Due to differences in their genetic make-up, individuals exposed to a carcinogen will not all have the same probability of getting cancer. Certain proteins from the liver, called enzymes, can break down or activate the carcinogen at different speeds in different people to either render it harmless or promote it to cause cancer. These enzymes will either destroy or activate carcinogens to varying degrees according to inherited tendencies. Some foods can induce certain enzymes to destroy certain carcinogens. The most potent food sources to induce these enzymes are vegetables of the Brassicaceae family, which includes Brussels sprouts, cabbage, and broccoli. (16)

Environmental Risk Factors
Environmental factors may be just as important as genetic factors for cancer. For example, Japanese men and women who leave Japan and settle in Hawaii or the continental United States have a lower risk of Stomach cancer than those who remain in Japan. Stomach cancer in the United States has been steadily decreasing with the advent of refrigeration and the consequent removal of carcinogenic chemicals as food preservatives.

Air pollution may be a risk factor for cancer, especially lung cancer. Those living in cities encounter many sources of pollution. More people in cities smoke cigarettes than in rural areas. Carcinogens derived from car emissions, industrial activity, burning of solid wastes and fuels remain in the air from four to forty days and thereby travel long distances. (17) And asbestos, a potent carcinogen, can also be found airborne in cities.

Our drinking water contains a number of carcinogens, including asbestos, arsenic, metals, and synthetic organic compounds. (18,19) Asbestos and nitrates are associated with gastrointestinal cancers; arsenic is associated with skin cancer; and synthetic organic chemicals are associated with cancers of the gastrointestinal tract and urinary bladder.

As with many carcinogens, the time between exposure to the carcinogen and actual development of cancer may be quite long. Hence, the cause of a cancer initiated by trace amounts of either airborne or waterborne carcinogens years before the cancer appears may be attributed to an unrelated or unknown cause at the time of diagnosis. We are able to detect many carcinogens in our environment, but many others exist in low concentrations. These environmental carcinogens may themselves cause cancer in certain individuals, or they may interact with other risk factors to initiate, or promote, cancers. Therefore, our imperfect environment, a risk factor for cancer, must be modified. If we avoid introducing harmful substances into the environment, it will remain clean.

Radiation Risk Factors
Human studies show that the more radiation a person is exposed to, the higher is the risk of developing cancer, especially if the radiation exposure is to bone marrow, where the blood cells are made. People who received radiation to shrink enlarged tonsils or to treat acne have a higher risk of developing cancer of the thyroid and parathyroid glands located in the neck. Survivors of the bombings of Hiroshima and Nagasaki have had an increased incidence of leukemia, Iymphoma, Hodgkin's disease, multiple myeloma, and other cancers. People who used to paint radium on wristwatch dials have a high incidence of osteogenic sarcoma, a bone cancer. Chronic exposure of fair-skinned, easily sunburned people to sunlight (ultraviolet light) will lead to a higher rate of skin cancer.

There has been mounting concern that people who work in or live near nuclear power plants have a higher risk of developing cancer. In the United Kingdom a higher incidence of childhood leukemia has been reported in children living near several nuclear facilities, most notably a fuel reprocessing plant located at Sellafield in northwest England. (20) The results of another study involving over 8,000 men who worked in the Oak Ridge National Laboratory in Tennessee between 1943 and 1972 show that these men had a higher risk of developing cancers, especially leukemia. (21) Another study shows no such increase in cancer incidence. (22)

Women with tuberculosis who received many chest X rays to follow the progress of treatment had an increased incidence of breast cancer with as little as 17 cGy total dose. A cGy, or centiGray, is a defined amount of energy absorbed by a certain amount of body tissue. One chest X ray using modern equipment delivers about 0.14 cGy. Riding in an airplane at 35,000 feet for six hours exposes a person to 0.01 cGy.

A study by Matanoski published in the Proceedings of the 1980 International Conference on Cancer indicates that radiologists, besides their well-known increased risk of developing cancer, may also have a 30 percent increased risk of death from cardiovascular disease and stroke due to radiation exposure. Workers in many industries are chronically exposed to low-dose radiation and hence may be at risk for heart disease and cancer. We may therefore have to reexamine standards for acceptable radiation levels in industry.

Occupational Risk Factors
About 10 percent of all cancers are related to exposure to carcinogens on the job The relationship between a person's job and cancer was noted in the eighteenth century when it was observed that the incidence of cancer of the scrotum was very high in chimney sweeps. Many associations between exposure to carcinogens at work and cancer have been made since then. Most recently the boot and shoe manufacture and repair industry and the furniture and cabinet-making industry were shown to be risk factors for cancer of the nasal sinuses. (23)

Preliminary studies indicate that butchers and slaughterhouse workers are at risk for lung cancer and cancer of other parts of the respiratory system as well as some leukemias. (24,25) However, these findings need to be confirmed and controlled for those persons who also smoke before this industry can be labeled as a definite risk factor for cancer. Other occupations and their associated human cancers are listed in Table 2.1.

Age as a Risk Factor
The older you get, the higher is your risk of developing cancer. Your risk for cardiovascular disease also increases with age, but to a greater degree. The Biometry Section of the National Cancer Institute has presented studies which show that with every five-year increase of age there is a doubling in the incidence of cancer. (26) The elderly often Suffer from nutritional deficiencies, and they have an increased num-ber of infections, autoimmune diseases, and infantile disease patterns, as well as cancer. Werner's syndrome, which prematurely ages very young children so that they die in early adolescence, is characterized by an impaired immune system. These facts suggest that the immune system (see Chapter 4) in the elderly is working inefficiently, partly due to poor nutrition. (27) Because the gastrointestinal tract absorbs nutrients less efficiently with age, the elderly need more nutrients in their diets.

As you get older, your risk of getting cancer increases not only because of your age but also due to the amount of time you have been exposed to external risk factors. For example, cigarette smoking increases your chances of getting lung cancer. The longer you smoke, the greater is the likelihood and incidence of lung cancer. For men between 55 and 64, the annual lung cancer mortality rate is five times higher if they started to smoke before age 15 than if they started to smoke after age (25-28) If a person stops smoking, there is a decreased risk of developing lung cancer, but this risk does not go back to zero.

Table 2.3 lists the number of people aged 65 or more for approximately each decade beginning with 1980. By 2030 the number of Americans in this age bracket will more than double the number in this group today.