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Overview

Factors that increase the chance of developing cancer are called risk factors. There are 2 types of risk factors: Modifiable and Non-modifiable. Modifiable risk factors are attitudes or circumstances that people can avoid, for example smoking. Non-modifiable risk factors are conditions that can't be changed, for example genetic predisposition to certain diseases. Factors that decrease the chance of developing cancer are called protective factors.

Many other factors in our environment, diet, and lifestyle may cause or prevent cancer. This summary reviews only the major cancer risk factors and protective factors that can be controlled or changed to reduce the risk of cancer. Risk factors that are not described in the summary include certain sexual behaviors, the use of estrogen, and being exposed to certain substances or chemicals.

Factors That are Known to Increase the Risk of Cancer

Tobacco

• Tobacco use is strongly linked to an increased risk for many kinds of cancer. Cigarette smoking is the leading cause of the following types of cancer:

• Acute myelogenous leukemia (AML)
• Bladder cancer
• Esophageal cancer
• Kidney cancer
• Lung cancer
• Oral cancer
• Pancreatic cancer
• Stomach cancer

• Not smoking or quitting smoking lowers the risk of getting cancer and dying from cancer.
• Cigarette smoking causes about 30% of all cancer deaths in the United States.
• Cigarette smoking causes an estimated 443,000 deaths each year, including approximately 49,000 deaths due to exposure to secondhand smoke.
• Lung cancer is the leading cause of cancer death among both men and women in the United States, and 90% of lung cancer deaths among men and approximately 80% of lung cancer deaths among women are due to smoking.

Infections

Certain viral and bacterial infections are able to cause cancer. Infection as a cancer cause is more common in developing countries (about 1 in 4 cases of cancer) than in developed countries (less than 1 in 10 cases of cancer). Examples of infection caused cancers:

• Human papillomavirus (HPV):

• Cervical cancer
• Penis cancer
• Vaginal cancer
• Anal cancer
• Oropharyngeal cancer

• Hepatitis B and hepatitis C viruses increase the risk for liver cancer
• Epstein-Barr virus increases the risk for Burkitt lymphoma
• Helicobacter pylori increases the risk for gastric cancer

Two vaccines to prevent infection by cancer-causing agents have already been developed and approved by the U.S. Food and Drug Administration (FDA):

• Hepatitis B vaccine
• Human papillomavirus vaccine

Radiation

There are two main types of radiation linked with an increased risk for cancer:

• Ultraviolet radiation from sunlight: This is the main cause of nonmelanoma skin cancers.
• Ionizing radiation including:

• Medical radiation from tests such as x-rays, CT scans, fluoroscopy, and nuclear medicine scans.
• Radon gas found in homes.

• Radiation exposure from diagnostic X-rays increases the risk of cancer in patients and X-ray technicians.
• The growing use of CT scans over the last 20 years has increased exposure to ionizing radiation. The risk of cancer also increases with the number of CT scans a patient has and the radiation dose used each time.

Ionizing radiation has been associated with:

• Leukemia
• Thyroid cancer
• Breast cancer
• Myeloma
• Lung cancer
• Gastric cancer
• Colon cancer
• Esophageal cancer
• Bladder cancer
• Ovarian cancer

Immunosuppressive Medicines

Immunosuppressive medicines are linked to an increased risk of cancer because these medications lower the immune system's ability to stop cancer from forming.

Asbestos exposure

• Asbestos has been classified as a known human carcinogen by the U.S. Department of Health and Human Services, the EPA, and the International Agency for Research on Cancer.
• Studies have shown that exposure to asbestos may increase the risk of lung cancer and mesothelioma.
• Mesothelioma is the most common form of cancer associated with asbestos exposure.
• Though inconclusive, studies have shown an association between asbestos and other cancers:

• Gastric cancer
• Colorectal cancer
• Throat cancer
• Kidney cancer
• Esophageal cancer
• Gallbladder cancer

• Smokers who are also exposed to asbestos have a risk of developing lung cancer that is greater than the individual risks from asbestos and smoking added together.

Formaldehyde

• The International Agency for Research on Cancer (IARC) and the National Toxicology Program classify formaldehyde as a human carcinogen.
• Several National Cancer Institute (NCI) surveys suggested that professionals who are potentially exposed to formaldehyde in their work, such as anatomists and embalmers, have an increased risk of leukemia (particularly myeloid leukemia) and brain cancer compared with the general population.
• Several epidemiologic studies have shown the risk of formaldehyde exposure and cancer. This risk was associated with increasing peak, average levels and duration of exposure, but it was not associated with cumulative exposure.
• A 10-year follow-up study showed the risk of leukemia was highest earlier in the follow-up period. Risks declined steadily over time, such that the cumulative excess risk of myeloid leukemia was no longer statistically significant at the end of the follow-up period.
• Several case-control studies, as well as analysis of the large NCI industrial cohort, found an association between formaldehyde exposure and nasopharyngeal cancer.
• Data from extended follow-up of the NCI cohort found that the excess of nasopharyngeal cancer observed in the earlier report persisted.

