R. L, BLANCHARD *10Po in the tissues for much longertimes thanis reflected bythe effective half-life of the particular organ. Inthe case of the caribou meatcaters as observed here, only *4°Po was ingested in larger than “‘normal” amounts. Consequently, there is no skeletal reservoir of “°Po supported by %°Pb, and once the subject ceases to eat and J. B. MOORE 131 to only a few hundrea mrem/year. Although these dose rates may be snaller than previously estimated for those cating caribou meat, it should be remembered that, except for #7, the subjects available for this investigation were not eating caibou or reindeer meat on a daily basis at the time of their death. Assuming the caribou or reindeer, the excess 7!©Po is excreted clfective half-life of “Po in the liver and kidney For example, the ICRP lists the effective hali- centration of “Po in these two tissues of subject #8 three months prior to this death when he is reported to have stopped eating caribou quite rapidiy froin the soft ussucs of the body. lives of *Po in liver, kidney and spleen as 32, 46 and 42 days, respectively.“ This probably explains why the body burden appears “normal” for subject #3 who ate caribou regularly while residing at Barrow, but not while residing in Anchorage during the three years preceding death. Except for the concentrations in the kidneyand liver which appearto be significantly higher than normal, the same reasoning can explain the apparent ‘‘normal’” 7!Po concentrations in tissues of subjects #4 and #5. In addition, the #°Po tissue concentration of subject #8 who ate reindecr regularly until three months before his death was undoubtedly much higher while eating caribou than was observed at the time of his death, The body burden of *!°Po was estimated for subject #8 by summing the products of the concentration observed in each tissue multiplied by the tissue mass, based on the 70 kg “‘standard man’’.23) Onthis basis, approximately 60% of the total body mass was analyzed. ‘The concentration in the remaining 40% was assumed equal to that in muscle. ‘Phe *!°Po body burden of this subject, so calculated, was estimated at death to be 1.7nCi. Taking 7p the ICRP recommended value for an occupational exposed population as the maximum permissible body burden for the general population, the estimated body burden at death, 1.7 nCi, is about one-half the maximum burden if the spleen is assumedthecritical organ." If it is assumed that the 2!°Po is distributed uniformly within the organ and if 10 is used as the RBE for 7!°Po alpha particles, then the dose rate in mrem/year is numerically equivalent to the concentration of 21°Po in the units of pCi/kg of tissue. Consequently, the dose rates delivered by #®Po to these tissues may be read directly from Table 2. Thesoft tissues which contain the higher levels, kidney and liver, are exposed as 32 days and 46 days, respectively, the con- meat was about 1780 pCi/kg and 840 pUi/ke, respecuvely. rate ‘This of about two corresponds and =! to reimf/yr, a dose respec- tively, if the assumptions menuoned above are correct. In making the above extrapolationto estimate the issue concentration 3 months prior to death, it was assumed that the metabolism during the terminal 3 monthsofillness was norinal, the diet during this interval contained no food with abnormally high levels of #4Po, and that the biological parameters given by ICRP for 74°Po are accurate. The first assimption mav not be truc and the 7!°Po cxeretion rate may have been different than that of a healthy person. It is, however, unlikely that this individual consumed food during hospitalization which contained high concentrations of 7!°Po, and the [CRP values, although possibly requiring somerevision, are the best presently available. Consequently, the person who consumes caribou meat daily will probably receive larger dose rates from the *10Po during the time of ingestion than is indicated bythe results shown in Table 2. Tissue levels for such individuals would be extremely valuable; however, autopsies are rarely performed on subjects of this population, and to obtain autopsy tissue samples will be extremely difficult. Lichens and caribou In order to determine if thermonuclear explosions in the arctic contributed significantly to the “Pb levels in the arctic ecosystem, samples of lichen and caribou (Rangifer tarandus) bones which had been collected betore the advent of nuclear testing in the arctic (1951), were analyzed for 74°Pb andRa, The collection data and analytical results for the lichen