Most cancer risk factors are initially identified in epidemiology studies. In these studies, scientists look at large groups of people and compare those who develop cancer with those who don’t. These studies may show that the people who develop cancer are more or less likely to behave in certain ways or to be exposed to certain substances than those who do not develop cancer.
Such studies, on their own, cannot prove that a behavior or substance causes cancer. For example, the finding could be a result of chance, or the true risk factor could be something other than the suspected risk factor. But findings of this type sometimes get attention in the media, and this can lead to wrong ideas about how cancer starts and spreads.
When many studies all point to a similar association between a potential risk factor and an increased risk of cancer, and when a possible mechanism exists that could explain how the risk factor could actually cause cancer, scientists can be more confident about the relationship between the two.
The list below includes the most-studied known or suspected risk factors for cancer. Although some of these risk factors can be avoided, others—such as growing older—cannot. Limiting your exposure to avoidable risk factors may lower your risk of developing certain cancers.
Advancing age is the most important risk factor for cancer overall, and for many individual cancer types. 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. This means that half of cancer cases occur in people below this age and half in people above this age. One-quarter of new cancer cases are diagnosed in people aged 65 to 74.
A similar pattern is seen for many common cancer types. For example, 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.
But the disease can occur at any age. For example, bone cancer is most frequently diagnosed among people under age 20, with more than one-fourth of cases occurring in this age group. And 10 percent of leukemias are diagnosed in children and adolescents under 20 years of age, whereas only 1 percent of cancer overall is diagnosed in that age group. Some types of cancer, such as neuroblastoma, are more common in children or adolescents than in adults.
Drinking alcohol can increase your risk of cancer of the mouth, throat, esophagus, larynx (voice box), liver, and breast. The more you drink, the higher your risk. The risk of cancer is much higher for those who drink alcohol and also use tobacco.
Doctors advise people who drink to do so in moderate amounts. The federal government’s Dietary Guidelines for Americans defines moderate alcohol drinking as up to one drink per day for women and up to two drinks per day for men.
It has been suggested that certain substances in red wine, such as resveratrol, have anticancer properties. However, there is no evidence that drinking red wine reduces the risk of cancer.
Cancer-Causing Substances in the Environment
Cancer is caused by changes to certain genes that alter the way our cells function. Some of these genetic changes occur naturally when DNA is replicated during the process of cell division. But others are the result of environmental exposures that damage DNA. These exposures may include substances, such as the chemicals in tobacco smoke, or radiation, such as ultraviolet rays from the sun.
People can avoid some cancer-causing exposures, such as tobacco smoke and the sun’s rays. But others are harder to avoid, especially if they are in the air we breathe, the water we drink, the food we eat, or the materials we use to do our jobs. Scientists are studying which exposures may cause or contribute to the development of cancer. Understanding which exposures are harmful, and where they are found, may help people to avoid them.
The substances listed below are among the most likely carcinogens to affect human health. Simply because a substance has been designated as a carcinogen, however, does not mean that the substance will necessarily cause cancer. Many factors influence whether a person exposed to a carcinogen will develop cancer, including the amount and duration of the exposure and the individual’s genetic background.
Coal Tar and Coal-Tar Pitch
Crystalline Silica (respirable size)
Hexavalent Chromium Compounds
Indoor Emissions from the Household Combustion of Coal
Mineral Oils: Untreated and Mildly Treated
Secondhand Tobacco Smoke (Environmental Tobacco Smoke)
Strong Inorganic Acid Mists Containing Sulfuric Acid
Inflammation is a normal physiological response that causes injured tissue to heal. An inflammatory process starts when chemicals are released by the damaged tissue. In response, white blood cells make substances that cause cells to divide and grow to rebuild tissue to help repair the injury. Once the wound is healed, the inflammatory process ends.
