E. Strontium-90 and Strontium-89 Background Information Strontium-90 has a half-life of about 28 years. deposited in the bones. It is selectively Chemically it is similar to calcium. These facts have led to the use of the "strontium unit" defined as one picocurie (2.2 disintegrations per minute) of strontium-90 per grams of calcium, Strontium-90 may become associated with foodstuffs by surface contamination of plants or by uptake from the soil. During years of rela- tively heavy fallout surface contamination accounts for more of the 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 springs and summers 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 show higher levels of strontium-90. Milk is one of the best indicators of strontiwm-90 in the food supply, yet at the same time it is one of the best sources of calcium. (Remember it is not just the amount of strontium-90 that is important but also how much there is present in relation to calcium). In fact, the total diet has roughly 1.5 times as great a strontium-90/calcium ratio as dots milk alone 12 . .