Anything that can cause cancer in humans or animals, or helps it to grow is known as a carcinogen. Carcinogens can be naturally occurring or synthetic. They can be physical, from chemical compounds to fibrous materials; they can be solid, liquid or gas. They can be biological, like the virus that causes cervical cancer. Many chemical carcinogens are molecular substances found in liquids or solids like pesticides, or in the air, like cigarette smoke and components of smog.
Carcinogens usually act directly on a cell’s DNA to cause a mutation. The mutation will cause the cell to grow in an uncontrolled way. Some carcinogens act indirectly by causing normal cells to grow more rapidly, leading to a higher chance of a cancer-causing mutation.
Carcinogens are discovered in a number of ways. Lab studies may show that a particular substance causes cancer in animals. It is often suspected that chemicals with similarities to known carcinogens might also cause cancer, and this may be tested in animals. It is assumed that what causes cancer in animals may cause it in humans.
Population studies can correlate clusters of cancer, and use available data to try and get an idea of what might be causing the particular cancers.
Scientists usually use information from epidemiologic studies as well as laboratory studies to determine if something is carcinogenic. Sometimes the evidence is strong and sometimes it is not.
There are a number of agencies that list carcinogens. The International Agency for Research on Cancer which is part of the World Health Organization is one such agency. They place possible cancer-causing agents into one of 5 groups, from carcinogenic to probably not carcinogenic.
The U.S. National Toxicology Program which was formed by parts of the NIH, the CDC and the FDA release reports about every two years. They make two classifications, “known to be human carcinogens” or “reasonably anticipated to be human carcinogens.”
The Environmental Protection Agency has its own 5 categories, from carcinogenic to not likely to be carcinogenic.
The lists of these groups are not identical. The IARC lists about 100 known human carcinogens, the NTP lists about 50.
Both include groups of chemicals, medications, radiation, certain dusts and fibers and viruses.
Asbestos is on all the lists of known carcinogens.
Dr. Kaplan
