What You Need to Know. What You Can Do.
What substances in the environment are known to cause or are likely to cause cancer in humans? Where are they found?
Upon completion of this section, you will be able to
Every 2 years, scientists from a wide range of government agencies and educational institutions collaborate with scientists from the National Toxicology Program (NTP) in Research Triangle Park, NC, to publish the Report on Carcinogens. The report identifies substances either known to cause or suspected of causing cancer in humans and to which a significant number of people in the United States are exposed. It is the source for the agents listed in this training.
This training does not include all of the more than 200 agents listed in the Report on Carcinogens. The 50 or so discussed below are those for which there is a great deal of public interest.
Exposure to the carcinogens in tobacco products accounts for about one-third of all cancer deaths in the United States each year.
are all linked to increased cancer risks.
Cigarette, cigar, and pipe smoking have been associated with cancers of the
Smokeless tobacco has been linked to cancers of the mouth; and ETS has been implicated in lung cancer. Cigarette smoke contains more than 100 cancer-causing substances. The risk for cancers of the mouth, voice box, and esophagus is further increased among smokers who also drink more than two drinks per day.
|Diet, Weight, Physical Inactivity||
Because there are few definite relationships between food and cancer, the Report on Carcinogens does not refer to the cancer-related effects of specific foods. However, several studies show that heavy consumption of red and preserved meats, salt-preserved foods, and salt probably increase the risk of colorectal and stomach cancers. Evidence also suggests that a diet rich in fruits and vegetables may decrease the risks of esophageal, stomach, and colorectal cancers.
Being overweight or obese appears after tobacco as one of the most important modifiable causes of cancer. Large population studies show a consistent association between obesity and certain kinds of cancer. The strongest links are with breast cancer in older women, and cancers of the endometrium, kidney, colon, and esophagus.
Strong evidence links physical inactivity with increases in colon and breast cancer risk. The beneficial effect of exercise is greatest among very active people. Together, it is estimated that inactivity and obesity account for 25 to 30 percent of the cases of several major cancers—colon, breast (postmenopausal), endometrial, kidney, and cancer of the esophagus.
Heavy drinkers (more than two drinks per day) have an increased risk of cancer, particularly among those who also smoke. Cancers associated with heavy drinking include cancers of the mouth, throat, voice box, liver, and esophagus. Some evidence also links alcohol and breast cancer.
Ultraviolet (UV) radiation from the sun, sunlamps, or tanning beds causes premature aging of the skin and DNA damage that can lead to melanoma and other forms of skin cancer. The incidence of skin cancers is rapidly increasing.
Viruses and Bacteria
Infectious agents such as viruses and bacteria clearly contribute to the development of several types of cancer. A sexually transmitted virus called human papillomavirus (HPV) is the primary cause of cervical and anal cancer. Women who begin sexual intercourse at age 16 or younger or have many sexual partners have an increased risk of infection. Infection with HPV is increasingly common. Still, although HPV infection is the primary cause of cervical cancer, most infections do not result in cancer.
There is a new vaccine that prevents the types of genital human papillomavirus (HPV) that cause most cases of cervical cancer and genital warts. The vaccine is routinely recommended for 11 and 12 year old girls. It is also recommended for girls and women age 13 through 26 who have not yet been vaccinated or completed the vaccine series.
Hepatitis B (HBV) and hepatitis C (HCV) viral infections are major causes of liver cancer. In Asia and Africa, HBV is usually acquired in childhood and carries a high risk of liver cancer; HBV infection is, however, less common in the United States. Risk factors for HBV include occupational exposure to blood products, injection drug use, and high-risk sexual behavior (unprotected sex with multiple partners). A vaccine is available to prevent infection with HBV. The rising incidence of liver cancer in the United States is thought to be due to HCV. The strongest risk factor for HCV infection is injection drug use, but sexual transmission is also possible. People who received a blood transfusion before 1989 may also be infected with this virus. Currently, no vaccine is available for HCV.
