Environmental Factors in Cancer Development
Environment as a cause of cancer is a complex and often misunderstood topic. The term environment has several different meanings when referring to causation of cancer. Originally, the term "environmental cause of cancer" was used to refer to all cancers that were not caused by hereditary or inherited factors. This definition included all cancers caused by lifestyle practices such as diet and tobacco use, viruses, and many other causes. For purposes of clarification, the term "environment" has been further defined by adding the labels "personal environment" and "external environment" when referring to causes of cancer.
By this definition, causes of cancer related to an individual's personal environment includes lifestyle choices such as diet, use of tobacco, and other factors which may place the individual at high risk for the development of cancer. External environmental causes of cancer refer to factors in the environment such as environmental pollutants that increase risk for cancer. Up to 85%
Epidemiologists are scientists who research and identify factors which are common to cancer patients' histories and lifestyles, and then evaluate those factors within the context of current biological and disease causation theories. Eventually, evidence may persuade epidemiologists to conclude that one or more factors or characteristics shared by a study group "caused" a disease such as a type of cancer. The science of epidemiology enables researchers to determine causes of diseases such as specific types of cancer, and also to estimate or project numbers of deaths that can be attributed to the cause on an annual basis.
Personal environment/Lifestyle choices
Tobacco is known as the one of the most potent carcinogens in humans. Tobacco causes more than 148, 000 deaths each year in the form of various cancers such as lung, trachea, bronchus, larynx, pharynx, oral cavity, and esophagus. Other cancers linked to tobacco use include cancers of the pancreas, kidney, bladder, and cervix. Cigarette smoking is more common among men; however, because of the increase in the number of women who smoke, more women die from lung cancer each year than from breast cancer. The life span of an individual who smokes is shortened by an average of 12 years.
Diet and nutrition
According to the American Cancer Society, the single most important dietary intervention to lower risk for cancer is eating five or more servings of fruits and vegetables daily. Consuming a diet rich in plant sources provides phytochemicals that are non-nutritive substances in plants that possess health protective benefits. A diet rich in foods from plant sources reduces the risk for development of cancers of the gastrointestinal tract, respiratory tract, and colon. Increased consumption of fruits and vegetables is also associated with decreased risk for lung cancer.
Diets high in fat have been associated with increased risk for colon, rectal, prostate, and endometrial cancers. The association between high-fat diets and the development of breast cancer is much weaker. Specific recommendations are to replace high-fat foods with fruits and vegetables, eat smaller portions of high fat foods, and limit consumption of meats—especially those that are considered high fat. Foods from animal sources remain a staple in American diets. Consumption of meat, especially red meats such as beef, pork, and lamb, have been associated with increased risk of colon and prostate cancer.
Obesity is often the result of meat-based, high-fat diets. Obesity has been linked to cancers at several sites including colon and rectum, prostate, kidney, endometrium, as well as breast cancer in postmenopausal women.
Drinking alcohol increases the risk of developing cancers of the mouth, esophagus, pharynx, larynx, and liver in men and women; and increases the risk of breast cancer in women. Cancer risk increases as the amount of alcohol consumed increases. An individual who both smokes and drinks alcohol greatly increases the risk of developing cancer when compared to either smoking or drinking alone.
Current recommendations related to physical exercise include engaging in moderate levels of activity for at least 30 minutes on most days of the week. Studies have revealed an association between physical activity and a reduced risk of the development of certain types of cancers, including colon, breast, and prostate cancer.
Only high frequency radiation such as ultraviolet (UV) radiation and ionizing radiation (IR) has been proven to cause cancer in humans. A source of UV radiation is sunlight. Prolonged exposure to UV radiation is the major cause of basal and squamous cell skin cancers. UV radiation is also a major cause of melanoma.
IR has cancer-causing capability, as proven by studies on atomic bomb survivors and other groups. Virtually any part of the body can by affected by IR, but the areas most affected are the bone marrow and the thyroid gland. IR is released in very low levels from diagnostic equipment such as medical and dental X-ray equipment. Much higher levels of IR are released from machines delivering radiation therapy to patients. Great precautions are taken not to expose patients or staff unnecessarily to the effects of IR.
Exposure to radon, a form of IR, can increase risk for lung cancer, especially among smokers. Radon is a naturally occurring radioactive gas formed by the decay of uranium in rocks and soil. The gas is odorless, colorless, tasteless and cannot be detected by sight. Radon seeps up
Reproductive and gynecologic factors
Lifestyle choices linked to breast cancer include diet, alcohol consumption, oral contraceptives, estrogen replacement therapy, postmenopausal obesity, and nulli-parity (a woman who has never had a child). The relationship between dietary fats and breast cancer continues to be studied. Women who consume more than two alcoholic drinks per day are at higher risk. Oral contraceptive use has been linked to the development of breast cancer. Nulliparous women who began using oral contraceptives prior to the age of 18 years and continued uninterrupted use for more than eight years have a minimally increased risk. Risk related to use of estrogen replacement therapy seems to be most significant for those women who used hormone replacement therapy prior to 1958, who used replacement therapy for eight years or more, and for who also used oral contraceptives. Weight gain in early adulthood seems to increase risk for breast cancer, especially if the gain occurred in the third decade of life. The highest risk related to obesity and the development of breast cancer is in postmenopausal women.
