There are numerous chemicals known to be carcinogenic. Exposure to these chemicals does not always lead to cancer formation. The mode of contact (ingested orally versus applied to the skin) is often important as well as the length of exposure. Common chemical carcinogens include tobacco smoke and benzenes. Laboratory studies on cells and animals are often 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. Importantly these strains of Salmonella have been mutated such that they do not produce their own histidine which is required for their growth. The chemical in question is applied to the plate. After 48 hours of incubation with limited amounts of histidine and the chemical, colonies are counted. Only bacteria that mutated 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 a human, as we have enzymes in our body that may change the chemical structure of a substance. This could make the metabolite carcinogenic even if the original chemical was not.