Almost all adults are infected with Epstein-Barr virus (EBV), which is linked to some types of lymphoma. EBV is the virus that causes mononucleosis. Another type of virus, Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is linked to a particular type of sarcoma called Kaposi’s sarcoma. KSHV infection only occurs through close person-to-person contact. In Mediterranean and African countries, KSHV infection in childhood is common. In the United States, KSHV infection is most common in homosexual men. The risk of cancer for people infected with either KSHV or EBV is low, except for those whose immune systems are weakened, such as people infected with the human immunodeficiency virus (HIV), the virus that causes AIDS.
Infection with Helicobacter pylori, a bacterium, is widespread and is the primary cause of peptic ulcers and chronic gastritis (inflammation of the stomach). H. pylori contributes to the development of stomach cancer. Most H. pylori infections, however, result in neither symptoms nor cancer.
Ionizing radiation is invisible, high-frequency radiation that can damage the DNA or genes inside the body.
Everyone is exposed to very small doses of ionizing radiation from cosmic rays (rays that enter the earth’s atmosphere from outer space). Radiation from this source may account for a very small percentage (about 1 percent) of our total cancer risk.
Some homes have elevated levels of radon, a naturally occurring radioactive gas found at low levels in most soil. Radon is produced by the breakdown of uranium, which naturally releases low levels of ionizing radiation. Higher levels of radon can be found in certain types of rocky soil. The health effects of radon were first seen in the elevated levels of lung cancer found in underground uranium miners in the United States and around the world. Radon gas seeps into homes from the surrounding soil through cracks and other openings in the foundation. About 1 of 20 homes has elevated levels of radon. Although the cancer risks for radon exposure in the home are much lower than for radon-exposed miners, an estimated 20,000 lung cancer deaths every year are caused by radon exposure in homes. Various strategies are available for reducing residential radon exposure.
Another source of ionizing radiation is the radioactive substances released by exploding nuclear weapons known as “fallout.” The doses of ionizing radiation received by the atomic bomb survivors in Japan resulted in increased risks of leukemia and cancers of the breast, thyroid, lung, stomach, and other organs. Radioactive substances were also released in the aboveground atomic bomb testing conducted by the U.S. government in the late 1950s and early 1960s in Nevada. People exposed, especially as children, to one radioactive iodine isotope, Iodine-131 or I-131, which collects in the thyroid gland, may have an increased risk of thyroid disease, including thyroid cancer. For more information visit: cancer.gov/i131.
People are also exposed to ionizing radiation during certain medical procedures. Some patients who receive radiation to treat cancer or other conditions may be at increased cancer risk. For example, persons treated with radiation in childhood to treat acne, ringworm, and other head and neck conditions have been shown to be at increased risk for thyroid cancer and other tumors of the head and neck. X-rays used to diagnose or screen for a disease are also forms of ionizing radiation. The dose of radiation from procedures used to diagnose or screen for a disease is much lower than the dose to treat a disease. Most studies on the long-term effects of exposure to radiation used to diagnose or screen for cancers or other diseases have not shown an elevated cancer risk, but a small risk associated with this exposure is possible. That said, children whose mothers received diagnostic X-rays during pregnancy were found to have increased risks of childhood leukemia and other types of cancer. This finding led to the current ban on diagnostic X-rays in pregnant women. Several other studies of women who received small weekly X-ray doses to the chest over extended periods to monitor treatment for tuberculosis showed a radiation-related increased risk of breast cancer.
Of the nearly 900 active ingredients in registered pesticides in the United States, about 20 have been found to be carcinogenic in animals, although not all have been tested. In the United States, a number of pesticides have been banned or their use has been restricted. These include ethylene oxide, amitrole, some chlorophenoxy herbicides, DDT, dimethylhydrazine, hexachlorobenzene, hexamethylphosphoramide, chlordecone, lead acetate, lindane, mirex, nitrofen, and toxaphene. Studies of people with high exposures to pesticides, such as farmers, pesticide applicators, crop duster pilots, and manufacturers, have found high rates of
So far, human studies have not enabled researchers to sort out exactly which pesticides are linked to which cancers. Therefore, most of these pesticides are still listed in the Report on Carcinogens as likely to be cancer-causing, rather than as known carcinogens. For more information, visit: www.aghealth.org.
