British Columbia (Atomic Energy of Canada Limited, 1957) and from North Dakota (Harley et al., 1956). The Sr®/sr*° ratio in the October milk samples varies from 12 (Yorkshire) to 24 (Somer - set), which compares with a ratio of 22 found in Chilton grass (Table 3). There is a marked reduction in Sr® activity from the October to the December 1956 sampling, due mainly to the change over from fresh grass eaten by the cows in the open autumn of 1956 to hay, silage, and other stored foods in winter time. If the cows were eating in December hay or silage cut the previous June, the Sr®™ content would have experienced a radioactive decay of three half lives, reducing it by a factor 8, during the storage period. The stable Sr/Ca ratios in the December series of samples were measured spectrographically, and show a range from 200 to 410 micrograms Sr per gram Ca. 6 RELATIVE IMPORTANCE OF MILK AND OTHER SOURCESOF Sr?® Milk is the main source of calcium for growing children in Britain but it does not necessarily follow that it is the main source of strontium or of radiostrontium. Strontium-90 reaches the earth in air and rain, neither of which contains appreciable calcium, and it is therefore possible for Sr®® to enter the human body by routes different from those of calcium. Some possible alternatives are considered in turn. 6.1 Inhalation Stewart et al. (1956) give the mean Sr®® concentration in air at ground level in the years 1952-1955 as 4x 1078 yc/ec. A person breathing at the “standard man” rate of 20 m?/day would have inhaled 10 ypc in the four years. Taking the fraction transferred to bone as 0.22 (ICRP, 1955), the resulting body burden would be 0.002 S.U. The same calculation would apply to a child, except insofar as the breathing rate of an active child may be greater in proportion to its body weight. Inhalation cannot therefore be a major factor in determining the body burden Logg ety th of Sr°*°. 6.2 Drinking Water The mean Sr*® content of rain in 1952-1955.was 1.7 pye/litre. (Stewart et al., 1956). In 1956 it was about 2.5 puc/litre. A litre of liquid milk contains about 1 gram of Ca, so the figures quoted above for the S.U. content of milk can also be read as puc/litre. The Sr*’ content of milk is thus about twice that of rain water, volume for volume. 6.3 Cereals and Vegetables If the plants eaten by man have S.U. levels equal to those found in grass they are potentially an important source of Sr*’, since they enter into diet without the discrimination against strontium which occurs in the production of milk by cows. Adequate data are lacking, but there are indications that the Sr®° levels in cereals and vegetables in Britain are at present about the same as in milk, and considerably lower than in grass.* The reasons for this include the ef- fects of cultivation and the protection afforded against foliar uptake by the outer leaves and husks of vegetables and cereals, and by washing and other preparatory processes. The addition of mineral calcium will depress the S.U. ratio in flour. The relative importance of milk and other sources of Sr*” may change with time, as the importance of foliar uptake and of the effect of cultivation will lessen as the cumulative fallout increases and the Sr*’ becomes more evenly distributed in the top soil. *Hiyama (1957) has reported that brown rice samples in Japan gave 12 8.U. in July 1956, and 104 8.U. in November 1956, There is some doubt whether the latter estimate is of Sr® or of Sr™ and Sr®*. Since the Sr®’/Sr*° ratio in grass towards the end of 1956 was about 20:1, the distinction is an important one. 219 4%