Anything that may cause a normal body cell to develop abnormally potentially can cause cancer. Many things can cause cell abnormalities and have been linked to cancer development. Some cancer causes remain unknown while other cancers may develop from more than one known cause. Some may be developmentally influenced by a person's genetic makeup. Many patients develop cancer due to a combination of these factors. Although it is often difficult or impossible to determine the initiating event(s) that cause a cancer to develop in a specific person, research has provided clinicians with a number of likely causes that alone or in concert with other causes, are the likely candidates for initiating cancer. The following is a listing of major causes and is not all-inclusive as specific causes are routinely added as research advances:
Chemical or toxic compound exposures: Benzene, asbestos, nickel, cadmium, vinyl chloride, benzidine, N-nitrosamines, tobacco or cigarette smoke (contains at least 66 known potential carcinogenic chemicals and toxins), and aflatoxin
Ionizing radiation: Uranium, radon, ultraviolet rays from sunlight, radiation from alpha, beta, gamma, and X-ray-emitting sources
Pathogens: Human papillomavirus (HPV), EBV or Epstein-Barr virus, hepatitis viruses B and C, Kaposi's sarcoma-associated herpesvirus (KSHV), Merkel cell polyomavirus, Schistosoma spp., and Helicobacter pylori; other bacteria are being researched as possible agents
Genetics: A number of specific cancers have been linked to human genes and are as follows: breast, ovarian, colorectal, prostate, skin and melanoma; the specific genes and other details are beyond the scope of this general article so the reader is referred to http://www.cancer.gov/cancertopics/genetics for more details about genetics and cancer.
It is important to point out that most everyone is exposed to cancer-causing substances (for example, sunlight, cigarette smoke, and X-rays) during their lifetime but many individuals do not develop cancer. In addition, many people have the genes that are linked to cancer but do not develop it. Why? Although researchers may not be able give a satisfactory answer for every individual, it is clear that the higher the amount or level of cancer-causing materials a person is exposed to, the higher the chance the person will develop cancer. In addition, the people with genetic links to cancer may not develop it for similar reasons (lack of enough stimulus to make the genes function). In addition, some people may have a heightened immune response that controls or eliminates cells that are or potentially may become cancer cells. There is evidence that even certain dietary lifestyles may play a significant role in conjunction with the immune system to allow or prevent cancer cell survival. For these reasons, it is difficult to assign a specific cause of cancer to many individuals.
Proving that a substance does not cause or is not related to increased cancer risk is difficult. For example, antiperspirants are considered to possibly be related to breast cancer by some investigators and not by others. The official stance by the NCI is "additional research is needed to investigate this relationship and other factors that may be involved." This unsatisfying conclusion is presented because the data collected so far is contradictive. Other claims that are similar require intense and expensive research which may never be done. Reasonable advice might be to avoid large amounts of any compounds even remotely linked to cancer, although it may be difficult to do in complex, technologically advanced modern societies.