In chronic inflammation, the inflammatory process may begin even if there is no injury, and it does not end when it should. Why the inflammation continues is not always known. Chronic inflammation may be caused by infections that don’t go away, abnormal immune reactions to normal tissues, or conditions such as obesity. Over time, chronic inflammation can cause DNA damage and lead to cancer. For example, people with chronic inflammatory bowel diseases, such as ulcerative colitis and Crohn disease, have an increased risk of colon cancer.
Many studies have investigated whether anti-inflammatory medications, such as aspirin or non-steroidal anti-inflammatory drugs, reduce the risk of cancer. However, a clear answer is not yet available.
Many studies have looked at the possibility that specific dietary components or nutrients are associated with increases or decreases in cancer risk. Studies of cancer cells in the laboratory and of animal models have sometimes provided evidence that isolated compounds may be carcinogenic (or have anticancer activity).
But with few exceptions, studies of human populations have not yet shown definitively that any dietary component causes or protects against cancer. Sometimes the results of epidemiologic studies that compare the diets of people with and without cancer have indicated that people with and without cancer differ in their intake of a particular dietary component.
However, these results show only that the dietary component is associated with a change in cancer risk, not that the dietary component is responsible for, or causes, the change in risk. For example, study participants with and without cancer could differ in other ways besides their diet, and it is possible that some other difference accounts for the difference in cancer.
When evidence emerges from an epidemiologic study that a dietary component is associated with a reduced risk of cancer, a randomized trial may be done to test this possibility. Random assignment to dietary groups ensures that any differences between people who have high and low intakes of a nutrient are due to the nutrient itself rather than to other undetected differences. (For ethical reasons, randomized studies are not generally done when evidence emerges that a dietary component may be associated with an increased risk of cancer.)
Scientists have studied many additives, nutrients, and other dietary components for possible associations with cancer risk. These include the following:
Alcohol. Although red wine has been suspected of reducing cancer risk, there is no scientific evidence for such an association. Also, alcohol is a known cause of cancer. Heavy or regular alcohol consumption increases the risk of developing cancers of the oral cavity (excluding the lips), pharynx (throat), larynx (voice box), esophagus, liver, breast, colon, and rectum. The risk of developing cancer increases with the amount of alcohol a person drinks.
Antioxidants are chemicals that block the activity of other chemicals, known as free radicals, that may damage cells. Laboratory and animal research has shown that exogenous antioxidants can help prevent the free radical damage associated with the development of cancer, but research in humans has not demonstrated convincingly that taking antioxidant supplements can help reduce the risk of developing or dying from cancer. Some studies have even shown an increased risk of some cancers.
Artificial sweeteners. Studies have been conducted on the safety of several artificial sweeteners, including saccharin, aspartame, acesulfame potassium, sucralose, neotame, and cyclamate. There is no clear evidence that the artificial sweeteners available commercially in the United States are associated with cancer risk in humans.
Calcium is an essential dietary mineral that can be obtained from food and supplements. Research results overall support a relationship between higher intakes of calcium and reduced risks of colorectal cancer, but the results of studies have not always been consistent. Whether a relationship exists between higher calcium intakes and reduced risks of other cancers, such as breast and ovarian cancer, is unclear. Some research suggests that a high calcium intake may increase the risk of prostate cancer.
Charred meat. Certain chemicals, called HCAs and PAHs, are formed when muscle meat, including beef, pork, fish, and poultry, is cooked using high-temperature methods. Exposure to high levels of HCAs and PAHs can cause cancer in animals; however, whether such exposure causes cancer in humans is unclear. For more information, see the Chemicals in Meat Cooked at High Temperatures and Cancer Risk fact sheet.
Cruciferous vegetables contain chemicals known as glucosinolates, which break down into several compounds that are being studied for possible anticancer effects. Some of these compounds have shown anticancer effects in cells and animals, but the results of studies with humans have been less clear.
Fluoride in water has shown no association between fluoridated water and cancer risk.
