Cancer and the Environment

by National Cancer Institute

Cancer: Inside and Outside Factors

Cancer is a renegade system of growth inside the human body. The changes that must occur inside for cancer to flourish are genetic changes, but factors outside the body also play a role.

Humans do not exist in contaminant-free surroundings. Over a lifetime, a person's internal genetic makeup persistently interacts with external factors. Factors outside the body such as diet, smoking, alcohol use, hormone levels, or exposures to certain viruses and cancer-linked chemicals (carcinogens) over time may collectively conspire with internal genetic mutations to destabilize normal checks and balances on growth and maturation.

What Is the Environment?

When most people think of the word "environment," they think of forests, oceans, or mountains. In cancer research, however, scientists define the environment as everything outside the body that enters and interacts with it. This interaction is called an exposure. So, environmental exposures can include such factors as sunshine, radiation, hormones, viruses, bacteria, and chemicals in the air, water, food, and workplace, as well as lifestyle choices like cigarette smoking, excessive alcohol consumption (more than 2 drinks/day), an unhealthful diet, lack of exercise, or sexual behavior that increases one's exposure.

Researchers have estimated that as many as 2 in 3 cases of cancer (67 percent) are linked to some type of environmental factor, including use--or abuse--of tobacco, alcohol, and food, as well as exposures to radiation, infectious agents, and substances in the air, water, and soil.

Avoidable Environmental Factors

The good news is that the major environmental factors that are linked to cancer deaths can be modified, because most of them involve lifestyle choices. Almost one-third of all cancer deaths could be prevented by eliminating the use of tobacco products, for example, and making better dietary choices could prevent many more premature deaths from this disease. Our knowledge and certainty about diet is much less firm than it is for tobacco. Diets are very complex and we need to know what people ate in the past that impacted their cancer diagnoses today.

Influencing Rates and Risks

The environment influences cancer rates and risks. We can see this by comparing cancer rates in different countries, and how rates change when people move from one country to another.

For example, U.S.-born Japanese men have twice the rate of colon cancer as native-born Japanese men, and U.S.-born Japanese women have colon cancer rates 40 percent higher than their counterparts born in Japan. So scientists study what exposures or characteristics differ between Japanese immigrants and their descendants in the U.S. to better understand the environmental factors that may be influencing their colon cancer rates and risks.

Different Exposures, Different Rates and Risks

Certain types of exposures are linked to specific cancers. For example, exposure to asbestos is linked to lung cancer, and exposure to benzidine (a chemical found in some dyes) is linked to bladder cancer. Exposure to carcinogens from tobacco use is linked to several types of cancer, including cancers of the lung, bladder, mouth, lip, throat, voice box, and esophagus.

The Inside Matters: Random Gene Changes

Of course, environmental exposures by themselves do not cause cancer. Cancer is complex and involves many gene-gene interactions that occur inside you and are not well understood. For example, certain randomly occurring gene changes may be accumulating in your body's cells right now. And these same kinds of changes may not be occurring in your friends, your coworkers, or even your family members, even though all of you remain in a similar environment most of the time. Over your lifetime, random gene changes are passed along as your body cells grow and divide, so they accumulate. The unique patterns that evolve over time may make some people more likely than others to increase their risk for cancer after exposure to a particular chemical or after choosing a particular behavior.

The Inside Matters: Other Factors

You might wonder why some families are more cancer prone than others. In part, inheritance is involved in some of these cases. This is because, at birth, some offspring unknowingly inherit gene changes that can make them more susceptible to cancer. But this explains only a very small percent of new cancer cases, no more than 5 percent.

Others factors that may change your cancer risk include having stronger or weaker immune systems, variations in detoxifying enzymes or repair genes, or differences in hormone levels.

Familial Rates and Risk: Those We Are Beginning To Understand

Rarely, several generations of the same family will develop the same type of cancer at rates much higher than those that occur in the population overall. Often, the family members are passing on mutated genes that impart a higher than average risk for developing this particular cancer. By studying the genetic profiles of these affected families, researchers are learning which genes are involved in cancer's development. Kidney cancer families are a good example of this. When scientists discovered the gene changes involved in the inherited form of renal cancer, they were able to use this information to better detect and diagnose sporadic or randomly occurring new cases of this cancer type.

Only about 2 to 5 percent of cancers run in families this way.

Familial Rates and Risk: Those We Still Don't Understand

Some families will exhibit higher than average rates of a particular cancer, yet when scientists search their genomes, they are unable to find the usual genomic alterations suspected of increasing cancer risk. These cases seem to point to gaps in our understanding of the full set of mutations required for cancer's development. They also prompt researchers to probe deeper in search of possible environmental exposures suffered by the clan collectively.

Faulty Gene Repair Activities

Normally, if environmental exposures cause an unwanted molecule to bind to a gene, excision repair proteins rapidly remove that damaged area of the gene. Because the genes in the body that produce these repair proteins can themselves have mutations, people can differ from one to another in their gene repair activities. Unfortunately, genetic variations can make a person's gene repair activities less efficient or more error-prone than normal, and this faulty condition can be passed from generation to generation.

About the Author

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