Full-term pregnancy seems to exert a deterrent effect on the development of breast cancer. Women who become pregnant after the age of 30 years, or who never become pregnant, are at higher risk. Historically, lactation and breast-feeding have been recognized as protective mechanisms for breast cancer development. A correlation between the development of breast cancer and abortion has been documented in several studies conducted in the United States. A large study conducted in Denmark to investigate the correlation found no increased risk of breast cancer among women who had undergone abortions.
Precancerous and cancerous lesions of the cervix are associated with many personal risk factors, including a higher incidence in women who become sexually active prior to age 17, have many sex partners, and are multi-parous (have borne at least one living child). An association has also been described between type of employment and increased cervical cancer mortality. Women with higher mortality rates include women once employed in farm work, manufacturing, personal services, or who worked as nurses' aides. Women who are infected with the human immunodeficiency virus (HIV) are at higher risk for the development of squamous intraepithelial lesions of the cervix. Cervical cancer is not often diagnosed in women who are nulliparous, those who are lifetime celibates, or who are lifetime monogamous (having sex with only one person).
Stress is known to activate the body's endocrine or hormonal system which in turn causes changes in the immune system. There is no specific evidence that changes in the immune system caused by stress directly cause cancer. However, the relationship between stress and the development of breast cancer has been recently studied. Some studies report significantly higher rates of breast cancer in women who experienced stressful life events and losses in the years immediately preceding the diagnosis of breast cancer. Other studies do not support the association between stress and breast cancer development.
Cellular phone use
Studies in the United States and Denmark in 2000 and 2001 revealed there is no link between cellular telephone use and tumors of the brain, salivary gland, leukemia, or other cancers. The type of telephone, the duration of the cell phone use, or age of the phone user had no effect on cancer risk.
Chemicals and other substances
Exposure to certain chemicals, pesticides, and metals can increase an individual's risk for cancer. Carcinogens
Environmental tobacco smoke (secondhand smoke)
Environmental tobacco smoke (ETS) is a combination of two forms of smoke from tobacco products— sidestream smoke and mainstream smoke. Sidestream smoke is smoke released between puffs of a burning cigarette, cigar, or pipe. Mainstream smoke is the smoke that is exhaled by the smoker. Sidestream smoke contains essentially the same compounds as those in the mainstream smoke inhaled by the smoker. Tobacco smoke is known to contain at least 60 different carcinogens. Non-smokers who are exposed to ETS absorb nicotine and other harmful compounds from the smoke. ETS can cause lung cancer in healthy adults who are nonsmokers and there is a 20% increased risk of lung cancer in non-smokers exposed to ETS. ETS has been linked to other cancers, including cancers of the nasal cavity, cervix, breast, and bladder. In 1992, the U.S. Environmental Protection Agency classified ETS as a Group A carcinogen. Group A is reserved by the EPA to categorize only the most dangerous cancer-causing agents to humans.
Asbestos is a group of minerals which occur naturally as strong, flexible fibers that can be separated into threads and then woven. Asbestos fibers cannot conduct electricity and are not affected by heat or chemicals. Because of these properties there have been many industrial applications of asbestos. Some of the applications include insulation, fireproofing, and absorption of sound. Serious health risks related to asbestos occur as a result of exposure to the dust that is formed when the fibers break into tiny particles. These asbestos particles can then be inhaled or swallowed. Exposure to asbestos can lead to lung, larynx, and gastrointestinal tract cancers, as well as to the rare cancer— mesothelioma. Individuals at highest risk include those with the combination of asbestos exposure and smoking. Smoking increases risk for lung cancer by 10 times more than for the nonsmoker also exposed to asbestos. Due to government regulations and public concerns about the health hazards from asbestos exposure, the use of asbestos in the United States has declined significantly. Workplace practices involving asbestos are highly regulated by industry and government to minimize worker exposure.
Electric and magnetic field (EMF) exposure
EMFs are emitted from devices that produce, transmit, or use electric power and arise from the motion of electrical charges. Examples of these devices include power lines, transmitters, and household products such as microwave ovens, electric blankets, televisions, and computers. EMFs are considered forms of nonionizing radiation. According to the National Cancer Institute (NCI), public concern has increased over the health effects of EMFs, particularly in relation to the risk of developing cancer in both children and adults exposed to EMFs. Numerous studies have been conducted in the past 15 years to evaluate risk of cancer from exposure to EMFs. As of 2001, the results have been inconsistent according to the NCI. One large NCI/Children's Cancer Group study sought to determine whether exposure to magnetic fields contributed to the development of acute lymphoblastic leukemia (ALL) in children under the age of 15 years. The results of this study revealed little evidence of a relationship between risk for ALL in children and exposure to magnetic fields.
Other studies focused on determining links between magnetic fields and central nervous system (CNS) tumors such as brain cancers continue to be actively researched. To date, expert panels that have reviewed the existing data have concluded that data are insufficient to support the conclusion that magnetic fields cause cancer.
Nuclear facility exposure
An NCI study published in 1991 concluded there was no general increased risk of death from cancer for people living in more than 100 U.S. counties containing, or closely adjacent to, nuclear facilities. A British survey of cancer mortality in areas around nuclear facilities in the United Kingdom did report an increase in deaths from childhood leukemia near some of the facilities. Other smaller surveys of cancer deaths around nuclear facilities in both countries yielded conflicting results.
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Melinda Granger Oberleitner, R.N., D.N.S.