Scientists know that many cancers have a genetic cause which limits their ability to prevent the disease. However, scientists have identified several factors that can increase a person's chances of developing cancer. They have also discovered substances that may protect DNA and keep cancer from developing. Many scientists believe that people can decrease their overall chances of developing cancer by adopting a healthy lifestyle and avoiding contact with known carcinogens.

Avoiding carcinogens. Smoking is one of the most widespread and avoidable environmental causes of cancer. Scientists think that smoking causes about one-third of all cases of cancer, including most lung cancers. Smoking also causes many cancers of the mouth, larynx, trachea, esophagus, pancreas, kidney, bladder, and cervix. Cigarette smoke can even cause cancer in nonsmokers who live or work closely with smokers. Smokers who quit can significantly reduce their chance of developing lung cancer. People can also reduce their risk of developing skin cancer. The sun is one of the most important sources of cancer-causing radiation. Most cases of skin cancer-including deadly melanoma-are caused by an invisible portion of the sun's radiation called ultraviolet rays. Physicians advise against sunbathing and recommend that people who work outdoors wear protective clothing or apply sunscreens that block ultraviolet rays.

Nutrition. Some chemicals naturally present in food may become a cancer threat if consumed in large quantities. Diets high in fats, for example, have been associated with cancers of the breast, colon, and prostate gland. Some studies have linked eating large amounts of salt-cured, salt-pickled, and smoked foods to cancers of the digestive system. Excessive alcohol consumption has been linked to cancer of the mouth, pharynx, larynx, esophagus, and liver.

Many scientists believe that certain foods contain substances that may help prevent cancers in people. Such foods include broccoli, cauliflower, cabbage, spinach, tomatoes, carrots, fruits, and whole-grain breads or cereals.