dioactivity was deposited in ected at increased distances west by the water currents. ratios of Co57/Zn** gradually alue of 5.5 at 480 miles, and 2, Thus, with increased time alt in the plankton increased Zn* and Fe5 . turn are eaten by the pelagic e rate of movement of these ankton, and thus they would nulated by feeding upon the we a high turnover rate for 1, they would tend to excrete ing their migrations through lioactive zinc and iron. Thus able to the plankton, whereas _ up in a reservoir. The ratio therefore increase with time o fallout contamination, the ew weeks’ interval belonged sms these radioelements are Sy90__90 and (s137__Bal37m e land plants. Thus either a for the transition elements, onversely, in the case of Sr®? ision occurs in the sea. Strom lout samples from the 1956 rays which had been exposed average ratio of abundance 1/4, and the Co®/Cs13" ratio has a half-life approximately resence of Sr® would increase Sin the sea, one would expect times or more that of Co®, and land animals. However, - iniwetok at levels of approxi80 is absent, or present in only ut, zinc and manganese, the of Sr® and Cs!3? in marine , Ca(OH,), and calcite, would sion-product elements. Thus wllout in the sea in the parti- petition by similar elements stable counterparts, neither manganese and zinc would to a finely-divided form with thermocline, and thus these vailable to the plankton in Many of the radioisotopes concentrated by plankton organisms are biologically important to man. However, they do not necessarily have a long biological half-life in man, as has been stated elsewhere (2). Of the isotopes present in plankton in significant amounts for the first 48 hours after contamination, only Cel#!-144__py141-144 (biological half-life 500 days) and Np?#*—Pu*9 (biological half-life 4.3 x 104 days)* have a biological half-life in man greater than one year. The other isotopes present include Mo*®—Tc®™ (150 days), Te}32__J132 (15 days), Ba!4°—La!*° (approximately 200 days) and Rul03—105—106— Rh}03-105-106 (20 days), Of the isotopes present in plankton six weeks after contamination, only Ce!44—Pr!44 has a long biological half-life. All of the other radioisotopes found in plankton at this time have biological half-lives of 65 days or less. These include Zr®5—Nb®5 (50 days), Ru!°®—Rh}06 (20 days), Zn (23 days) Co47 58 6 (9 days), Fe*5,(65 days), and Mn** (5 days) (Handbook-52). Two isotopes which would be of long-term consequence as hazards to man because of their long biological half-life if taken up by plankton are Sr%_Y2 (3.9 x 108 days) and Eu15 (1.4 x 108 days). However, these radioisotopes are not accumulated in any significant amount by marine plankton. The biological half-lives of all of the isotopes concentrated by plankton are short in comparison with those of two radioisotopes commonly used in medicine, Ca*® (18 x 105 days) and P%? (3.2 x 10% days). However, the biological half-life is not the only criterion for determination of hazards of radioisotopes in humans. Another important consideration is that of radioactive half-life. The two isotopes Ca*5 and P®2 are relatively innocuous because of their short physical half-lives. In fish tissues the predominant radioisotopes have short biological halflives in man. They may therefore be tolerated in food or water for human consumption at a much higher level than Sr®°9—Y®, Fe55 may be present at a level of 8,000 times, Zn®> 1,000 times, Co5” 5,000 times, Co**® 1,000 times, and Co® 500 times that allowed for Sr®® (Handbook 69). The most important fission products in fallout on land, from the standpoint of hazard to human consumption, are Sr9°-—Y°°, Cs!37__Ba}3™ are potential hazards to a lower degree (21, 22, 23). The principal route for these radioelements is from the soil through the plants to man. The plants at Eniwetok and in the near vicinity tend to accumulate Sr9°—Y%and Cs!3?Bal3’m from the soil with greater efficiency than they do the other radioisotopes (Table IV). In soil from Eniwetok Atoll, Sr°°9—Y° accounted for 5.9% and Cs!3?Bal37m for 3.8% of the total radioactivity: In three plants from the same island, sr%°_Y® accounted for an average of 7.3° and Cs!*’—Ba13’m for 85% of the total radioactivity. On a direct comparison basis of disintegration rates of these isotopes in soil and in wet plant material, however, Sr® was present in the plants at a level approximately one-sixth of that in the soil, and Cs!*7 at a level, on the average, of approximately BS times that in the soil. In 12 samples of scaevola leaves collected at Rongelap Atoll in March 1958, Sr°°_y% accounted for approximately 26° and Cs!3’-Ba!3’™ for 34% of the total radioactivity. In pandanus plant tissues from Rongelap collected in 1958, the leaves contained the lowest amounts of Sr®°—Y° (0.37 to 3.1%) and Cs13?7__Bal3™m (1.2 to 3.5%). In fruit, Sr® contributed 5—7°%, of the total radioactivity and Cs!8? 77—95%. Wood from the pandanus tree contained the * Pu) in the soluble form. 9 TTAeT ENT eTRR “aeme . . : . . 129