Numerous chemicals are known to be carcinogenic. Exposure to these chemicals does not always lead to cancer formation, but it does increase risk. The mode of contact (ingested orally versus applied to the skin) and the length of exposure are often necessary. Common chemical carcinogens include tobacco smoke and benzenes. Laboratory studies on cells and animals are frequently used to categorize a substance as mutagenic, meaning that it leads to mutations in DNA. Substances determined to be mutagenic in those studies are likely carcinogenic in humans.
The most widely used test to determine if a chemical is mutagenic is the Ames test. The Ames test uses different strains of Salmonella bacteria growing in culture. Significantly, these strains of Salmonella have been mutated so that they do not produce their histidine, which is required for their growth. A chemical tested as a toxin or carcinogen is applied to the bacteria growing on a plate. After 48 hours of incubation with limited amounts of histidine and in the presence of the chemical, colonies are counted. Only bacteria that mutate to produce histidine will continue to grow. The mutagenicity of the substance is determined by the number of colonies that were able to grow.
This test has limitations in determining the carcinogenicity of a substance in humans since humans have much more complex detoxification pathways and metabolism. These differences could make a compound less carcinogenic or more carcinogenic (if humans metabolize a substance into a carcinogen).
