Smoking accounts for 30 percent of all cancer deaths. Early onset of smoking increases risk of lung cancer and continuing to smoke in the adult years promotes the final stages of cancer formation. An estimated 30 percent of all US cancer deaths can be attributed to diet, including its effect on obesity. Two alcoholic drinks a day can increase breast cancer risk by about 25 percent. Physical inactivity is associated with a twofold increase in the risk of colon cancer.
Colon cancer is responsible for approximately as many deaths as breast or prostate cancer.
It is estimated that only 5 to 10 percent of most types of cancer are due to defects in single genes that run in families. Poverty increases the risk of certain cancers, notably cancer of the stomach and liver. In addition, poor people have fewer opportunities to seek preventive care, resulting in higher risk. Almost all skin cancer, including malignant melanoma, is caused by damage to skin cells by ultraviolet light. While it is never too late to reduce sun exposure, for most people more than half of their lifetime exposure to sunlight occurs during childhood and adolescence.