ENVIRONMENTAL FACTORS AND CANCER
It is true that smoking cigarettes, harmful chemicals in our environment, and nuclear radiation all cause cancer. Nonetheless, all of these factors first act by causing mutations. Because cancers are primarily the result of mutations in somatic cells, those chemicals that cause mutations, that is, mutagens, can also cause cancer. Radiation that causes DNA damage similarly leads to mutations and cancers. Not every mutation will actually cause cancer. Most mutations do not even affect transcribed regions of DNA, and even if they do affect a particular gene, it is not likely to be involved in controlling cell division.
In practice, some cancer-causing mutations are due to environmental factors, whereas others occur spontaneously as a result of mistakes made during replication of the cell’s DNA. Cancer-causing agents are often called carcinogens. Almost all carcinogens are also mutagens. Occasional discrepancies occur due to metabolism within the body, usually in the liver. Certain chemicals that do not react with DNA themselves may be altered by the body, giving rise to derivatives that do react with DNA. In this case, the original compound is by definition a carcinogen but not, strictly speaking, a mutagen.
Approximately 80% of cancers are derived from the epithelial cells that form the outer covering of tissues. Epithelial cells are the surface cells of the skin and are also found lining the intestines and the lungs. Because the outermost layers are constantly worn away, the underlying layers must keep dividing. Cells from tissues where cell division is rare only occasionally become cancerous. (Nerve and muscle cancers account for only 3% to 4% of the total.) In addition, the surface cells are much more likely to suffer exposure to dangerous chemicals and harmful radiation.