But sometimes the repair process fails. Exposure to our own hormones, to infectious organisms that have penetrated our immune defenses, and to environmental toxins can also create free radicals that scramble genetic codes and eventually damage cells. If cells are damaged, cell growth may produce tumors rather than copies of healthy cells. It's estimated that three-quarters of all cancers occur largely because of external influences, not our genes. What you can do 1 Don't smoke.
Tobacco use causes more cancer here and in the rest of the world than anything else.
The longer you smoke, and the more you smoke, the likelier it is to be lethal. Besides lung cancer, smoking increases the risk of cancer of the bladder, cervix, mouth, throat, pancreas, kidney, and stomach. It may also promote colon and even breast cancer. About 3 million people die of smoking-related causes every year around the world, and that number will rise to 10 million in the next century if the number of smokers continues to increase. If all tobacco users in this country quit, total deaths from cancer would eventually drop by at least one-third.