Benzo[a]pyrene is a polycyclic aromatic hydrocarbon found in coal tar with the formula C20H12. Its metabolites are mutagenic and highly carcinogenic, and it is listed as a Group 1 carcinogen by the IARC. The compound is one of the benzopyrenes, formed by a benzene ring fused to pyrene, and is the result of incomplete combustion at temperatures between 300 °C and 600 °C.
Frequent skin cancers were noted among fuel industry workers in the 19th century. In 1933, benzo[a]pyrene was determined to be the compound responsible for these cases, and its carcinogenicity was demonstrated when skin tumors occurred in laboratory animals repeatedly painted with coal tar. Benzo[a]pyrene has since been identified as a prime carcinogen in cigarette smoke. When the body attempts to metabolise benzo[a]pyrene, the resulting diol epoxide reacts and binds to DNA, resulting in mutations and eventually cancer.