Garlic. Some studies have suggested that garlic consumption may reduce the risk of developing several types of cancer, especially cancers of the gastrointestinal tract. However, the evidence is not definitive. For more information, see the Garlic and Cancer Prevention fact sheet.
Tea contains polyphenol compounds, particularly catechins, which are antioxidants. Results of epidemiologic studies examining the association between tea consumption and cancer risk have been inconclusive. Few clinical trials of tea consumption and cancer prevention have been conducted and their results have also been inconclusive.
Vitamin D helps the body use calcium and phosphorus to make strong bones and teeth. It is obtained primarily through exposure of the skin to sunlight, but it can also be obtained from some foods and dietary supplements. Epidemiologic studies in humans have suggested that higher intakes of vitamin D or higher levels of vitamin D in the blood may be associated with a reduced risk of colorectal cancer, but the results of randomized studies have been inconclusive.
Estrogens, a group of female sex hormones, are known human carcinogens. Although these hormones have essential physiological roles in both females and males, they have also been associated with an increased risk of certain cancers. For instance, taking combined menopausal hormone therapy (estrogen plus progestin, which is a synthetic version of the female hormone progesterone) can increase a woman’s risk of breast cancer. Menopausal hormone therapy with estrogen alone increases the risk of endometrial cancer and is used only in women who have had a hysterectomy.
A woman who is thinking about menopausal hormone therapy should discuss the possible risks and benefits with her doctor.
Studies have also shown that a woman’s risk of breast cancer is related to the estrogen and progesterone made by her ovaries (known as endogenous estrogen and progesterone). Being exposed for a long time and/or to high levels of these hormones has been linked to an increased risk of breast cancer. Increases in exposure can be caused by starting menstruation early, going through menopause late, being older at first pregnancy, and never having given birth. Conversely, having given birth is a protective factor for breast cancer.
Diethylstilbestrol (DES) is a form of estrogen that was given to some pregnant women in the United States between 1940 and 1971 to prevent miscarriages, premature labor, and related problems with pregnancy. Women who took DES during pregnancy have an increased risk of breast cancer. Their daughters have an increased risk of a cancer of the vagina or cervix. The possible effects on the sons and grandchildren of women who took DES during pregnancy are being studied.
Many people who receive organ transplants take medications to suppress the immune system so the body won’t reject the organ. These "immunosuppressive" drugs make the immune system less able to detect and destroy cancer cells or fight off infections that cause cancer. Infection with HIV also weakens the immune system and increases the risk of certain cancers.
Research has shown that transplant recipients are at increased risk of a large number of different cancers. Some of these cancers can be caused by infectious agents, whereas others are not. The four most common cancers among transplant recipients and that occur more commonly in these individuals than in the general population are non-Hodgkin lymphoma (NHL) and cancers of the lung, kidney, and liver. NHL can be caused by Epstein-Barr virus (EBV) infection, and liver cancer by chronic infection with the hepatitis B (HBV) and hepatitis C (HCV) viruses. Lung and kidney cancers are not generally thought to be associated with infection.
People with HIV/AIDS also have increased risks of cancers that are caused by infectious agents, including EBV; human herpesvirus 8, or Kaposi sarcoma-associated virus; HBV and HCV, which cause liver cancer; and human papillomavirus, which causes cervical, anal, oropharyngeal, and other cancers. HIV infection is also associated with increased risks of cancers that are not thought to be caused by infectious agents, such as lung cancer.
Certain infectious agents, including viruses, bacteria, and parasites, can cause cancer in infected people or increase the risk that cancer will form. Some viruses can disrupt normal controls on cell growth and proliferation. They may also increase the chance that a person will be affected by other cancer risk factors, such as UV radiation or substances in tobacco smoke that cause cancer. Some viruses, bacteria, and parasites also cause chronic inflammation, which may lead to cancer.