Some drugs used to treat cancer (e.g., cyclophosphamide, chlorambucil, melphalan) have been shown to increase the occurrence of second cancers, including leukemia. Others used as immunosuppressants, such as cyclosporin and azathioprine for patients having organ transplants, are also associated with increased cancer risks, especially lymphoma. But the Food and Drug Administration has determined that the life-saving benefits of these drugs outweigh the additional cancer risks that might arise years later. Nevertheless, people should consult a physician or other health care specialist and weigh the risks and benefits concerning the use of a drug.
Some medicines have been linked to reduced risk of cancer. For example, some studies find a reduced risk of colon cancer in persons who regularly take aspirin or other nonsteroidal, anti-inflammatory medicines. Evidence for protection of other cancers such as breast cancer or prostate cancer is inconsistent.
Estrogens used to treat symptoms of menopause and other gynecological conditions have been shown to increase the incidence of endometrial cancer. In addition, some studies have shown an increased risk of breast cancer with estrogen use, but a reduced risk of colon cancer. Progesterone is another hormone now taken in combination with estrogen for hormone replacement therapy in older women. It helps to protect against the increased endometrial cancer risk with estrogen alone. Nevertheless, increased risks of breast cancer, heart disease, stroke, and blood clots have recently been shown to be associated with the use of estrogen plus progestin, a synthetic form of progesterone. Long-term users of combination oral contraceptives have substantially reduced risks of endometrial and ovarian cancers, but may experience increases in early-onset breast cancers and liver cancer. The amount of estrogen and progesterone in oral contraceptives is substantially less than in previous years, which means that the risk of the current formulations is likely to be less than those used in the past.
Tamoxifen use is associated with increased risks of endometrial cancer as well as increased risks of stroke and blood clots. Tamoxifen is a synthetic hormone used to prevent the recurrence of breast cancer after breast cancer surgery. It is also used to prevent breast cancer in women who, because of family history or other factors, are at high risk for the disease. Again, people should consult a physician or other health care specialist and weigh the risks and benefits concerning the use of a drug.
Diethylstilbestrol (DES) is a synthetic form of estrogen prescribed to pregnant women from the early 1940s to 1971. Their daughters who were exposed to DES before birth were determined to have an increased chance of developing a rare type of cervical and vaginal cancer. In addition, women who took DES during pregnancy may have a slightly higher risk for developing breast cancer. Because of these findings, DES is no longer prescribed, and it has been banned as a cattle feed additive.
Several solvents used in paint thinners, in paint and grease removers, and in the dry cleaning industry are, based on in animal studies, known or suspected of causing cancer. These include benzene, carbon tetrachloride, chloroform, dichloromethane (methylene chloride), tetrachloroethylene, and trichloroethylene. Except for benzene (a known carcinogen), human studies are suggestive, but not conclusive. Therefore, with the exception of benzene, these substances are listed as likely to cause cancer in humans.
Benzene, however, is known to cause leukemia in humans. Its use is widespread as a solvent in the chemical and drug industries and as a gasoline component. After 1997, it was banned as an ingredient in pesticides. Workers employed in the petrochemical industry, pharmaceutical industry, leather industry, rubber industry, gas stations, and in the transportation industry are exposed to benzene. Inhaling contaminated air is the primary method of exposure. Because benzene is present in gasoline, air contamination occurs from vapors around gas stations and from automobile exhaust in congested areas. It is also present in cigarette smoke. Half of the estimated exposure to benzene in the United States is from cigarette smoking. About half of the U.S. population is exposed to benzene from industrial sources, and virtually everyone in the country is exposed to benzene in gasoline.
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