3. The exeretion rate is quite important. A single dose in- gested will be 50% excreted in approximately two weeks and 95% excreted within a year. The variously estimated 2-5% remaining may be considered permanently fixed in the bone. 4. 19.9 Strontium-90 has a half-life of 20_to-30-years and decays through the radioactive isotope yttriun-90 to the stable element zirconium-90 by twobetaemissions. Strontium-89 has a half-life of 55 days and decays by a singlebetaehiission’to stable yttrium-89. A graphical comparison in Figure 2) shows that the strontium-89/ strontium-90 ratio of initial intensities is approximately 130/l. If this is reduced by a factor of 2 to take into account the two to 1 ratio of beta particles emitted per disintegration by strontium-90, it results ina 65/1 initial radiation intensity ratio. This ratio by virtue of the relatively long half-life of strontium-90, may be considered as being reduced by a factor of 2 every 55 days. The curves in Figure 21 indicate the relative intensities of strontium-89 and strontium-90 present at times after deposition in bone assuming the limiting extreme of immediate deposition after detonation. With proper graphical pre- sentation, the areas under the curves give the relative proportions of strontium-89 and strontium-90 radiations that will occur in the bone. Calculations based upon the above considerations indicate that strontium-90 will be the greater contributor of the total dose to bone. Pathway Through the Biosphere. Some qualitative and quanti- tative understanding of the behavior of strontium-90 as it passes through the physical er.vvironment is necessary in order to interpret the data relating to pathway through the biosphere. No significant studies have appeared which suggest that inhalationpathway into the human is important. Therefore, the mechanical and biological transport of strontium-90 through the food chain will be given primary consideration. Intake of water contaminated with strontium-90 will be considered as augmenting the food chain. 102