Table 1. Radiation Exposures from Natural Background and Medical Sources Natural background (annuel exposures) Total _ Roentgens 0.085 - 0.20 Gamma rays (from territorial sources) and cosmic rays O.1 (varies) Potassium-40 (internal) 0.018 (varies) Carbon-14 7001 Medical Exposures Chest X-ray (per exposure) 0.2 Back X-ray (per exposure) O.4 Photoflurogram (per exposure) _ about 0.5 - 2.0 Gastro-intestinal series about 30. 59. c¢