Biological effects and potential genetic/epigenetic adaptive responses to chronic natural background radiation

Natural background radiation (NBR) delivers chronic low-dose-rate ionizing radiation worldwide, with a global average annual effective dose of ∼2.4 mSv. In certain high-background radiation areas (HBRAs), annual doses range from 10 to > 200 mSv. While the linear no-threshold (LNT) model assumes any radiation dose carries some cancer risk, epidemiological studies in HBRAs (Kerala-India, Ramsar-Iran, Yangjiang-China, Guarapari-Brazil) have consistently failed to detect the predicted cancer excess. Some cytogenetic and molecular investigations suggest enhanced DNA repair proficiency or lower-than-expected chromosomal damage in chronically exposed residents. This narrative review critically synthesizes epidemiological, cytogenetic, and limited molecular evidence from HBRAs, evaluates the ability of current dose-response models (LNT versus non-linear alternatives) to explain these observations, identifies major knowledge gaps, and discusses implications for low-dose radiation risk assessment and public health policy.