Lung cancer is the deadliest cancer in Europe and worldwide. While 80–90% of cases are linked to tobacco, other risk factors include occupational exposures, air pollution, and radon. Radon is a natural radioactive gas, odorless and colorless, produced by uranium decay. It accumulates mainly in poorly ventilated buildings, accounting for over 50% of total exposure. Radon exposure causes genomic damage (DNA mutations, breaks) and immune system dysfunction (chronic inflammation, immunosuppression), promoting lung carcinogenesis. Epidemiological studies show a clear link between radon exposure and lung cancer risk, with a 10–20% increased risk per 100 Bq/m ³ increment. This risk is independent of smoking but synergistic with it. Prevention focuses on building ventilation, sealing, and systematic radon measurement in high-risk areas. Legal thresholds exist (300 Bq/m ³ in France) but enforcement varies. Prospective studies, such as Bioradon, aim to clarify biological mechanisms and refine screening strategies.