144 Health Physics The method of estimating external doses can be based on either historical data of ground-level exposure rates, or, alternatively, on data on the deposition density of particular radionuclides contained in the fallout, such as °’Cs, combined with information on the ratio of the nuclide activity at the time of fallout to the exposure rate at that time. Crude exposure estimates can also be made from retrospective estimates of '’Cs or *’Sr inventories measured in soil samples, provided one can estimate the relative contributions from each of the tests to the total measured inventory. The reliability of dose estimates is dependent, however, on having reliable estimates of the time of transport of the fallout from the detonation point to the receptor point. Those data, called the fallout “time-of-arrival” (TOA, measured in h), can consider- ably affect the dose estimates for locations relatively close to the detonation point (i.e., within a few hours transit time for the fallout). In a separate paper, Beck et al. (2010) describe available post-test data on measured exposure rates and provide estimates of both TOA and ground deposition densities (Bq m*) of '*’Cs based on those and other types of data. '*’Cs deposition density estimates were developed for each of 32 atolls and separate reef islands of the Marshall Islands from each of the 20 tests that took place at Bikini or Enewetak that resulted in measurablefallout on the atolls.* Estimates of fallout TOA were also developed for those tests and atolls so that estimates of dose from external irradiation could be reconstructed using either method. The names, dates, and yields of the 20 tests that deposited fallout on any of the inhabited atolls or separate reef islands, other than the test site atolls themselves, are provided in Simon et al. (2010a, Table 1). In a companion paper (Simon et al. 2010b), the doses from internal irradiation also are estimated for all the tests and atoll populations that are considered in this paper. The risks of cancer resulting from the doses arising from exposure to radioactive fallout from regional nuclear testing in the Marshall Islands, taking into consideration age andatoll of residenceat the time of the tests, are assessed in another companion paper (Land et al. 2010). MATERIALS AND METHODS The doses from external irradiation were estimated in three basic steps: * The reader will note that this work does notattempt to quantify the deposition on the test site atolls (Bikini and Enewetak). Not only was the contamination on the islands of those atolls very heterogeneous, but they were monitored extensively for many years and those data are reported elsewhere. Moreover, those atolls were never inhabited during the testing years. August 2010, Volume 99, Number 2 1. estimation of the outdoor exposure rates at 12 h at each atoll after each test and of the temporal variation of the exposure rate after eachtest; 2. estimation of the total external exposures from fallout from TOA to infinity, obtained by integrating the estimated exposure rates over time assuming continuous residence on the atoll (with corrections for relocated populations); and 3. estimation of whole-body and organ doses by applying conversion factors from outdoor exposure to tissue dose. Estimation of the outdoor exposure rates The outdoor exposures at each atoll following each test have been assessed in one of two ways depending on whether measured exposure rates were available for the times and locations of interest. If historical data on exposure rate were available, the data were assessed and a best estimate of the island- or atoll-average exposure rate at 12 h post detonation (termed £12) was made. Because the quality of the exposure-rate measurements varied by test and location, expert judgments were often used to determine the appropriate weighting of measurements of varying quality. As discussed in Beck etal. (2010), many of the reported measurements were made before all the fallout from a test was deposited, while other measurements were obtained many weeksafter the test when the exposure rate had been attenuated due to weathering of the fallout by rainfall or humanactivities. Of course, neither would have been as preferable as high-quality ground-level exposure-rate measurements made soon after deposition was complete. If no reliable exposure-rate data were available to estimate F12 directly, then the dose estimation method used was that developed by the Off-Site Radiation Exposure Review Project (ORERP) for estimating external whole-body and organ doses from fallout originating at the Nevada Test Site (NTS) (Hicks 1982). That method relates the '*’Cs deposition densities and fallout TOA values to £12 using ratios of '’Cs to E12 for a range of times developed specifically for some of the tests considered in this paper (Hicks 1984). The types of data provided by Hicks (1981, 1982, 1984) are: (1) calculated exposure rates from all radionuclides in the fallout debris relative to a reference exposure rate of | mRh'atH+12(12h post detonation), at 31 times after detonation, ranging from 1 h to 50 y, and (ii) related radionuclide ground deposition densities, expressed in pCi m*, for more than 60 of the most important fission and activation products (the numbervaries from onetest to another). Activities of fission products per unit of