DATA BASES © The exposure pathways for persons living at or resettling an atoll consist of two major categories: external and internal exposure. , The specific pathways in each categoryare as follows. (1) External exposure (a) Natural background (b) Man-made gammaand beta rays (2) Internal exposure (a) Radionuclides inhaled (b) Radionuclides in drinking water (c) Radionuclides in terrestrial foods (d) Radionuclides in marine foods The natural background at the atolls is 3.5 uR/h (microroentgen per hour) or 22 mrem/y (milliroentgen equivalent man per year) and results primarily from cosmic radiation. The natural background is not included in the doses presented here. EXTERNAL EXPOSURE--IN SITU MEASUREMENTS External exposure rates for 137 Cs, 60 Co, and 241 Am were obtained from in situ measurements performed by EG&G as part of the Northern Marshall Islands survey.!! These measurements were made with 40 !2.7-cm-diameter by 5.l-cm-thick sodium iodide scintillation detectors mounted on two pods on a Sikorski SH-3 helicopter. Flight lines were on a 46-m grid at an altitude of 38 m over the islands. For a detailed description of this methodology, see Ref. 11. The average external exposure for Bikini Island is 31 —uR/h fo r 13765 and 1.9 uR/h for 606, and for Eneu Island it is 2.3 and 0.2 uR/h, respectively. The external gamma doses presented here are based on the island average external exposure. However, the Marshallese spend considerable time (30 to 50%) in or around the housing area. As a result, the housing provides shielding that reduces the average outside exposure by a factor of 2. Also, coral gravel spread 20 to 40 ft around houses, a common practice in the Marshall Islands, will reduce the external exposure by another factor of 2 (see Ref. 6). The result is that the external gamma doses presented here are probably upper limits because, depending on how much time one wishes to estimate is spent in and around the housing area, the external exposures will be considerably reduced because of shielding Il