Isoprene: One of the major components that makes up natural rubber and is used to make synthetic rubbers. It is also emitted from plants and trees, has been detected in tobacco smoke and automobile exhaust.
Isoprene is a possible carcinogen (cancer-causing agent). The US government in 2000 classified it as "reasonably anticipated to be a human carcinogen." In laboratory animal studies of isoprene, cancer was observed in multiple organ sites following long-term inhalation exposures.