Carcinogens are agents that initiate or promote uncontrolled neoplastic (cancerous) cell growth by causing DNA damage, chromosomal mutations, or disrupting cell cycle regulation. Examples include ionising radiation, tobacco smoke (polycyclic aromatic hydrocarbons), asbestos, benzene, and certain viruses (HPV, HBV). Teratogens cause birth defects; mutagens cause genetic mutations (which may or may not lead to cancer); heavy metals can be carcinogenic but the broad term is carcinogen.