According to the National Cancer Institutes 2001 Cancer Progress Report, two kinds of factors can affect a person's risk of getting cancer: behaviors and exposures to chemicals in the environment. Choosing the right behaviors and preventing exposures to certain chemicals may help to prevent cancers before they can start.

Behavioral Factors:
Scientists estimate that as many as 50 percent to 75 percent of cancer deaths in the United States are caused by human behaviors such as smoking and dietary choices. The first part of the Prevention section describes trends in the following behaviors that can help to prevent cancer:

Smoking causes about 30 percent of all U.S. deaths from cancer. Avoiding tobacco use is the single most important step Americans can take to reduce the cancer burden in this country.

Adult Smoking:
Cigarette smoking is the most preventable cause of death in the United States. It causes nearly one-third (163,000) of all U.S. cancer deaths each year and is the leading cause of lung cancer deaths.

Cigarette smoking also causes cancers of the larynx, mouth, esophagus, pharynx, and bladder. In addition, it plays a role in cancers of the pancreas, kidney, and cervix.

Cigar smoking has been found to cause cancers of the larynx, oral cavity (lip, tongue, mouth, and throat), esophagus, and lung.

Although the rate of smoking has dropped by nearly half since the Surgeon General’s first report on smoking in 1964 (42 percent of adults were current smokers in 1965), progress has slowed in recent years. Further decreases in tobacco use could vastly improve the public’s health.

From 1993 to 1997, U.S. cigar sales soared by almost 50 percent, mostly due to increased sales of large cigars. This followed new cigar marketing approaches that began in 1992.

Youth Smoking:
For most of the 1990s, about 3,000 youth under 18 became regular cigarette smokers each day. This has declined recently to just over 2,000 each day. Of these 2,000, nearly 700 will die early due to lung cancer or other tobacco-related diseases.

Other forms of tobacco used by young people include smokeless tobacco (chewing tobacco and snuff, also known as spit tobacco), cigars, and bidis (small, brown, hand-rolled, flavored cigarettes). Each of these also can cause cancer.

Since 1997, current smoking leveled off or possibly began to decline among 9th–11th graders. However, it has risen steadily among 12th graders since 1991.

In 1999, 13 percent of middle school students (grades 6 to 8) reported using some form of tobacco in the past month. Cigarettes were the most popular, followed by cigars.

Bidis—increasingly popular among young people—can be even more dangerous than cigarettes. Bidis produce higher levels of carbon monoxide, nicotine, and tar than cigarettes. Also, bidi smokers tend to inhale more often and more deeply than cigarette smokers.

Age of Smoking Initiation:
The younger a person starts smoking, the greater the lifelong risk of developing smoking-related cancers. That is because young smokers are more likely to become addicted, and the more years one smokes, the greater the risk of cancer.

Most smokers try their first cigarette before the age of 18 and become addicted during adolescence.

Second Hand Smoke:
Secondhand smoke—also known as environmental tobacco smoke—is what comes from a burning cigarette, pipe, or cigar, plus what the smoker exhales. Tobacco smoke is known to contain at least 60 carcinogens. People who are exposed to secondhand smoke inhale these chemicals, just as smokers do, although at lower levels.

In 1993, the U.S. Environmental Protection Agency (EPA) reported that secondhand smoke is a "known human carcinogen." The EPA also reported that secondhand smoke causes some 3,000 lung cancer deaths each year among U.S. nonsmokers.

Although secondhand smoke remains a major public health concern, nonsmokers' exposure to tobacco smoke declined more than 75 percent from 1988-1991 to 1999.

In 1999, nearly 7 out of 10 U.S. workers reported a smoke-free policy in their workplace.

State laws that protect against secondhand smoke slowly became more common in the past decade. It appears that greater improvement came from voluntary or local efforts during that time.

Quitting:
Ten years after quitting smoking, a person's risk of getting lung cancer is about one-third to one-half that of people who continue to smoke. The longer the time off cigarettes, the lower the risk. Quitting also reduces the risk of getting cancers of the larynx, esophagus, pancreas, bladder, and cervix.

Also, the sooner one quits smoking, the better. Long-term smokers who stop smoking at around 50 or 60 years of age are less likely to get lung cancer than are people who continue to smoke. Quitting at around age 30 lowers this risk even more.

The quickest non-cancer health benefit of quitting is a lower risk of coronary heart disease. This risk is cut in half after 1 year of quitting. After 15 years, the chance of getting the disease is similar to that of people who never smoked.

Studies show that most smokers want to quit.

Efforts to reduce smoking are most effective when multiple techniques are used, including educational, clinical, regulatory, and economic interventions (for example, increasing excise taxes), along with media campaigns and other social strategies.

Efforts to help young people delay or avoid smoking may help to prevent some cancers.

