What causes cancer?

It is rarely possible to find the cause of a cancer in an individual, but studies on groups of people with cancer have shown specific risk factors to be associated with specific cancers. This suggests that different types of cancer probably have different causes. These studies also indicate that cancer formation is a multi-step process, and that for most cancers the time from a cancer-causing exposure to a clinically diagnosable cancer averages about 20 years.

Among the known risk factors for cancer, tobacco stands out. Cigarette smoking is associated with lung cancers, and with a substantial proportion of cancers of the bladder, mouth and throat, stomach, pancreas and others. Diet is also a risk factor; higher cancer rates are seen in people who eat a diet high in fat and low in fresh vegetables and fruits. It is estimated that diet and tobacco together account for approximately two out of three cancers.

The well-established risk factors for breast cancer, the most common cancer in women, are not easily modifiable. About 50% of breast cancers are thought to be explained by known risk factors such as a family history of cancer and hormonal functions associated with early onset of menstruation and late menopause, delayed childbearing and having fewer children. Few risk factors have been defined for the most common cancer in men, prostate cancer.

Occupational studies have shown certain chemicals and other substances to be carcinogenic;these include asbestos, benzene, arsenic, vinyl chloride and other industrial products. Exposure to these substances is thought to account for about 5% of all cancers. 

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