“ae TT ee py Pe Poree f U.S. PUBLIC HEALTH SERVICE, DIVISION OF RADIOLOGICAL HEALTH PHOTO FIGURE 6.—Counting a sample of milk for iodine 131. The procedure is quick and simple—the milk is merely poured inte a plastic container and set into the counter. In contrast, analysis of milk for strontium 90 may require weeks including radiochemical preparation of the sample. sures that have hitherto been considered of prime interest for local fallout. However, the total potential doses that may be accrued will require the drinking of the milk over periods of weeks. Up-to-date techniques and equipment now permit a relatively easy and early surveillance of iodine 131 in the milk supply providing an opportunity for whatever action may be appropriate (Figure 6). E. STRONTIUM 90 AND STRONTIUM 89 Background Information Strontium 90 has a half-life of about 28 years. It is selectively deposited in the bones. Chemically it is related to calcium. This similarity has led to the use of the “strontium unit” defined as one picocurie (2.2 disintegrations per minute) of strontium 90 per gram of calcium. Strontium 90 may become associated with foodstuffs by surface contamination of plants or by uptake of the strontium 14 90 from the soil. During years of relatively heavy fallout, surface contamination has accounted for the larger partof the strontium activity in plants but in the absence of atmospheric 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 contamination contributes more to the total strontium 90 activity found in plant life than does soil uptake during these periods. (Incidentally, the cesium 137 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 timeit is one of the better sources of calcium. Rememberit is not just the amount of strontium 90 that is important but also how muchthereis present in relation 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 may be only one-quarter or 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 atmosphere below 100,000 feet (based on measurements using air- craft and balloons)!” with some additional amounts abovethis 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 15