148 Health Physics August 2010, Volume 99, Number 2 Table 2. Variation of the exposure (mR) with increasing TOA(h) and influence of the weathering effect. The exposure rate is normalized to 1 mR h7' at H+12 andtherelative degree of fractionation (R/V) is assumedto be 0.5. Exposure (mR) Exposure (mR) from TOAto: Weathering? (Y/N) TOA = Oh TOA = 4h TOA = 6h TOA = 12h TOA = 22h TOA = 40 h TOA = 68 h TOA = 162 h 1 wk 1 wk 1 mo 1 mo ly ly 10y 10y T0y T0y Y N Y N Y N Y N Y N 143.0 143.3 154.0 154.5 158.0 162.1 159.1 162.1 159.1 165.2 42.8 43.0 53.3 54.2 58.2 61.4 58.5 62.1 58.5 62.6 37.0 37.3 A474 48.3 52.3 55.5 52.6 56.4 52.7 56.8 28.5 28.8 39.0 39.8 43.9 47.0 44.2 47.9 44.2 48.3 21.3 21.5 31.8 32.6 36.7 39.7 37.0 40.6 37.1 41.1 14.7 14.8 25.2 25.9 30.1 33.0 30.5 33.8 30.5 34.4 9.1 9.1 19.7 20.3 24.6 27.4 24.8 28.2 25.0 28.8 0.34 0.34 11.1 11.5 16.1 18.6 16.5 19.4 16.5 20.0 10' I island to another. Over time, however, exposure rates in 40° oeaeeeneeee nn a crieioone cna iemea ewe: scheoec cue eeeeemed meemaumceneeeiCe = 2 IE faeennsnncnnunnn esonatennnes gpm ants ncaa 4 , areas near the shore became lower compared to exposure rates in the center of islands as a consequence of weathering, human activity, and intermittent flooding mo Ol ee wettdct cesses teeeeeedeeseeeeteeesaees = from storms. The exposure rates were also much lowerin subsequent years than during the first year after fallout. B 40° L- In this work, we have assumedthat our estimated outdoor . 3 Q , —o— No weathering, R/V=0.5] | S& —*— Weathering, R/V=0.5 —— No weathering, R/V=1.0 10° || —#— Weathering, RIV=1.0 40° 10° RR a Dl | 0 ey seeps teaewu eeeueen aera. tape ocoemneceuueeet wep neneuUaneucemeur emus cule 10° 10° | | ! | 10' 10° 10° 10° 10° Time post-detonation (h) Fig. 3. Exposure rate as a function of time after detonation with and without a weathering correction at two fractionation levels, R/V = 0.5 and R/V = 1.0. necessarily reflect the variation from one area of an island to another or to indoor conditions. The potential reduction of exposure due to shielding by building materials when inside traditional Marshallese houses would be small as suggested by measurements made after Bravo that indicated that native housing did not appear to substantially attenuate the fallout radiation (Sharp and Chapman 1957; Conard et al. 1975). Contemporary measurements of outdoor exposurerates (Fig. 4), however, show substantial variation from one area of any island to other areas. At downwind distances where most atolls were located, fallout debris clouds were, for the most part, larger than individual islands. For that reason, we believe that fallout deposition was usually relatively homogeneous over any given island. Therefore, during the first year after fallout, when over 97% of the lifetime exposure occurred (Table 2), there waslittle difference in the exposure rates from one area of any exposure rates, based on the original fallout levels, were representative of the average conditions under which people lived during the periods of maximum exposure, but we recognize that this assumption may haveresulted in a very slight overestimation of the cumulative exposure as Marshallese spend muchoftheir time in village areas that are typically near the lagoon shore. In order to calculate the organ andtissue doses from the free-in-air exposure data, one must first convert exposure to dose in air using a factor of 8.75 X 10° Gy R|. Then, a factor of 0.75 Gy Gy' wasused to convert from dose in air to dose in tissue or organ. This factor of course varies with the energy of the radiation and the orientation with respect to radiation incidence (NCRP 1999; Eckerman and Ryman 1993; ICRP 1996), as well as with the organ and tissue that is considered and with the anthropometric characteristics of the person. Because there is little difference between the values of this conversion factor for one organ to another for gamma-ray energies of a few hundreds of keV that are typical for fission products (Jacob et al. 1990; ICRP 1996), the same value was used for all organs and tissues that were considered in this study and also would be used if the effective doses were to be calculated. The conversion factor from dosein air to effective dose was taken as 0.75 Sv Gy| by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1993) and by the National Council on Radiation Protection and Measurements (NCRP 1999) for adults exposedto fall- out. The net conversion from exposure in air to tissue or