Cancer develops over many years and has many causes. Several factors both inside and outside the body contribute to cancer development. In this context, scientists refer to everything outside the body that interacts with humans as the “environment.”

In the United States, exposure to a wide variety of natural and manufactured substances in the environment accounts for at least two-thirds all cancer cases. These environmental factors include lifestyle choices such as

• Cigarette smoking,
• Excessive alcohol consumption,
• Poor diet,
• Lack of exercise,
• Excessive sunlight exposure, and
• Sexual behavior that increases exposure to certain viruses (see viruses and bacteria).

Other factors include exposure to certain medical drugs, hormones, radiation, viruses, bacteria, and environmental chemicals possibly present in the air, water, food, and workplace. The cancer risks associated with many environmental chemicals have been identified through studies of occupational groups that have higher exposures to these chemicals than does the general population.

The importance of the environment can be seen in the differences in cancer rates throughout the world and the change in cancer rates when groups of people move from one country to another. For example, when Asians—who have low rates of prostate and breast cancer and high rates of stomach cancer in their native countries—immigrate to the United States, their prostate and breast cancer rates rise over time until they are nearly equal to or greater than the higher levels of these cancers in the United States. Likewise, their rates of stomach cancer fall, becoming nearly equal to the lower U.S. rates. Lifestyle factors such as diet, exercise, and excessive weight are thought to play a major role in the trends for breast and prostate cancers, and infection with the Helicobacter pylori bacterium is an important risk factor for stomach cancer. Recently, the rapid rise in the rates of colorectal cancer in Japan and China suggests an environmental cause such as lifestyle factors.

Different environmental exposures are linked to specific kinds of cancer. For example, exposure to asbestos is linked primarily to lung cancer, whereas exposure to benzidine, a chemical found in certain dyes (see Benzidine), is associated with bladder cancer. In contrast, smoking is linked to cancers of

• The lung,
• Bladder,
• Mouth,
• Colon,
• Kidney,
• Throat,
• Voice box,
• Esophagus,
• Lip,
• Stomach,
• Cervix,
• Liver, and
• Pancreas.

Certain factors inside the body make some people more likely to develop cancer than others. For instance, some people either inherit or acquire the following conditions: altered genes in the body’s cells, abnormal hormone levels in the bloodstream, or a weakened immune system. Each of these factors may make a person more susceptible to cancer.

One of the ways scientists know that genes play an important role in the development of cancer is from studying certain rare families where family members over several generations develop similar cancers. These families are apparently passing on an altered gene that carries a high chance of cancer growth. Several genes that greatly increase a person’s chance of developing certain cancers (e.g., colon, breast, and ovary) have been identified. Only a very small percentage of people in the general population have abnormal copies of these genes: known as familial cancers, such genes account for only two to five percent of all cancers.

Gene alterations may also contribute to individual differences in susceptibility to environmental carcinogens (cancer-causing substances). For instance, people differ in their ability to eliminate from their bodies cancer-causing agents to which they have been exposed or to repair DNA damage caused by such agents. Subsequent-generation family members may inherit these gene alterations, which may also account for higher rates of cancer in these families. Higher rates of cancer in families may also be related to shared environmental exposures such as diet or workplace exposure to carcinogens.

Figure 1. Tightly coiled strands of DNA, which carry the instructions that allow cells to make proteins, are packaged in units called chromosomes. Subunits of DNA are known as genes.

One of the main objectives of a growing field in cancer research called molecular epidemiology is to identify gene alterations that increase or decrease a person’s chance of developing cancer after an environmental exposure.

Throughout our lives, environmental factors such as viruses, sunlight, and chemicals interact with our cells. Mechanisms to repair damage to our genes and healthy lifestyle choices (e.g., wearing protective clothing for sun exposure or not smoking) help to protect us from harmful exposures. But over time, substances in the environment may cause gene alterations that accumulate inside our cells. While many alterations have no effect on a person’s health, permanent changes in certain genes can lead to cancer.

The chance that a person will develop cancer in response to a particular environmental agent, however, depends on several interacting factors

• How long and how often a person is exposed to a particular substance,
• His/her exposure to other agents,
• Genetic factors,
• Diet,
• Lifestyle,
• Health,
• Age, and
• Sex.

Diet, alcohol consumption, and certain medications can, for example, affect the levels of chemicals in the body that break down cancer-causing substances.

Because of the complex interplay of many factors, to predict whether a specific environmental exposure will cause a particular person to develop cancer is not possible. We know that certain genetic and environmental factors increase the risk of developing cancer, but we rarely know exactly which combination of factors is responsible for a person’s specific cancer. This also means that we usually do not know why one person’s body develops malignant tumors and another does not.

Key Points

• Cancer develops over many years and has many causes.
• Exposure to a wide variety of natural and manufactured substances in the environment accounts for at least two-thirds of all the cases of cancer in the United States.
• The importance of the environment can be seen in the differences in cancer rates throughout the world and the change in cancer rates when groups of people move from one country to another.
• Different environmental exposures are linked to specific cancers.
• Certain factors inside the body make some people more likely to develop cancer than others.
• One of the ways scientists know that genes play an important role in the development of cancer is from studying certain rare families where over several generations family members will develop similar cancers.
• Gene alterations may also contribute to individual differences in susceptibility to environmental carcinogens.
• Exposure to cancer-causing substances is only a part of what determines who will get cancer.
• Mechanisms to repair damage to our genes and healthy lifestyle choices (wearing protective clothing for sun exposure or not smoking) help to protect us from harmful exposures.
• The chance that a person will develop cancer in response to a particular environmental agent depends on several interacting factors: how long and how often a person is exposed to a particular substance, his/her exposure to other agents, genetic factors, diet, lifestyle, health, age, and sex.
• Because of the complex interplay of many factors, to predict whether a specific environmental exposure will cause a particular person to develop cancer is not possible.
• Particular patterns of gene alterations and environmental exposures render people more susceptible or more resistant to cancer.