Carcinogens
Carcinogens are substances, usually chemicals, that can cause cancer. Although chemical carcinogens are often linked with industry and technology, it is important to recognize that many carcinogens are naturally occurring substances.
Researchers have identified four factors, each of which is able to trigger cancer. These factors are chemicals, radiation (including ultraviolet radiation in sunlight), certain viruses, and inherited genes. In some cancer cases, more than one cause is responsible for generating the disease. For example, it is possible that certain chemicals might only cause cancer if a person has a genetic susceptibility to the disease or because there is a genetic error which prevents DNA repair.
Carcinogens are generally divided into two classes: those that are organic compounds and those that are inorganic compounds. Organic compounds always contain the element carbon. Chemicals in this class of carcinogens include solvents, certain pesticides, and naturally occurring chemicals produced by some plants and molds. Some organic compounds are not carcinogenic until they have been biochemically altered, or metabolized, in the body. Inorganic compounds that are carcinogenic include arsenic, asbestos, certain forms of chromium, and nickel. There is less evidence for beryllium, cadmium, lead, and silica, but these chemicals have been shown to cause cancer in animals.
The term cancer was first used by Hippocrates in the fifth century B.C. to describe the abnormal growth of tissues in the body. Cancer can occur in almost every tissue in the body. Cancers are classified according to the type of tissue that is affected. The three classifications are: carcinoma, sarcoma, and lymphomas and leukemias.
Carcinomas are the most common types of cancers. These cancers affect epithelial cells-- the cells that cover internal and external surfaces in the body. Common carcinomas include lung, breast, and colon cancers. Sarcomas are less common and affect tissues that serve as cell and tissue framework. These cancers include those that affect bone, cartilage, connective tissue, and muscles. Lymphomas and leukemias affect blood and lymph cells.
Normally, the cells in a tissue have a balanced cycle of cell division, cell maturation, and cell death. This balanced cycle allows new cells to be created at the same rate that old cells need to be replaced. Under some circumstances, certain cells develop an unbalanced cycle, and more new cells are produced than are necessary to replace the old ones. As a result, an abnormal mass of tissue, called a tumor, forms.
Not all tumors are cancerous, however. Benign tumors are confined to a specific area and are rarely life-threatening. Malignant tumors, however, can be dangerous. Malignant tumor cells have abnormal properties that allow them to spread from the site of the original tumor. The cancer cells can spread into nearby tissues or migrate to other areas of the body and establish tumors elsewhere.
The link between chemicals and cancer was first noted in the early eighteenth century. In 1761, John Hill (1707?-1775), a doctor living in London, noted that people who used snuff (a form of tobacco that is snorted) had a high incidence of nasal cancer. Later, in 1775, an English physician and surgeon named Percival Pott (1714-1788) wrote about the high incidence of cancer of the scrotum among chimney sweeps. Due to the nature of their work, chimney sweeps had a lot of contact with soot, and Pott suggested that some element in the soot caused the chimney sweeps to develop cancer.
Throughout the nineteenth century, the industrial use of chemicals sky-rocketed and reports linking certain types of cancer with specific industries accumulated. For example, many scientific reports detailed increased incidence of skin and bladder cancer in connection to the dye industry. However, factories continued to use the chemicals, because the link between chemicals and cancer was not clear. Cancer itself was not a well-understood disease.
By the early twentieth century, researchers were beginning to study the link between chemicals and cancer in a systematic way. The first detailed studies involved applying coal tar to the skin of animals. The animals developed malignant skin tumors, which proved that there was something in the coal tar that caused cancer. It wasn't until the early 1930s that the substance was shown to be dibenz(a, h)anthracene. Following this discovery, further animal studies were completed in which other substances were tested and shown to be carcinogens.
For ethical reasons, such experimental studies would never be conducted on humans. But researchers can observe the incidence of cancer among groups of people and try to establish if a common factor exists. This kind of study is called an epidemiological study. Epidemiologists, the scientists who conduct epidemiological studies, must have a very good understanding of statistics. They must consider many different factors that can affect people's exposure to the suspected carcinogen and they must also consider alternate explanations for their observations.
Through epidemiological studies, scientists showed that cigarette smokers developed lung cancer at a much higher rate than people who did not smoke. This finding alone was not enough to prove the connection between cigarettes and cancer, but it served as very good evidence. Scientists determined which chemicals were in cigarette smoke and did experimental studies in animals to test which were carcinogens. They found that tobacco smoke contains a complex mixture of more than 30 carcinogens, each of which is capable of causing cancer on its own.
Carcinogens trigger cancer by interacting with cellular molecules. There are two ways in which carcinogens can act: genetic and non-genetic. Genetic effects occur with a two- stage attack on the DNA. The first stage is called initiation. During initiation, the carcinogen causes an irreversible change in the DNA. The second stage is called promotion. During promotion, the cell with the abnormal DNA is stimulated to multiply. Because the DNA is damaged, cell multiplication is uncontrolled and a tumor can form. Many years can pass between initiation and promotion, and the effects of being exposed to a carcinogen may not be apparent for decades. Carcinogens can also cause cancer by non-genetic means. They act by altering cell signaling pathways or by encouraging the growth of abnormal cells.
Although epidemiological studies and animal experiments have identified many carcinogens, they tend to be time-consuming and expensive. Further, they are not helpful in predicting which substances may be carcinogens. By the time a substance is discovered to be a carcinogen, it may have already affected many people.
To address these problems, a test was developed by an American molecular geneticist, Bruce Ames (1928-), in the early 1970s. The so-called Ames test is based on the fact that most carcinogens are mutagens (substances that damage DNA).
The Ames test uses a strain of bacteria, Salmonella typhimurium, that has been genetically altered so that it cannot produce histidine. Bacteria that cannot produce histidine will not grow. If the bacteria grow after being exposed to the suspected carcinogen, it is very likely that the substance has altered the DNA and may be considered a carcinogen.
Once a carcinogen has been identified, scientists and others investigate how much threat it poses to people and what can be done to lessen the risks. In workplaces, carcinogens are replaced with other chemicals where possible. If substitution is not possible, workers wear protective gear and use the substances under strictly regulated conditions.
Environmental carcinogens are a source of much concern because it can be difficult to cleanse the environment of the compound. Carcinogens may be present in the environment through both human activities and natural causes. Many countries now have laws that restrict the release of carcinogens into the environment; however, other countries either do not have such laws or do not enforce them rigorously.
Exposure to naturally occurring carcinogens can be difficult to control. Humans and other organisms have evolved mechanisms to prevent or repair damage due to many naturally occurring carcinogens; however, these mechanisms cannot handle all naturally occurring carcinogens. For example, arsenic in drinking water is a continuing problem in several countries including areas in China, India, and Bangladesh. People who rely on wells for their drinking water receive continued exposure to arsenic, resulting in an increased incidence of cancer in those areas.
Individuals can decrease their cancer risk from certain carcinogens by making lifestyle changes. Cancer researchers estimate that at least 30% of the cancer deaths in the United States can be linked to smoking. The longer a person smokes, the greater his or her risk of developing cancer. Other cancer deaths seem associated with diets heavy in fat and low in fruits and vegetables. Fruits and vegetables contain compounds called antioxidants. Antioxidants may help the body's natural cancer-fighting mechanisms.
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