Causes and Risk Factors of Cancer

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Most people don't realize that cancer is preventable in many cases. Learning what causes cancer and what the risk factors are is the first step in cancer prevention. Many cancer risk factors can be avoided on the one hand, or changed in another, to reduce your risk.

Common Causes

Cancer develops when there are changes to the DNA (mutations) within a cell that allows it to grow out of control. The cancerous cells then can crowd out normal cells or invade body tissues. Radiation, cancer-causing substances, infections, and your genetic makeup can raise your risk of these mutations resulting in cancer.

According to the American Cancer Society and the National Cancer Institute, the most common causes of cancer are:

  • Smoking and tobacco use
  • Alcohol
  • Lack of physical activity
  • Being overweight or obese
  • Poor diet
  • Sun exposure
  • Radiation exposure
  • Virus infections and other infections
  • Exposure to cancer-causing substances
  • Family history and genetics
  • Chronic inflammation
  • Hormones
  • Immunosuppression
  • Age

Environmental Exposures

Your surroundings can raise your risk of developing cancer. Carcinogens—substances and exposures that can lead to cancer—can be found in the home, in the workplace, and outdoors. Tobacco use and smoking are part of this category.

Another example is exposure to asbestos, a group of minerals found in housing and industrial building materials which can cause mesothelioma, a cancer of the lung lining. People who are exposed to a high amount of benzene (found in gasoline, tobacco smoke, and pollution) are at risk for cancer.

There are many other substances in our environment that could put you at risk. Be careful with chemicals in your home, and always take time on the job to read the information provided about the chemicals you work with.

Radiation Exposure

The most common form of radiation exposure is from the sun. Another environmental exposure is radon gas, which may be present in the soil and can build up in your home. You may also be exposed due to medical imaging or treatment.


Infections can increase cancer risk in several ways. Some viral infections directly affect the DNA to produce cancerous changes. Other infections may lead to long-term inflammation, which raises risks. Other infections, such as HIV, suppress the immune system so it doesn't protect against cancer growth.

Human papillomavirus (HPV) increases your risk for cervical, anal, vulvar and vaginal cancer.  Recent studies suggest that HPV also plays an important role in many head and neck cancers, and research is ongoing looking at its possible role in other cancers as well. The HPV vaccine is recommended for girls and boys starting at age 11 or 12.


While cancer can occur at any age, the median age for a cancer diagnosis is between 65 and 74, depending on the type. Over the years you have had more exposure to carcinogens and inflammatory processes and there has been more time for slow-growing cancers to become symptomatic. Your body also becomes less efficient at finding and destroying cancerous and pre-cancerous cells. However, there are some forms of cancer that are more common in children, including bone cancer and some forms of leukemia.


Genes are the ultimate cause of all cancers, but in the vast majority of cases, these are acquired mutations that are not passed on to your children. You have "healthy" genes but a mutation happens to one cell and then is able to grow out of control. It is common to have an underlying susceptibility to cancer due to oncogenes and inactivated tumor suppressor genes.

But in 5% to 10% of cases, cancer is due to a family cancer syndrome that can be inherited. If you have a family history of cancer, such as breast cancer, taking extra precautions is vital. Genetic tests are available for some hereditary cancers. Keep in mind that if you have a family history of cancer, it does not mean you will develop it. You only have a greater chance of developing it (a genetic predisposition).

Lifestyle Risk Factors

The good news is that many of the major risk factors for cancer are ones under your control.


Smoking causes 30% of all cancer deaths in the U.S. and is responsible for 80% of cases of deaths attributable to lung cancer. Not only does it affect the lungs, but smoking increases the risk of many cancers. Quitting smoking immediately decreases your risk factor for cancer.


Alcohol is an irritant that can damage cells and promote the production of carcinogenic chemicals in the colon. To reduce your cancer risk from alcohol, the American Cancer Society suggests limiting alcohol to one drink per day for women and two drinks per day for men.

Lack of Physical Activity

Exercising at least 30 minutes a day, five days a week greatly reduces your cancer risk. Exercise like yoga, aerobics, walking, and running are great activities to lower your risk. You don't need to run marathons. It's been found that even light exercise—such as working in the garden a few days a week—reduces the risk of lung cancer, among other types.


Obesity is a leading cause of cancer. It raises the risk of breast cancer, colon and rectal cancer, endometrial cancer, esophageal cancer, pancreatic cancer, and kidney cancer, among others. Excess fat cells produce more of the hormones estrogen and insulin, which promote cancer growth. Achieving or maintaining a healthy body weight can reduce your risks.


A diet that focuses on plants, including vegetables, whole fruit, whole grains, and protein from peas and beans, is best for reducing the risk of cancer. Processed meats, red meat, sugar-sweetened beverages, and refined carbohydrates should be limited.

Sun Exposure

Skin cancer may be caused by excess exposure to UV rays from the sun. Many people don't realize that a sunburn—even a tan—is actually the result of skin damage caused by the sun. Many cases of skin cancer can be prevented through a little planning. Wearing sunscreen can help, but practice safe sun exposure as well. Avoid direct sunlight mid-day (from 10 a.m. to 2 p.m.), sit under an umbrella, wear protective clothing, and don't forget sunglasses to protect your eyes. One form of skin cancer—melanoma—has a tendency to affect the eyes.

Unsafe Sex

Practicing unsafe sex can increase your risk of HPV, HIV, and hepatitis B, all of which raise your risk of cancer.

A Word From Verywell

The rate of new cancer diagnoses is falling in the U.S., but over 1.7 million people will be diagnosed with cancer this year. The good news is that due to early detection and better treatment, the cancer death rate has declined 27% in 25 years and continues to go down 1.8% per year. While you can't avoid all causes of cancer, you can significantly reduce your risks by making lifestyle changes. These measures also reduce your risks of other major diseases and are worth the effort.

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