Cancer Risk Factors and Causes

Cells may become cancerous due to the accumulation of defects, or mutations, in their DNA. Certain inherited genetic defects (for example, BRCA1 and BRCA2 mutations) and infections can increase the risk of cancer.

It is usually not possible to know exactly why one person develops cancer and another doesn't. But research has shown certain risk factors may increase a person's chances of developing cancer. (There are also factors linked to a lower risk of cancer, sometimes called protective risk factors or just protective factors.)

Cancer risk factors include exposure to chemicals or other substances, as well as certain behaviors. They also include things people cannot control, such as age and family history. A family history of certain cancers can be a sign of a possible inherited cancer syndrome.

Most cancer risk (and protective) 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 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 exposure to avoidable risk factors may lower your risk of developing certain cancers.

1. Age
2. Alcohol
3. Cancer-Causing Substances
4. Chronic Inflammation
5. Diet
6. Hormones
7. Immunosuppression
8. Infectious Agents
9. Obesity
10. Radiation
11. Sunlight
12. Tobacco

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 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 62 years for breast cancer, 67 years for colorectal cancer, 71 years for lung cancer, and 66 years for prostate cancer.

But cancer can occur at any age. For example, bone cancer is most frequently diagnosed among people under age 20, with more than one-quarter of cases occurring in this age group. And 4.8 percent of leukemia and lymphoma cases were 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 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 does not mean 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.

• Aflatoxins
• Aristolochic Acids
• Arsenic
• Asbestos
• Benzene
• Benzidine
• Beryllium
• 1,3-Butadiene
• Cadmium
• Coal Tar and Coal-Tar Pitch
• Coke-Oven Emissions
• Crystalline Silica (respirable size)
• Erionite
• Ethylene Oxide
• Formaldehyde
• Hexavalent Chromium Compounds
• Indoor Emissions from the Household Combustion of Coal
• Mineral Oils: Untreated and Mildly Treated
• Nickel Compounds
• Radon
• Secondhand Tobacco Smoke (Environmental Tobacco Smoke)
• Soot
• Strong Inorganic Acid Mists Containing Sulfuric Acid
• Thorium
• Vinyl Chloride
• Wood Dust

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 (IBD), 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 (NSAIDs), reduce the risk of cancer but, 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:

• Acrylamide: Acrylamide is a chemical found in tobacco smoke and some foods. It can be produced when certain vegetables, such as potatoes, are heated to high temperatures. Studies in animal models have found that acrylamide exposure increases the risk for several types of cancer but, there is no consistent evidence dietary acrylamide exposure is associated with the risk of any type of cancer in humans.
• 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: 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 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 the artificial sweeteners available commercially in the United States are associated with cancer risk in humans.
• Calcium: 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.
• Cruciferous vegetables: 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: Fluoride in water helps to prevent and can even reverse tooth decay. Many studies, in both humans and animals, have 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.
• Processed meat: Processed meats such as hot dogs, sausage, bacon, deli meats, canned meats, and chicken nuggets have been classified by the World Health Organization (WHO) as a Group 1 human carcinogen, a substance known to cause cancer.
• Tea: 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: 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) may slightly increase a woman's risk of breast cancer. A meta-analysis of published findings on breast cancer risk associated with menopausal hormone therapy are inconsistent, and there is limited information on long-term effects. 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 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 human immunodeficiency virus (HIV) also weakens the immune system and increases the risk of certain cancers.

Research has shown 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 or increase the risk that cancer will form. Some viruses can disrupt signaling that normally keeps cell growth and proliferation in check. Also, some infections weaken the immune system, making the body less able to fight off other cancer-causing infections. And 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. As described below, you can lower your risk of infection by getting vaccinated, not having unprotected sex, and not sharing needles.

Epstein-Barr virus (EBV)

EBV, a type of herpes virus, causes mononucleosis as well as certain types of lymphoma and cancers of the nose and throat. EBV is most commonly transmitted by contact with saliva, such as through kissing or by sharing toothbrushes or drinking glasses. It can also be spread by sexual contact, blood transfusions, and organ transplantation. EBV infection is lifelong. More than 90% of people worldwide will be infected with EBV during their lifetime, and most do not develop any symptoms. There is no vaccine to prevent EBV infection and no specific treatment for EBV infection.

