Carcinogens in the environment: A major cause of cancer?

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.

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.

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.

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