288 BULLETIN OF THE TORREY BOTANICAL CLUB (You. 91 elements (Blomeke 1955). Over half of these are radioactive, and most are relatively short-lived. With the exception of [-131 which has a half-life of 8.04 days and a few bone-seekers such as Ba-140 which has a half-life of 12.8 days, short-lived radionuclides are not generally considered to be important as potential biological hazards. The fission products generally considered to constitute a long term biological hazard are Sr-90 and Cs-137. The biochemical behavior of Sr-90 is similar to that of caleium; and when assimilated by animals, it is deposited in the skeleton. The behavior of Cs-137 is similar to that of potassium and therefore, when assimilated by animals, it contributes to the radioactivity of muscle tissue. There are a variety of other fission products (Caster 1959) which accumulate in different animal organs, but their quantities are rather small in comparison to those of Sr-90 and Cs-187. Carbon-14 is produced by both ‘‘atomic’’ and ‘‘hydrogen’’ bombs while Table 5. Estimated production of major radionuclides by nuclear weapons tests up to the end of 1961. Radio- nuclides Sr-90 Cs-137 C-14 H-3 Halflife 28 ¥ 33 y 5500 y 12.5 y Estimated Yield in eurles per K'T “Fission” 143 c* 253 e* 23.4 e** —_— “Fusion” — — 149 c** 6720 c** Estimated Weapons Yield “Fission” “Fusion” 127.5 MT* 18 x 108%¢ 32x 106¢ 3x 106e@ _ 162.5 MT* _ _— 24x 106¢ 1092 x 106 ¢ * Estimates based on data given by Dunning (1962). ** Estimates based on data cited by Leipunsky (1959). H-3 is produced only by thermo-nuclear devices. The assimilation of these radioisotopes by plants and animals has already been mentioned. Table 5 provides a rough estimate of the amounts of Sr-90, Cs-137, C-14, and H-3 produced by nuclear detonations prior to January 1962. These estimates do not necessarily indicate the amounts of radionuclides potentially available for assimilation by plants and animals. There is considerable uncertainty as to the amounts of these materials currently stored in the stratosphere, and the amounts eventually to be deposited as fallout will not be uniformly distributed. Also, as will be discussed in later sections, biological availability is dependent upon the interaction of a great many biotic and abiotic factors which are quite variable from place to place in the biosphere. Assuming the area of the earth to be 2 x 10° square miles, there has been enough Sr-90 and Cs-137 produced by nuclear detonations up to the end of 1961 to result in the world-wide deposition of 90 me (millicuries) of Sr-90 and 160 me of Cs-137 per square mile. It has been estimated (Dunning 1962) that the total deposition of these isotopes in the United States may eventually amount to 125 me of Sr-90 and 230 me of Cs-137 per