Carcinogen

A carcinogen is a substance that causes a normal cell to change into a cancerous cell. The word "car cinogen" is derived from Greek and means in English, cancer-causing. Carcinogens fall into two broad categories, naturally occurring substances that are found in food or soil , or artificial substances created by chemists for various industrial purposes. Although the way carcinogens cause cancer is still not completely understood, cancer researchers believe that humans and other animals must be exposed to a carcinogen for a certain period of time and at a high enough concentration for cancer to occur.

Cancer is a group of diseases in which cells grow abnormally. Like all living things, normal cells grow, reproduce, and die. These processes are controlled by chemicals and reactions within the cell, which are in turn controlled by the cell's genetic material within its nucleus. In a cancerous cell, the genetic material is altered, and the genes which encode and direct the chemical reactions within the cell are mutated, or changed. Cancerous cells grow uncontrollably, forming a large mass of cells called a tumor , which invades tissues and kills non-cancerous cells. Sometimes cancerous cells "break off" from a tumor and enter the bloodstream, traveling to other parts of the body and infecting other organs and tissues in a process known as metastasis. In this way, a cancer can spread from an isolated tumor to the entire body.

Several agents, such as viruses, medication, and synthetic carcinogens can cause mutations within a cell's genetic material. Some kinds of cancers are caused by viruses. For example, a special kind of virus called a retrovirus causes a rare form of leukemia (cancer of the white blood cells). Radiation from naturally-occurring radioactive substances (such as uranium ) can disrupt a cell's genetic material and bring about cancer. Synthetic carcinogens are found in processed foods and industrial chemicals.

Carcinogens and cancer

For a carcinogen to cause cancer a person must be exposed for a certain length of time to the carcinogen and at a high enough concentration. Repeated exposure to a carcinogen over an extended period (such as 20 years) increases the likelihood that a normal cell's genetic material will mutate and initiate cancer. Cigarette smoke contains potent carcinogens; it can take many years and many repeated exposure to the carcinogens in smoke for smoking to cause cancer. Smoking-related lung cancers typically develop between 10 and 20 years of continuous smoking. In Hiroshima and Nagasaki in Japan, where atomic bombs were dropped in 1945, the leukemia rates in the surviving population increased dramatically some five years after the bombs were detonated.

Some carcinogens are more powerful, cancer-causing agents, than others. Powerful carcinogens are called tumor promoters, and can cause genetic mutations directly within cells. Less powerful carcinogens are called tumor initiators and can cause latent changes in the cell's genetic material. These changes are not enough to actually cause cancer, but sensitize the tissue for later exposures to tumor promoters. If a tumor initiator has already wrought some damage to the cell's genetic material, the likelihood that a tumor promotor will cause the cell to become cancerous is increased.

Cancerous tumors develop over many stages, and it is rare that exposure to a carcinogen is the sole cause of most cancers. Exposure to a carcinogen must usually be combined with other environmental factors for a cancer to develop. Some environmental risk factors are difficult to identify, while others such as prolonged, heavy cigarette smoking have been easy to identify. Heredity, such as whether close relatives have developed cancer, is another risk factor that is not as easily characterized.

Some carcinogens such as cigarette smoke can be avoided. Other factors, such as a diet, can be modified. Additional risk factors such as gender, immune status, metabolic rate , levels of certain enzymes, and age can neither be avoided nor modified.

Carcinogens used in industry

The idea that chemicals could cause cancer was first promoted in 1775 by Percivall Pott, a London physician. Dr. Pott noted that young chimney sweeps had a high incidence of scrotal cancer. Because most sweeps began their careers very early in life and seldom washed or changed clothes, the sweeps were exposed to soot repeatedly and for long periods of time, leading to scrotal cancer in young adulthood. Not until 150 years later were the actual carcinogenic substances in soot identified, but Dr. Pott made his case for more humane treatment and better working conditions for the chimney sweeps by noting the connections between cancer and this profession.

With the advent of industrial development in the nineteenth century, other connections between certain cancers and chemicals were noticed. Shale oil and coal and tar workers had a high incidence of skin cancer. Dyestuff workers developed bladder cancer. And a chemical called vinyl chloride, used in the manufacture of leather goods, caused a rare liver tumor.

