suggests that
heredity plays a role in developing cancer. Some gene mutations
associated with cancer are inherited. For example, inheriting mutated tumor
suppressor genes BRCA1 or BRCA2 greatly increases a woman’s chances of
developing breast cancer. 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.
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