Hepatitis B virus and hepatitis C virus (HBV and HCV)

Chronic infections with HBV or HCV can cause liver cancer. Both viruses can be transmitted via blood (for example, by sharing needles or through blood transfusions) and from mother to baby at birth. In addition, HBV can be transmitted via sexual contact.

Since the 1980s, infants in the United States and most other countries have been routinely vaccinated against HBV infection. Experts recommend that adults who have not been vaccinated against HBV and are at increased risk of HBV infection get vaccinated as soon as possible. Vaccination is especially important for health care workers and other professionals who come into contact with human blood.

The U.S. Centers for Disease Control and Prevention (CDC) also recommends everyone in the United States born from 1945 through 1965, and other populations at increased risk for HCV infection, be tested for HCV. Although there is not currently a vaccine against HCV, new therapies can cure people of 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 immunodeficiency virus (HIV)

HIV is the virus that causes acquired immunodeficiency syndrome (AIDS). HIV does not cause cancer itself, but infection with HIV weakens the immune system and makes the body less able to fight off other infections that cause cancer. People infected with HIV have increased risks of a number of cancers, especially Kaposi sarcoma, lymphomas (including both non-Hodgkin lymphoma and Hodgkin disease), and cancers of the cervix, anus, lung, liver, and throat.

HIV can be transmitted via blood and through sexual contact. Men who have unprotected sex with other men and people who share needles for injection drug use are at the highest risk of acquiring HIV infection; heterosexual individuals who have unprotected sex with multiple partners are at the next highest risk.

People can be infected with HIV for years before they begin to develop symptoms. If you think you may be at risk for HIV infection, ask your doctor about being tested. If you test positive, your doctor can prescribe highly effective antiviral treatment and can tell you how to keep from infecting other people.

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. High-risk HPVs spread easily through direct sexual contact, including vaginal, oral, and anal sex. Several vaccines have been developed that prevent infection with the types of HPV that cause most HPV-associated cancers. In the United States, experts recommend that children be vaccinated at age 11 or 12, but children as young as age 9 and adults as old as 26 can also be vaccinated.

Cervical cancer screening can be used to detect signs of HPV infections in the cervix. Although HPV infections themselves cannot be treated, the cervical abnormalities that these infections can cause over time can be treated.

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1)

HTLV-1 can cause an aggressive type of non-Hodgkin lymphoma called adult T-cell leukemia/lymphoma (ATLL). This virus spreads via blood (by sharing needles or through transfusions), through sexual contact, and from mother to child in the womb or via breastfeeding. Infection with this virus is more common in Japan, Africa, the Caribbean, and South America than in the United States. Most people with HTLV-1 infection do not have any symptoms or develop disease.

Blood is routinely screened for HTLV-1 in the United States. There is no vaccine to prevent infection with this virus and no treatment if you are infected. If you think you may be at risk for HTLV-1 infection, ask your doctor about being tested. If you test positive, your doctor can tell you how to keep from infecting other people and monitor you for HTLV-1-induced disease.

Kaposi sarcoma-associated herpesvirus (KSHV)

Kaposi sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), can cause Kaposi sarcoma. KSHV can also cause primary effusion lymphoma and multicentric Castleman disease.

KSHV is most commonly spread through saliva. It can also be spread through organ or bone marrow transplantation, and there is some evidence that it can be spread by blood transfusion, although this risk is minimized by practices followed in the United States such as blood storage and removal of white cells.

KSHV infection is generally limited to certain populations, and the way KSHV is spread varies among these populations. In sub-Saharan Africa and certain regions of Central and South America, where KSHV infection is relatively common, it is believed to spread by contact with saliva among family members. In Mediterranean countries (Italy, Greece, Israel, Saudi Arabia), where KSHV infection is present at intermediate levels, it is thought to spread by contact among children and by ill-defined routes among adults. Finally, in regions where KSHV infection is uncommon, such as the United States and Northern Europe, it appears to be mostly transmitted sexually, especially among men who have sex with men.

