Benzo[a]pyrene, C20H12, is a five-ring polycyclic aromatic hydrocarbon that is mutagenic and highly carcinogenic. It belongs to a class of polycyclic aromatic compounds known as benzopyrenes, which consist of a benzene ring fused to a pyrene molecule. Benzo[a]pyrene is a product of incomplete combustion at temperatures between 300 and 600 °C. Benzo[a]pyrene was determined in 1933 to be the component of coal tar responsible for the first recognized occupation-associated cancers, the sooty warts (cancers of the scrotum) suffered by chimney sweeps in 18th century England.
In the 19th century, high incidences of skin cancers were noted among fuel industry workers. By the early 20th century, the toxicity of benzo[a]pyrene was demonstrated when malignant skin tumors were produced in laboratory animals by repeatedly painting them with coal tar.