Cause of Cancer

The great majority of cancers, some 90–95% of cases, are due to environmental factors. The remaining 5–10% are due to inherited genetics.^[1] Environmental, as used by cancer researchers, means any cause that is not inherited genetically, such as lifestyle, economic and behavioral factors, and not merely lifestyle factors such as tobacco smoking and obesity, or external exposures in the surrounding world such as pollution and exposure to sun.^[2] Common environmental factors that contribute to cancer death include tobacco (25–30%), diet and obesity (30–35%), infections (15–20%), radiation (both ionizing and non-ionizing, up to 10%), stress, lack of physical activity, and environmental pollutants.^[1]

It is nearly impossible to prove what caused a cancer in any individual, because most cancers have multiple possible causes. For example, if a person who uses tobacco heavily develops lung cancer, then it was probably caused by the tobacco use, but since everyone has a small chance of developing lung cancer as a result of air pollution or radiation, then there is a small chance that the cancer developed because of air pollution or radiation.

Tobacco use is the cause of about 22% of cancer deaths.^[3] Another 10% is due to obesity, a poor diet, lack of physical activity, and drinking alcohol.^[3] Other factors include certain infections, exposure to ionizing radiation, and environmental pollutants.^[1] In the developing world nearly 20% of cancers are due to infections such ashepatitis B, hepatitis C, and human papillomavirus.^[3] These factors act, at least partly, by changing the genes of a cell.^[4] Typically many such genetic changes are required before cancer develops.^[4] Approximately 5–10% of cancers are due to genetic defects inherited from a person's parents.^[5]

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