Most people infected with KSHV do not develop cancer or show any symptoms, although those who also have HIV infection or are immunosuppressed for other reasons are more likely to develop KSHV-caused diseases. There is no vaccine to prevent KSHV infection and no therapy to treat infection. Men who have sex with men may be advised to avoid oral-anal contact (including use of saliva as a personal lubricant). And people infected with HIV can lower their risk of KSHV-related complications by using antiretroviral therapy.

Merkel cell polyomavirus (MCPyV)

MCPyV can cause Merkel cell carcinoma, a rare type of skin cancer. Most adults are infected with MCPyV, with transmission most likely occurring through casual direct (i.e., skin-to-skin) or indirect (i.e., touching a surface that an infected person has touched) contact in early childhood. The risk of Merkel cell carcinoma is greatly increased in elderly people and in younger adults with who are infected with HIV or are immunosuppressed for other reasons. Infection does not generally cause symptoms, and there are no treatments for MCPyV.

Helicobacter pylori (H. pylori)

H. pylori is a type of bacterium that can cause noncardia gastric cancer (a type of stomach cancer) and a type of lymphoma in the stomach lining, gastric MALT lymphoma. It can also cause stomach ulcers. The bacterium is thought to spread through consumption of contaminated food or water and direct mouth-to-mouth contact. The CDC estimates that approximately two-thirds of the world's population harbors H. pylori, with infection rates much higher in developing countries than in developed nations. In most populations, the bacterium is first acquired during childhood.

If you have stomach problems, see a doctor. Infection with H. pylori can be detected and treated with antibiotics.

Opisthorchis viverrini

This parasitic flatworm (fluke), which is found in Southeast Asia, can cause cholangiocarcinoma (cancer of the bile ducts in the liver). People become infected when they eat raw or undercooked freshwater fish that contain the larvae. Antiparasitic drugs are used to treat the infection.

Schistosoma hematobium

This parasitic flatworm (fluke), which lives in certain types of freshwater snails found in Africa and the Middle East, can cause bladder cancer. People become infected when infectious free-swimming flatworm larvae burrow into skin that has come into contact with contaminated fresh water. Antiparasitic drugs are used to treat the infection.

People who are obese 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 electromagnetic fields, do not damage DNA and have not been found to cause cancer.

Radon

Radon is a radioactive gas given off by rocks and soil. Radon is formed when the radioactive element radium breaks down. Radium in turn is formed when the radioactive elements uranium and thorium break down. People who are exposed to high levels of radon have an increased risk of lung cancer.

If you live in an area of the country that has high levels of radon in its rocks and soil, you may wish to test your home for this gas. Home radon tests are easy to use and do not cost much. Most hardware stores sell test kits. There are many ways to lessen the amount of radon in a home to a safe level.

X-rays and other sources of radiation

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 and skin tones 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. Even though skin cancer is more common among people with a light skin tone, people of all skin tones can develop skin cancer, including those with dark skin.

Follow these tips to protect your skin from sunlight:

• Wear a hat with a wide brim all around that shades your face, neck, and ears. Baseball caps and some sun visors protect only parts of your skin.
• Wear sunglasses that block UV radiation to protect the skin around your eyes.
• Wear long sleeves and long pants. Tightly woven, dark fabrics are best. Some fabrics are rated with an ultraviolet protection factor (UPF). The higher the rating, the greater the protection from sunlight.
• Use sunscreen products with a sun protection factor (SPF) of at least 15. (Some doctors suggest using a product with an SPF of at least 30.) Apply the product's recommended amount to uncovered skin 30 minutes before going outside, and apply again every two hours or after swimming or sweating.

Keep in mind that the sun's rays...

• Are strongest between 10:00 a.m. and 4:00 p.m.
• Can pass through light clothing, windshields, windows, and clouds
• Are reflected by sand, water, snow, ice, and pavement

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.