By Lisa Fayed 

Updated on September 07, 2021

 Medically reviewed by Doru Paul, MD

• How Cancer Is Diagnosed

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

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.¹

Verywell / Jessica Olah

Common Causes

According to the American Cancer Society and the National Cancer Institute, the most common causes and risk factors 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 fall in this category.¹

Another example is exposure to asbestos, a group of minerals found in some older housing and industrial building materials that 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.²

There are many other substances in the 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.¹

Infections

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 your risk. Other infections, such as HIV, suppress the immune system so it cannot effectively protect against cancer growth.³

Human papillomavirus (HPV) increases your risk for cervical, anal, vulvar, and vaginal cancer.  Studies suggest that HPV also plays an important role in many head and neck cancers, and ongoing research is looking at its possible role in other cancers as well.³ The HPV vaccine is recommended for girls and boys starting at age 11 or 12.

Age

While cancer can occur at any age, the median age for a cancer diagnosis is between 65 and 74, depending on the type.⁴

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.

That said, there are some forms of cancer that are more common in children, including bone cancer and some forms of leukemia.⁵

Genetics

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.⁶ 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.⁷ 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).

 Non-BRCA Gene Mutations That Raise Breast Cancer Risk

Lifestyle Risk Factors

Many of the major risk factors for cancer are ones that you can control. This can be especially empowering for those who are aware of certain predispositions.

Tobacco

Not only does smoking affect the lungs, but it increases the risk of many cancers. In fact, smoking causes 30% of all cancer deaths in the U.S. and is responsible for 80% of cases of deaths attributable to lung cancer specifically.

Quitting smoking immediately decreases your risk factor for cancer.⁸

Alcohol

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.⁹

Lack of Physical Activity

Exercising at least 30 minutes a day, five days a week greatly reduces your cancer risk.¹⁰ 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

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.¹¹ 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.

Diet

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.¹

Sun Exposure

Skin cancer may be caused by excess exposure to UV rays from the sun. 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.¹

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 each year. Due to early detection and better treatment, however, the cancer death rate has declined 27% in 25 years and continues to go down 1.8% per year.¹² 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.

FREQUENTLY ASKED QUESTIONS

What causes cancer tumors to develop and grow?

Tumors form when cells in the body that normally would die to make room for new ones instead grow out of control. This occurs due to genetic changes, or mutations, in the cells. The proliferation of cells that develops can be benign, meaning it will not reach beyond the area where it forms, or it can be malignant and capable of metastasizing, or spreading to other parts of the body.¹³

How does cancer spread?

Cancer can spread by growing into surrounding tissues. It can also metastasize: Malignant cells can break away from the original tumor and then be carried to other parts of the body via the bloodstream or the lymphatic system.¹³

Do I have to have a certain gene to get cancer?

No. Only between 5% and 20% of cancers are caused by inherited gene mutations, which are known as germline mutations. Such mutations can be passed from generation to generation. That said, it's important to be aware that just because you inherit a cancer gene it does not mean you'll definitely develop cancer.¹⁴

Can people who are healthy get cancer?

Yes. You can develop cancer even if you have not inherited a genetic propensity for the disease and you pass every health check-up and screening with flying colors. There are still risk factors for cancer you may not be aware of or that you were exposed to in the past, such as air pollution, second-hand cigarette smoke, exposure to ultraviolet rays, and so forth. That said, it's never too late to take measures to lower your risk of cancer.¹⁴