Most of the viruses that are linked to an increased risk of cancer can be passed from one person to another through blood and/or other body fluids. You can lower your risk of infection by getting vaccinated, not having unprotected sex, and not sharing needles.
Human Papillomaviruses (HPVs). Infection with high-risk types of HPV cause nearly all cervical cancers. They also cause most anal cancers and many, oropharyngeal, vaginal, vulvar, and penile cancers. In the United States, experts recommend that children ages 11 and 12 receive a vaccine that prevents infection with the types of HPV that cause most HPV-associated cancers. Children as young as age 9 and adults as old as 26 can also be vaccinated. HPV infections in the cervix can be found with specific tests. Although HPV infections themselves cannot be treated, the cervical cell changes these infections can cause over time can be treated.
Hepatitis B Virus and Hepatitis C Virus (HBV and HCV). Chronic infections with HBV or HCV can cause liver cancer. Since the 1980s, infants in the United States and most other countries have been routinely vaccinated against HBV infection. If you are an adult who has not been vaccinated against HBV and have an increased risk of HBV infection, experts recommend that you get vaccinated as soon as possible. Vaccination is especially important for healthcare workers and other professionals who come into contact with human blood.
The U.S. Centers for Disease Control and Prevention (CDC) also recommends that everyone in the United States born from 1945 through 1965 be tested for HCV, along with other populations at increased risk for HCV infection. If you think you may be at risk for HBV or HCV infection, ask your doctor about being tested. These infections do not always cause symptoms, but tests can show whether you have the virus. If so, your doctor may suggest treatment. Also, your doctor can tell you how to keep from infecting other people.
Human T-cell Leukemia/Lymphoma Virus Type 1 (HTLV-1). HTLV-1 can cause a type of leukemia and lymphoma.
Human Immunodeficiency Virus (HIV). HIV is the virus that causes AIDS. Infection with HIV weakens the immune system and makes the body less able to fight off other infectious agents that cause cancer. People infected with HIV have an increased risk of Kaposi sarcoma, lymphoma, and cancers of the cervix, liver, lung, and anus. HIV infection does not always cause symptoms. If you think you may be at risk for HIV, ask your doctor about being tested. If you test positive, your doctor may suggest treatment and can tell you how to keep from infecting other people.
Epstein-Barr Virus (EBV). Infection with EBV, a type of herpes virus, has been linked to an increased risk of lymphoma and cancers of the stomach and nasopharynx.
Human Herpesvirus 8 (HHV8). HHV8, also known as Kaposi sarcoma-associated herpes virus (KSHV), can cause Kaposi sarcoma.
Merkel Cell Polyomavirus (MCPyV). MCPyV can cause Merkel cell carcinoma, which is a rare type of skin cancer.
Helicobacter pylori (H. pylori). H. pylori is a type of bacteria that can cause stomach cancer and a type of lymphoma in the stomach lining. It can also cause stomach ulcers. If you have stomach problems, see a doctor. Infection with H. pylori can be found and treated. For more information, see the Helicobacter pylori and Cancer fact sheet.
Schistosoma hematobium. This flatworm (fluke), which is found in Africa and the Middle East, can cause bladder cancer.
Opisthorchis viverrini. This flatworm (fluke), which is found in Southeast Asia, can cause cholangiocarcinoma (cancer of the bile ducts in the liver).
Obesity and Type 2 Diabetes
People who are obese and/or have Type 2 diabetes may have an increased risk of several types of cancer, including cancers of the breast (in women who have been through menopause), colon, rectum, endometrium (lining of the uterus), esophagus, kidney, pancreas, and gallbladder.
Conversely, eating a healthy diet, being physically active, and keeping a healthy weight may help reduce risk of some cancers. These healthy behaviors are also important to lessen the risk of other illnesses, such as heart disease, type II diabetes, and high blood pressure.
