Carcinogen

For a carcinogen to cause cancer a person must be exposed for a certain length of time to the carcinogen and at a high enough concentration. Repeated exposure to a carcinogen over an extended period (such as 20 years) increases the likelihood that a normal cell's genetic material will mutate and initiate cancer. Cigarette smoke contains potent carcinogens; it can take many years and many repeated exposure to the carcinogens in smoke for smoking to cause cancer. Smoking-related lung cancers typically develop between 10 and 20 years of continuous smoking. In Hiroshima and Nagasaki in Japan, where atomic bombs were dropped in 1945, the leukemia rates in the surviving population increased dramatically some five years after the bombs were detonated.

Some carcinogens are more powerful, cancer-causing agents, than others. Powerful carcinogens are called tumor promoters, and can cause genetic mutations directly within cells. Less powerful carcinogens are called tumor initiators and can cause latent changes in the cell's genetic material. These changes are not enough to actually cause cancer, but sensitize the tissue for later exposures to tumor promoters. If a tumor initiator has already wrought some damage to the cell's genetic material, the likelihood that a tumor promotor will cause the cell to become cancerous is increased.

Cancerous tumors develop over many stages, and it is rare that exposure to a carcinogen is the sole cause of most cancers. Exposure to a carcinogen must usually be combined with other environmental factors for a cancer to develop. Some environmental risk factors are difficult to identify, while others such as prolonged, heavy cigarette smoking have been easy to identify. Heredity, such as whether close relatives have developed cancer, is another risk factor that is not as easily characterized.

Some carcinogens such as cigarette smoke can be avoided. Other factors, such as a diet, can be modified. Additional risk factors such as gender, immune status, metabolic rate, levels of certain enzymes, and age can neither be avoided nor modified.