4.1 External Dose The 1978 aerial survey (Tipton & Meibaum, 1981) provided DOE with important information on exposure to fallout in the Northern Marshall Islands. As the survey proceeded south and east from Bikini Atoll, the seat of the Bravo shot, the external exposure rate fell ( Table 4.1 #1). It was calculated for 1 meter above ground level. At Rongelap Atoll (Figure 4.1 #1), the islands fell into four exposure groups (microroentgens per hour) from north to south: Naen, Yugui, Lomuilal (28-43 pR/h), Eniaetok, Kgbelle, Gogan (10-27 pR/h); Busch, Borukka, Gabelle, Tufa (5-9 pR/h); Rongelap and Arbar (4.1-4.5 uR/h). The external dose (whole-body), was calculated from exposure by ny assuming 1 roentgen = 0.7 rem tissue dose (Kerr, 1980; U.N. 1982). For Rongelap Island the annual dose was .028 rem, well below the EPA guide of -170 rem/year; 8 other major islands were also below the guide (Table 4.1 #1).* The factor of 0.7 rem per roentgen was used to allow for the [possibly] smaller size of the Rongelap [population) and the many children. The conventional value for the 70 kg standard man is 0.61. There is also a shallow dose to be considered, that due to beta rays which travel for short distances (< 1 cm) into those parts of the body that are near or in close contact with the soil and that are unshielded. Their contribution is considered to be negligible (Note 7). These estimated external gamma-ray dose rates are maximal ones. Indoors the rate is reduced by about 50%. Likewise, the rate is reduced by about 50% in the immediate vicinity of houses owing to the coral Gravel that is spread around them (Shingleton et al, 1987 and Robison et al, 1982b). This, of course, is important in the case of infants and small children. Other annual contributions to external dosage which are not included come from cosmic radiation (.028 rem) and medical exposure. In summary, the contribution of fallout to the total external radiation dose at Rongelap Island in 1978 was approximately .028 rem per year uncorrected for the shielding within or around buildings, which would decrease the rate by 50%. The 30-year whole-body dose would be -590 rem allowing for spontaneous decay, but not shielding. Environmental decay such as leaching of radionuclides from the soil would reduce this estimate still more, but was not allowed for. . (* Based on the annual doses in Table 4.1 #1, the Lukuen group of northern islands exceed the radiation protection guide (Note 5) on the basis of external dose alone and the Eniaetok group approaches this limit (.17 rem/yr). With the internal dose also taken into account, I would recommend that no islands be inhabited north of Borukka and Eniaetok.] 24 9000631