Cancer is a mysterious group of diseases which often arouses fearful images of death. There are over one hundred different types of cancer, which can be distinguished by the type of cell or organ which is affected, the treatment plan employed, and the cause of the cancer. Any substance or agent that can cause cancer is called a carcinogen. One of the first people to suspect that certain substances in the environment can cause cancer was Sir Percivall Pott (1714-1788), who in 1775 published a paper on cancer occurrence in chimney sweeps. It has since been discovered that benzo(a)pyrene, a chemical foundin soot, is a potent carcinogen. Ionizing radiation was first suspected of being carcinogenic around the turn of the twentieth century, when physicians who developed the use of X-rays and radium in medicine showed a high incidenceof skin cancer. Later, in 1915, scientists in Japan documented carcinogens when they noticed that rabbits developed tumors when tar was applied to the inside of their ears. By 1930, Ernest Kennaway isolated polycyclic hydrocarbonsand proved that they were the carcinogenic agent.

Today, the media rarely misses an opportunity to report on newly discovered carcinogenic substances. Sometimes it seems as if everything causes cancer, but very few things are proven carcinogens. The two main categories ofcarcinogens are genetic and environmental. Specific environmental factors include tobacco, alcohol, diet, infection, sexual practices, occupation, geophysical phenomena, pollution, medications, food additives, and industrial products. Tobacco and diet together account for almost two-thirds of all cancer-related deaths. Stress and emotional factors should also be listed as elements that may contribute toward the development of some cancers.

Cigarette smoking is clearly the single most preventable cause of illness and premature death in the United States. The United States Surgeon General estimates that 30% of all cancer deaths are directly attributable to tobacco use. The carcinogens in tobacco include nicotine, tar, carbon monoxide, ammonia, formaldehyde, phenols, creosote, anthracene, pyrene, hydrocyanic acid,arsenic, and lead. Tar is the particulate matter derived from burning organic compounds and is the leading cancer-causing chemical in tobacco smoke. Researchers have also identified 4-(N- nitrosomethylamino)-1-(3-pyridyl)-1butanone (NNK), as a carcinogen that is formed in the production, curing, and agingof tobacco. Studies in animals have shown that it can cause benign and malignant tumors, as well as other forms of cancer. Pipe and cigar smoke contain essentially the same array of poisons although the relative amounts may vary. In 1997, scientists produced the first chemical evidence that an increased risk for lung cancer is associated with passive, second-hand, smoke inhalation. The investigators found the metabolite of a tobacco carcinogen in theurine of non-smokers who have been exposed to second-hand smoke. Smokeless tobacco, including chewing tobacco and snuff, are not alternatives to avoid cancer. These products contain large quantities of cancer-causing chemicals called n-nitrosamines. Lip, mouth, tongue, and throat cancers have been positively linked to the use of smokeless tobacco products. Furthermore, the risk of these types of cancers are increased when tobacco is used with alcohol. This phenomenon is known as synergism. While marijuana does not contain nicotine ortobacco, it does contain tar and other carcinogens.

Diet and nutrition have been recognized as major factors that influence the development of many cancers. The National Cancer Institute recommends a low fat, high fiber diet with adequate allowances of vitamin A and vitamin C as a specific way to reduce the risk of developing colon cancer. Cruciferous vegetables like cabbage and brussels sprouts may also help reduce the chances of developing some stomach and colon cancers. Charred foods and alcohol contain carcinogens and should be avoided or consumed in moderation.

Infection may also lead to cancer. This usually occurs when a virus, bacteria, or parasite is contracted. Only about 10 percent of all cancers are believed to be caused by these organisms. Retroviruses such as the herpes virus andthe virus that causes AIDS have been implicated in certain types of cancer. According to the oncogene theory of virus-mediated cancer, when a virus infects a cell, it may enter the genes of the host cell that control growth and division. The infected cell may begin to divide uncontrollably, resulting in theformation of a tumor. Changes in a single oncogene rarely lead to malignantcancer. Rather, it usually requires many of the more than 100 oncogenes working in concert to initiate this chain of events. Parasitic and bacterial-induced cancer may be due to the chronic irritation, either internal or external,caused by these organisms over long periods of time. It also appears that other co-factors need to be present before many of these cancers can fully develop.

Changes in the body resulting from sexual intercourse, pregnancy, andchildbirth are obviously in a different class of carcinogens than those produced by exposure to chemicals. They are, however, considered environmental since they are not controlled solely by one's own genes. This is not to suggest that childbirth causes cancer. In fact, pregnancy and childbirth may actually help prevent cancer of the uterus, ovary, and breast. However, frequent sexual intercourse with large numbers of partners has been positively linked to an increased risk of cervical cancer. Researchers think that theprimary carcinogenic agent in this example may actually be an unknown virus.

