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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:

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):

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):

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:

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:

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

Drinking alcohol is related with an increased risk of the following types of cancers:

Oral cancer
Esophageal cancer
Breast cancer
Colorectal cancer in men
Risk of liver cancer and colorectal cancer in women may also be increased

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:

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.