Polycyclic aromatic hydrocarbons (PAH) are ubiquitous environmental contaminants with potential health consequences. Essentially all the cancer risk from exposure to coal tar, cigarette smoke and other products of incomplete combustion is thought to reflect the contributions of PAHs with 4 or more fused rings. In risk assessment as currently practiced the major contributor to overall risk is the 5-ring PAH benzo(a)pyrene (B(a)P).However, recent studies using a lifetime feeding protocol indicate that B(a)P does not meet criteria for application to risk assessment since it does not induce lung tumors, the endpoint upon which risk assessments should be based. It is important to identify the chemical primarily responsible for lung tumor induction in order to establish risk of PAH. We have recently discovered that a 4-ring PAH, 7H-benzo(c)fluorene, induces damage to lung DNA consistent with tumor induction. Evidence supporting a role for 7H-benzo(c)fluorene in assessing PAH risk will be presented.