Hair dye

• Studies have shown that professional hairdressers have an increased risk of bladder cancer that may be due to occupational exposure to hair dye. However, the information is inconclusive.
• A review of 14 studies of female breast cancer and hair dye published between 1977 and 2002 found that dye users had no increase in the risk of breast cancer compared with nonusers.
• Studies of the association between personal hair dye use and the risk of leukemia have had conflicting results.

Acrylamide

• The National Toxicology Program and the International Agency for Research on Cancer consider acrylamide to be a “probable human carcinogen,” based on studies in laboratory animals.
• One study showed that women with higher levels of acrylamide bound to the hemoglobin in their blood, had a statistically significant increase in risk of estrogen receptor-positive breast cancer.
• A questionnaire-based cohort study performed the Netherlands found an excess of endometrial and ovarian cancer associated with higher levels of acrylamide exposure

Factors That May Affect the Risk of Cancer

Diet

• Diet is being studied as a risk factor for cancer.
• Some studies show that fruits and non-starchy vegetables may protect against oral cancer, esophageal cancer, gastric cancer and lung cancer
• Results from studies on the effect of a diet high in fat, proteins, calories, and red meat as a risk factor for colorectal cancer are controversial. There are studies that have showed a clear relationship and studies that haven't.

• It is not known if a diet low in fat and high in fiber, fruits, and vegetables lowers the risk of colorectal cancer.

Alcohol

• Based on extensive reviews of research studies, there is a strong consensus about the association between alcohol drinking and several types of cancer.
• The National Toxicology Program of the US Department of Health and Human Services lists consumption of alcoholic beverages as a known human carcinogen.
• Evidence indicates that the more alcohol a person drinks -particularly the more alcohol a person drinks regularly over time- the higher his or her risk of developing an alcohol-associated cancer.
• Drinking alcohol is related with an increased risk of the following types of cancers:

• Head and neck cancer: Alcohol consumption is a major risk factor for certain head and neck cancers, particularly cancers of the oral cavity, pharynx, and larynx.

• People who consume 50 or more grams of alcohol per day (approximately 3.5 or more drinks per day) have at least a two to three times greater risk of developing these cancers than nondrinkers.
• The risks of these cancers are substantially higher among persons who consume this amount of alcohol and also use tobacco.

• Esophageal cancer: Alcohol consumption is a major risk factor for esophageal squamous cell carcinoma. People who inherit a deficiency or polymorphisms in ALDH2 (an enzyme that metabolizes alcohol) have been found to have substantially increased risks of alcohol-related esophageal squamous cell carcinoma.
• Breast cancer: More than 100 epidemiologic studies have looked at the association between alcohol consumption and the risk of breast cancer in women. ::* These studies have consistently found an increased risk of breast cancer associated with increasing alcohol intake. A meta-analysis of 53 of these studies (which included a total of 58,000 women with breast cancer) showed that women who drank more than 45 grams of alcohol per day (approximately three drinks) had 1.5 times more risk of developing breast cancer as nondrinkers.

• The risk of breast cancer was higher across all levels of alcohol intake: The Million Women Study in the United Kingdom showed an estimate of breast cancer risk at low to moderate levels of alcohol consumption: every 10 grams of alcohol consumed per day was associated with a 12 percent increase in the risk of breast cancer.

• Colorectal cancer: Alcohol consumption is associated with a modestly increased risk of cancers of the colon and rectum.

• A meta-analysis of 57 cohort and case-control studies that examined the association between alcohol consumption and colorectal cancer risk showed that people who regularly drank 50 grams or more of alcohol per day (approximately 3.5 drinks) had 1.5 times the risk of developing colorectal cancer as nondrinkers or occasional drinkers.
• For every 10 grams of alcohol consumed per day, there was a small 7% increase in the risk of colorectal cancer.

• Liver cancer: Alcohol consumption is an independent risk factor for and a primary cause of liver cancer.

• Multiple studies have shown that increased alcohol consumption is associated with a decreased risk renal cell carcinoma and Non-Hodgkin's Lymphoma (NHL). A meta-analysis of the NHL studies found a 15% lower risk of NHL among alcohol drinkers compared with nondrinkers. The mechanisms by which alcohol consumption would decrease the risks of either renal cell carcinoma or NHL are not understood.

Physical Activity

Studies have shown that physically active people have a lower risk of certain cancers than those who are not. It is not known if physical activity itself is the reason for this. Physical activity has been associated with a lower risk of the following cancers:

• Colorectal cancer
• Postmenopausal breast cancer
• Endometrial cancer

Obesity

• Studies show that obesity is linked to a higher risk of the following types of cancer:

• Postmenopausal breast cancer
• Colorectal cancer
• Endometrial cancer
• Esophageal cancer
• Kidney cancer
• Pancreatic cancer
• Some studies have shown obesity a risk factor for gallblader cancer

• It is not known if losing weight lowers the risk of cancer

Environmental Risk Factors

Being exposed to chemicals and other substances in the environment has been linked to some cancers:

• Air pollution as a cancer risk factor has been demonstrated. These includes:

• Association between lung cancer and secondhand smoke, outdoor air pollution, and asbestos
• Association between drinking water that contains a large amount of arsenic and skin cancer, bladder cancer, and lung cancer
• There are inconclusive results from studies performed to see if pesticides and other pollutants increase the risk of cancer. The results of those studies have been unclear because other factors can change the results of the studies.