Eras, . yy wee me HM ee evnade tHegeererenapniomage em a ee 90 from the soil. During years of relatively heavy fallout, surface contamination has accounted for the larger part of the strontium activity in plants but in the absence of atmospherie nuclear testing the avenue of soil uptake predominates. The periods showing the highest amount of strontium 90 in the food supply have been invariably the spring and summer months following years of heaviest testing. This is because of meteorological factors and also the fact that surface con- tamination contributes more to the total strontium 90 activity found in plant life than does soil uptake during these me tai sree kehMallbealrhe diet ne periods. (Incidentally, the cesium 1387 content of plant life is even more dependent on surface contamination since only very small amounts are taken up from the soil.) Areas of heavier rainfall consistently show higherlevels of strontium 90. Milk is one of the best indicators of strontium 90 in the food supply, yet at the same time it is one of the better sources of calcium. Rememberit is not just the amount of strontium 90 that is important but also how much there is present in rela- tion to calcium. In fact the total diet has had roughly 1.5 times as great a strontium 90/calcium ratio as did milk alone.'® Strontium 89 has the same chemical properties as strontium 90 and will follow the same metabolic paths. It is created in much larger quantities than strontium 90 but produces less of a problem since it has a shorter half-life (53 days) and emits beta particles with about one-half the energy of those from strontium 90 and its daughter product. For these reasons the strontium 89 content in milk may peak at values many times that of strontium 90 during the periods immediately following nuclear tests, yet the total radiation dose to the bone over a lifetime from strontium 89 maybe only one-quarteror less than that of strontium 90.! The Data About 20 million curies of strontium 90 have been created by atmospheric nuclear tests with about 17 million curies of © this being spread globally. The other 3 million curies fell quickly in areas local to the testing sites. To date, roughly 8-9 million curies of strontium 90 have been deposited globally, leaving a calculated 6 million curies in the region of the atmos- phere below 100,000 feet (based on measurements using aircraft and balloons)!’ with some additional amounts above this level. The discrepancy in total numbers is due in part to radiological decay of strontium 90 but more because of uncertainties in the estimates themselves. As expected, the peak value of “strontium units” in milk was passed in June of 1963 (32 “strontium units” as a national V0 ry 5004 15