Additional important steps are maintaining a healthy weight, being physically active, eating a low-fat diet and enough fruits and vegetables, avoiding too much alcohol, and protecting skin from sunlight.

Alcohol Consumption:
Drinking alcohol increases the risk of cancers of the mouth, esophagus, pharynx, larynx, and liver in men and women, and breast cancer in women. In general, these risks increase after about one daily drink for women and two daily drinks for men. (A drink is defined as 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of 80-proof liquor.)

Two drinks daily increase the risk of getting breast cancer by about 25 percent. The chances of getting liver cancer increase with five or more daily drinks.

The earlier that long-term, heavy alcohol use begins, the greater the cancer risk. Also, using alcohol with tobacco is riskier than using either one alone, because it further increases the chances of getting cancers of the mouth, throat, and esophagus.

People who drink and smoke may find it harder to stop either of these behaviors.

Drinking low levels of alcohol can have both negative and positive health effects: It raises the risk of getting breast cancer and lowers the risk of getting heart disease. Therefore, women who already are at low risk for heart disease could reduce their risk of breast cancer by avoiding regular alcohol use.

Sun Protection:
Skin cancers are most common in light-skinned people, although they also occur in people with darker skin. Studies suggest that reducing long-term exposure to the sun, to tanning booths, and to sunlamps can lower the risk of nonmelanoma skin cancer. Avoiding burns from these sources—especially by children and teens-may reduce the chances of getting melanoma skin cancer. The rates of new cases of melanoma increased from 1973—1998, although the rate of increase has slowed since 1981.

In general, increased exposure to the sun—especially without adequate use of sunscreen and protective clothing—increases the chances of getting skin cancer.

Some research suggests that people apply less than an adequate amount of sunscreen and fail to reapply it appropriately.

Physical Activity:
Physical activity at work or during leisure time is linked to a 50 percent lower risk of getting colon cancer. Both vigorous and moderate levels of physical activity appear to reduce this risk. Physical activity probably is connected with a lower risk of breast cancer and possibly prostate cancer. Studies continue to look at whether physical activity has a role in reducing the chances of getting other cancers.

Since the mid-1980s, fewer high school students have taken part in physical education classes.

Removing barriers (such as lack of physical education classes) and setting up supports (such as bicycle and walking paths) can help to promote physically active lifestyles.

Also, being overweight increases the chances of health problems, including heart disease, stroke, diabetes, and some cancers.

In women, overweight and obesity, weight gain, and increased amounts of fat at the waist or around the body's mid-section double to triple the chances of getting endometrial cancer. These factors also double the chances of getting breast cancer after menopause.

Obesity and increased body fat raise the risk of getting colorectal cancer. Overweight and obesity are linked to an increased risk of some types of esophageal and kidney cancers.

Daily physical activity balanced with appropriate calorie intake is one of the most effective ways to avoid weight gain. Lack of activity is believed to be one of the major reasons for the increase in overweight among U.S. youth and adults.

Increased TV watching is linked with excess weight.

Environmental Factors:
Certain chemicals in the environment are known to cause cancer.

Radon:
Radon—an invisible, odorless, tasteless gas that is released from rocks and soil—enters homes through cracks and holes in the foundation. Indoor radon is the most serious environmental cancer-causing agent to which the general public is exposed. The Environmental Protection Agency estimates that as many as 8 million homes in the United States have high levels of radon. State surveys show that one out of five homes have high levels.

Radon is second only to tobacco as the leading cause of lung cancer. Radon found in homes may contribute to as many as 20,000 lung cancer deaths each year. It is a more serious health threat to under-ground miners.

People who are exposed to both radon gas and tobacco smoke are more likely to get lung cancer than are people who are exposed to either one alone. Most radon-related deaths from lung cancer occur among smokers.

Researchers estimate that lowering indoor radon exposure would prevent about 30 percent of lung cancer deaths from radon. Of these, 86 percent would be among smokers or former smokers.

Benzene:
Benzene is a natural part of crude oil, gasoline, and cigarette smoke. It is also used as a gasoline additive and in the manufacture of a number of products.

The general population's main exposure to benzene is inhaling air that contains it. About half of human exposures to benzene come from smoking and secondhand smoke. Other sources include vapors from heavy traffic and gas stations. Long-term exposure to high levels of benzene in the air can cause leukemia.

The Environmental Protection Agency says that benzene concentrations in the air have declined because reformulated gasoline is being used in many parts of the United States. This is an example of how changes to the environment can help to lower cancer risk.

More measures of environmental chemical carcinogen exposures—such as those reported by the National Center for Environmental Health, Centers for Disease Control and Prevention—need to be tracked over time.

These environmental measures were chosen because of the availability of reliable national data showing trends over time. Additional environmental measures will be available for future editions of this report.

For more information on nutritional factors, see our pages on nutrition and the Food Guide Pyramid.