In response to growing concern about cancer in industrial workers, the International Agency for Research on Cancer and the National Toxicology Program formulated a system to classify chemicals according to their cancer-causing risk. A chemical could be classified either as a probable carcinogen, or as a non-carcinogen.

A problem with this and other classification systems is that much of the research is based on experiments on animals. If cancer can be induced in experimental animals with high levels of a chemical, it is sometimes assumed that humans could also be at risk even with lower exposure levels over a long period of time. The science of risk assessment investigates the possibility of developing cancer from very low levels of exposure to chemicals that cause cancer at very high levels.

Dioxin is a case in point. In the early 1980s, dioxin was sprayed on the roads of Times Beach, Missouri, to seal the pavement. Dioxin had been classified by the Environmental Protection Agency (EPA) as a probable carcinogen. When the townspeople discovered that dioxin had been sprayed on their roads, the town was abandoned and lawsuits against the road contractor were initiated which were challenged by the defendants.

Carcinogens in food

Some foods contain naturally-occurring carcinogens. Safrole, found in sassafras root; estragole, found in the herb tarragon; allyl isothiocynate, found in mustard seed; and benzene , found in eggs, fruits , vegetables , cooked meats, and fish are all carcinogens. However, these substances must be consumed in large amounts, over a long period to initiate cancer.

Processed foods such as bacon, sausages, and canned meats contain the preservative nitrite. Frying the cured bacon can convert some non-carcinogenic substances in the nitrites into potent carcinogens. Browning meats such as hamburger can also cause carcinogenic chemicals to be produced. However, in both cases, the amount of carcinogen is extremely small. Interestingly, microwave cooking does not release the carcinogens in beef.

Foods that involve fermentation in their production, such as beer, wine, bread, and yogurt, all contain mildly carcinogenic substances. Again, these foods must be eaten in large amounts over decades to possibly cause the genetic mutations related to cancer.

Other carcinogens

Radioactive substances found in rocks and soil are also considered potentially carcinogenic. While most people do not come in contact with radioactive chemicals on a day-to-day basis, these substances can emit radioactive particles that can be dangerous. Radon , a radioactive substance emitted by uranium, can seep from rocks into buildings. In some areas of the country, radon can be emitted in relatively large amounts into buildings which should be tested for radon. If the levels are found to be high, changes should be made to the building's ventilation system to reduce the amount of radon in the building.

In 1993, the EPA designated second-hand smoke from cigarettes to be a known human carcinogen. It is estimated that 2,000 lung cancer deaths a year are caused by second-hand smoke, which has led to the designation of many public areas as smoke-free zones.

Avoiding carcinogens

It is recommended that people eat a varied diet high in fresh fruits and vegetable and avoid excess consumption of foods high in nitrites. While it is not possible to completely eliminate one's exposure to carcinogens, it is possible to avoid the concomitant risk factors that may lead to cancer. Avoiding smoking, eating a varied, balanced diet that includes fiber, and limiting alcohol consumption are all associated with a lowered cancer risk.

See also Mutagen; Mutation.

Resources

Periodicals

Ashby, John. "Change the Rules for Food Additives." Nature 368 (April 1994).

Begley, Susan. "Don't Drink the Dioxin." Newsweek 124 (September 1994): 57.

Boyle, Peter. "The Hazards of Passive-and Active-Smoking." New England Journal of Medicine 328 (June 1993): 1708.

Moolenaar, Robert J. "Overhauling Carcinogen Classification." Issues in Science and Technology 8 (Summer 1992): 70.

Nesnow, Stephen. "Breakthroughs in Cancer Risk Assessment." EPA Journal 19 (Jan/Mar 1993): 27.

"Second-hand Smoke Designated as a Known Human Carcinogen." EPA Journal 19 (2): 5. April/June 1993.

Yuspa, S. H. "Overview of Carcinogenesis: Past, Present and Future." Carcinogenesis 21 (2000): 341–344.

Kathleen Scogna

KEY TERMS

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Cancer

—A disease in which cells grow abnormally.

Carcinogen

—Any substance capable of causing cancer by mutating the cell's DNA.

Mutation

—A change in the genetic material of a cell.

Risk assessment

—The study of the risk of exposure to certain levels of an agent that may lead to the development of a disease, such as cancer.

Risk factor

—Any habit, condition, or external force that renders an individual more susceptible to disease. Cigarette smoking, for example, is a significant risk factor for lung cancer and heart disease.