Carcinogens in the environment: A major cause of cancer?

Explore how carcinogens in the environment might contribute to cancer risk. Learn why it's difficult to tell what causes cancer and what doesn't.

In a study of cancer myths, 40 percent of people agreed that city air pollution was a bigger risk for lung cancer than was smoking. Pollution certainly may contribute to some cases of lung cancer, but not anywhere near the number caused by smoking. Yet the idea that chemicals in the environment are a major cause of cancer persists.

Researchers estimate that cancer-causing chemicals (carcinogens) in the environment cause fewer than 5 percent of cancer deaths in the United States. Most cancers are believed to be caused by lifestyle choices, such as what you eat, whether you maintain a healthy weight and whether you smoke. So why do some people still believe their environment is a major cause of cancer? Here's an in-depth look at the issue.

Why is there so much confusion over whether certain chemicals are carcinogens?

Most cancers take years to develop, making it difficult to determine if a chemical exposure today will cause cancer in the future. Tumors usually develop for 15 to 20 years before they become evident. Blood and lymph cancers, such as leukemia and lymphoma, usually take five to 10 years to develop.

Chemicals — both natural and man-made — are everywhere in our environment. People are exposed to so many chemicals and combinations of chemicals that it's nearly impossible to pinpoint one chemical that could have caused an individual's cancer. It's even more daunting when you factor in the nonchemical causes of cancer, such as family history and lifestyle choices.

It's unlikely that one carcinogen or a single lifestyle factor could be responsible for a person's cancer. Instead, genetics, lifestyle choices and environmental carcinogens work together. For instance, smokers who work around asbestos are more likely to develop lung cancer than are smokers who don't. Carcinogens in cigarette smoke and asbestos work together to increase the chance that a person will develop cancer — a process sometimes called synergy.

What evidence is needed to determine whether certain chemicals are carcinogens?

To reach a definitive answer, scientists would need to conduct a controlled clinical trial in which half the people were exposed to a suspected carcinogen. Most people would be unwilling to enroll in such a trial, and the institutional review boards that monitor trials to ensure safety wouldn't allow these types of trials.

With that in mind, scientists rely on other types of studies to decide whether chemicals are carcinogens. These include:

Human observation studies. These studies compare a group of people who are more likely to be exposed to potential carcinogens to a group of people in the general population. For instance, people who work around asbestos are more likely to be exposed to asbestos particles, so that group might be followed over a number of years. However, one observational study usually provides insufficient evidence to prove something does or doesn't cause cancer.
Animal studies. Scientists expose animals — usually mice or rats — to very high levels of suspected carcinogens to see how their bodies react. Whether diseases in animals are comparable to diseases in humans, however, is a subject of debate.

Two groups determine whether substances are carcinogens — the National Toxicology Program, run by the Department of Health and Human Services, and the International Agency for Research on Cancer, run by the World Health Organization. Both groups review the latest in scientific evidence and rule whether a chemical causes cancer, is likely to cause cancer or doesn't cause cancer. Sometimes there isn't enough evidence to make a ruling. Though both groups examine many of the same chemicals, they don't always agree.

Common misconceptions about environmental carcinogens: Recognize myths and explore realities

Because cancer is such a complicated process, it's difficult to know how to react to news reports of chemical spills and air pollution. Before you panic, get the facts about these common myths about carcinogens:

Even tiny amounts of carcinogens can cause cancer
Most carcinogens won't cause cancer unless you're exposed to a great deal of the substance. For instance, radiation causes cancer, but getting your arm X-rayed to look for broken bones isn't likely to cause cancer because you're exposed to X-ray radiation for a very short period of time. Other carcinogens require many years of daily exposure to cause cancer.

Man-made carcinogens are more dangerous than carcinogens that occur naturally
Carcinogens aren't solely man-made. They can also occur naturally in the environment. For instance, asbestos and cadmium — both listed as known carcinogens by the federal government — are both found naturally in the earth. Eliminating all man-made carcinogens wouldn't remove all the carcinogens in the environment.

Being near a carcinogen is all it takes to cause cancer
Most carcinogens are absorbed into your body in a very specific way. Cadmium, for example, only increases the risk of cancer if it's inhaled through polluted air or ingested through contaminated food or water. Touching a rock that contains cadmium won't increase your risk of cancer. Other carcinogens are absorbed through your bloodstream, your mucous membranes or your skin.

Cancer rates are on the rise because there are more carcinogens in the environment
Many factors contribute to the rate of newly diagnosed cancers. There's no evidence to suggest that environmental carcinogens have anything to do with the increases in newly diagnosed cancers that occurred from 1975 through the early 1990s. Most researchers attribute that increase to smoking and the fact that more people were being screened for cancer. In addition, the population is increasing and people are living longer — making them more likely to develop cancer. The incidence rate of cancer — the number of cancer cases per 100,000 people — has remained stable for most cancers.

How can you protect yourself from carcinogens?

It's impossible to stay away from all environmental carcinogens. While it may be scary to know that you can't control whether you get cancer, you can take measures to reduce your risk. You can:

Control what's within your reach. You make choices every day that could reduce your risk of cancer. Though eating a healthy low-fat diet full of fruits and vegetables can't guarantee that you won't get cancer, it may reduce your risk. Avoiding cigarette smoke, excessive alcohol consumption and excessive amounts of sunlight reduces your risk of certain cancers.
Know what carcinogens you work with. In the United States, your employer is required to inform you about the hazards of any chemicals in your workplace. Follow all safety precautions at your workplace. Ask your doctor what more you could do to protect yourself.
Get screened. Though cancer is difficult to prevent, most cancers can be treated if found at an early stage. Screenings for cancer can help your doctor identify cancers at a treatable stage.
Be conscious of chemicals around your home and use them properly. The bug spray in your home isn't likely to cause cancer, especially when used correctly. Follow the directions on any household chemical containers. Open a window in the room where you're using chemicals and wash up when you're done. If you're worried about household chemicals, choose alternatives, but know that the alternatives may do more to calm your mind than to actually reduce your risk of cancer. Use the National Library of Medicine's Household Products Database to learn more about the ingredients in household chemicals.