The percentage of all cancers that can be attributed to work-related influences varies from about 4 percent to as high as 15 percent. Since 1971, the International Agency for Research on Cancer (IARC) has been categorizing carcinogens and occupations associated with high cancer rates. Some chemicals used inshoe, tire, and furniture manufacturing, as well as nickel refining, dieselfuel, and dry cleaning have been identified as "anticipated" carcinogens. Arsenic, asbestos, benzene, benzidine, chromium, 2-Naphthylene, oils, and vinylchloride show occupational exposures causally associated with cancer in humans. Cadmium, DDT, and formaldehyde are other common chemicals that are suspected to cause cancer in humans.

Asbestos is perhaps the most familiar carcinogen in this category. This material, made of small silicate fibers, has been used in construction since the 1800s. Asbestos only becomes a cancer risk when the fibers are set free and inhaled during decomposition or renovation. The fibers irritate the body's alveoli and can lead to lung cancer. Asbestos exposure is now carefully regulatedby the Occupational Safety and Health Administration (OSHA).

One of the most infamous carcinogens is PCB or polycholrinated biphenyls. These man-made chemicals encompass 209 individual compounds with various toxicities. PCBs were once widely used as coolants and lubricants before their manufacture was halted in October 1977 due to their accumulations in the environment and resultant health hazards, including their role as carcinogens that cancause cancer. However, PCBs are still widespread throughout the environment,including contaminated water, sediment, and fish. Another group of contaminants that exist worldwide are polychlorinated dibenzo-para-dioxins, or simplydioxins. Dioxins are typically byproducts of chemical reactions used for other purposes, like the production of some herbicides. A working group for IARChas determined that one dioxin, 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD)is carcinogenic to humans, increasing risk of lung cancer and all cancers combined in those exposed to it. IARC continues to investigate the other dioxins. Benzene, a clear colorless liquid used widely as a solvent and reactant, has also been linked to leukemia, which is a type of blood cancer. Radiation refers to energy that is sent through space. It may be in the form of waves such as ultraviolet light, X-rays, and microwaves, or in the form of charged atomic particles including electrons, protons, and neutrons. Radiation can alter genes within chromosomes. Skin cancer in early X-ray workersand radium workers, bone sarcoma in luminous dial painters, and lung cancers in miners of radioactive ores have all been studied. Ironically, someforms of radiation are used to diagnose and treat cancer.

Radiation from the sun is an increasing concern and may be linked to a depletion of the protective ozone layer in the Earth's upper atmosphere. The sun isthe chief cause of nonmelanoma skin cancer. The amount of ultraviolet-beta radiation from the sun varies with location, altitude, sky cover, and the timeof year. Exposure levels can be reduced by using sun screen products and monitoring prolonged outdoor activities--especially between 11:00 a.m. and 1:00p.m. during the late summer months.

In recent years, the approval of irradiation of food has raised growing concerns about its possible association with cancer. Food irradiation uses ionizing energy to kill bacteria, mold and insects and can also prolong the food's shelf life. The major concern is that the process forms free radicals, or unidentified radiolytic products, which have been associated with the developmentof cancer. Those against irradiated foods argue that they contain high levels of carcinogens, including nitrosamines and formaldehyde. However, more than39 countries, including the United States. approve the controlled use of irradiation for one or more food items, including meats, poultry, seafood, grains, vegetables, nuts, and spices.

Historically, substances have been tested for carcinogenicity by using them on human or animal test subjects. In the late 1970s, however, Bruce Ames of the University of California at Berkeley developed a carcinogenicity test thatuses bacteria instead of human or animal subjects. The Ames test yields results in hours or days instead of the years required for human or animal tests,and it has a high rate of consistency with human tests. While this test is not accurate enough to give permanent proof of a substance's carcinogenicity, it does allow for speedy screening of new or previously unsuspected substances. The most difficult aspect of testing substances as potential carcinogens isdetermining what doses are actually harmful. For example, the Ames test on animals is performed with maximum tolerated doses (MTD), and some scientists question whether testing with MTD accurately reflet a substance's potential carcinogenic effect on humans.

Many environmental carcinogens are avoidable, and early diagnosis and treatment increases the recovery rate for all cancers. Genetically predisposed cancers are more difficult to regulate. They are also less common than many environmentally induced cancers. A growing area of interest are investigations of carcinogens like NNK and their ability to alter genes and cause cancer.

Research conducted by the World Health Organization (WHO) has shown that industrialized countries have high cancer rates compared with countries that have little industry. Despite the fact that industrialized countries makeup only one-fifth of the world's population, one-half of all the world's cancers are found in people living in these countries. While many people fear theoverall increase of chemical and physical carcinogens in the environment, itis especially important to prioritize risk factors of the highest magnitude.For example, smokers should be less concerned about food additives and moreconcerned about when they will stop smoking.

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