Radiation of certain wavelengths, called ionizing radiation, has enough energy to damage DNA and cause cancer. Ionizing radiation includes radon, x-rays, gamma rays, and other forms of high-energy radiation. Lower-energy, non-ionizing forms of radiation, such as visible light and the energy from cell phones and magnetic fields, do not damage DNA and have not been found to cause cancer.
High-energy radiation, such as x-rays, gamma rays, alpha particles, beta particles, and neutrons, can damage DNA and cause cancer. These forms of radiation can be released in accidents at nuclear power plants and when atomic weapons are made, tested, or used.
Certain medical procedures, such as chest x-rays, computed tomography (CT) scans, positron emission tomography (PET) scans, and radiation therapy can also cause cell damage that leads to cancer. However, the risks of cancer from these medical procedures are very small, and the benefit from having them is almost always greater than the risks.
Talk with your doctor if you think you may be at risk for cancer because you were exposed to radiation. People considering CT scans should talk with their doctors about whether the procedure is necessary for them and about its risks and benefits. Cancer patients may want to talk with their doctors about how radiation treatment could increase their risk for a second cancer later on.
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.
People of all ages should limit the amount of time they spend in the sun, especially between mid-morning and late afternoon, and avoid other sources of UV radiation, such as tanning beds. It is important to keep in mind that UV radiation is reflected by sand, water, snow, and ice and can go through windshields and windows.
The best way to lessen UV damage when spending time in the sun is to wear long sleeves, long pants, a hat with a wide brim, and sunglasses with lenses that absorb UV radiation. Sunscreen with a sun protection factor (SPF) of at least 15 may help prevent skin cancer but does not work as well as staying out of the sun and wearing protective clothing.
Tobacco use is a leading cause of cancer and of death from cancer. People who use tobacco products or who are regularly around environmental tobacco smoke (also called secondhand smoke) have an increased risk of cancer because tobacco products and secondhand smoke have many chemicals that damage DNA.
Tobacco use causes many types of cancer, including cancer of the lung, larynx (voice box), mouth, esophagus, throat, bladder, kidney, liver, stomach, pancreas, colon and rectum, and cervix, as well as acute myeloid leukemia. People who use smokeless tobacco (snuff or chewing tobacco) have increased risks of cancers of the mouth, esophagus, and pancreas.
There is no safe level of tobacco use. People who use any type of tobacco product are strongly urged to quit. People who quit smoking, regardless of their age, have substantial gains in life expectancy compared with those who continue to smoke. Also, quitting smoking at the time of a cancer diagnosis reduces the risk of death.
New research from Cancer Research UK shows that 40% of cancers in women and 45% in men could be prevented by a healthier lifestyle.
The Cancer Research UK report found more than 100,000 cancers each year in the UK are caused by four lifestyle factors - smoking, unhealthy diets, alcohol and people being too fat.
This rises to around 134,000 cases a year when 14 lifestyle and environmental factors are taken into account.
The key facts are:
- Smoking accounts for 23% of all cancers in men and 15.6% in women.
- As well as lung cancer, it is implicated in other forms of the disease including bladder, kidney, pancreatic and cervical cancer
- One in 25 cancers is linked to a person's job, such as being exposed to chemicals or asbestos, while one in 33 is linked to infections, such as the human papillomavirus (HPV), which causes most cases of cervical cancer
- 34% of cancers in 2010 (106,845) were linked to smoking, diet, drinking alcohol and excess weight
- In men, 6.1% (9,600) of cancer cases were linked to a lack of fruit and vegetables, 4.9% (7,800) to occupation, 4.6% (7,300) to alcohol, 4.1% (6,500) to overweight and obesity and 3.5% (5,500) to excessive sun exposure and sunbeds
- In women, 6.9% (10,800) were linked to overweight and obesity, 3.7% (5,800) to infections such as HPV, 3.6% (5,600) to excessive sun exposure and sunbeds, 3.4% (5,300) to lack of fruit and vegetables and 3.3% (5,100) to alcohol