Causes of Cancer


Cancers that run in families can be caused by an abnormal gene that is inherited. These abnormal genes fail to normally control our cell function, resulting in cancer. Only about 5-10% of all cancers result directly from genetic defects inherited from a parent. In other cases, certain cancers seem to run in families because members share certain behaviors, exposures or other factors that increase cancer risk, such as obesity.

We can also go through genetic changes that cause cancer throughout our lifetime. This can be a result of errors that occur as our cells divide. It could also be due to exposure to carcinogenic (cancer-causing) substances that damage our DNA, including radiation and certain chemicals. These changes to our genes that occur after our birth are called somatic changes.

Risk Factors

It’s usually not possible to know for sure what causes one person to develop cancer. However, research has shown that certain risk factors outside of genetics can increase a person’s chances of developing cancer. Although some risk factors are unavoidable, limiting your exposure to those that are avoidable may lower your risk of developing certain cancers.


Age is one of the most important risk factors for cancer overall. According to the most recent statistical data from NCI’s Surveillance, Epidemiology, and End Results program, the median age of a cancer diagnosis is 66 years. One-quarter of new cancer cases are diagnosed in people aged 65 to 74. However, cancer can still occur at any age. Some cancers are more common in children than in adults.

For many common cancer types, a similar trend in age is seen. The median age at diagnosis is 61 years for breast cancer, 68 years for colorectal cancer, 70 years for lung cancer, and 66 years for prostate cancer.



Cancer-Causing Substances

Carcinogens are substances that exist in our environment that can damage our DNA, resulting in genetic changes that impact cell division and can lead to cancer. Such exposures include tobacco smoke or alcohol.


Tobacco use is a leading cause of cancer and death from cancer.People who use tobacco products or who are regularly around secondhand smoke have an increased risk of cancer because tobacco products and secondhand smoke have many chemicals that damage DNA. It can cause many types of cancer, including lung, throat, bladder, kidney, liver, stomach, colon, cervix, and acute myeloid leukemia.


Having a poor diet that has low nutritional value can impact your health. People who are obese may have an increased risk of several cancer types, including breast, colon, kidney, and gallbladder.

For more information, please view the Obesity and Cancer and Physical Activity and Cancer fact sheets, courtesy of the NIH National Cancer Institute.


Certain types of radiation have enough energy to damage DNA and cause cancer. These harmful radiation types include radon, x-rays, and gamma rays. Lower-energy forms of radiation, such as visible light and the energy from cell phones, do not damage DNA and have not been found to cause cancer. However, the sun, sunlamps, and tanning booths all give off ultraviolet (UV) radiation. Exposure to UV radiation causes early aging of the skin and skin damage that can lead to skin cancer.