J. F. LOUTIT 



Stratospheric bank still to be deposited, the position is rightly of concern now. 

 If, on the other hand, these values in sheep bone are chiefly a function of the 

 rate of deposit, \vhich we know from Stewart's observations has been rela- 

 tively constant in the last few years at between 2 to 3 mC/km^ per year, one 

 should now begin to see some signs of equilibrium. Recent^ observations 

 suggest that such an equilibrium in sheep bone is being attained. The 

 same phenomenon, however, can be seen more readily and cjuickly as a 

 result of the analyses of milk. Bryant and his colleagues have since 1954 

 been analysing the dried milk from a factory in Frome, Somerset. There is 

 only one observation for 1954 but many for 1955, 1956 and 1957. Their 

 report^ shows that, while there is a considerable scatter in the figures in any 

 individual year, the mean or median is practically constant for those three 

 years in spite of the fact that the accumulated fall-out of ^"Sr has risen 

 seven-fold in that time. 



The same report gives the figures for the analyses of human bones derived 

 from autopsies made in 1956. The activity of ^°Sr in strontium-units was, 

 as one would expect, a function of age, the maximum concentration being 

 in the first few years of life. The activity in the bones of still-births was in the 

 region of 0-4 S.U., rising to about 0-8 at the age of two years and falling 

 off thereafter to much lower levels. The maximum in the first year or so of 

 life merely indicates that the child population has only been exposed 

 significantly to ingestion of fall-out for about two years. One would expect 

 that with the passage of time the peak would shift. There is evidence of this 

 given in a later report'^. A plateau now seems to extend between the ages 

 of a few months and three years. The mean and median values for still- 

 births are still about the same — at 0-5 S.U. The median level in the age 

 group of to 5 years was increased over the figure for 1956; but as the 

 population sampled was known to be different, being predominantly the 

 population in the west and wetter parts of the country, this is not surpi-ising, 

 nor are the few individual values which are higher than the highest for 1956. 

 As far as one can interpret the data therefore, the rate of deposition of ^''Sr 

 has been constant for three years; the concentration of ^°Sr in milk has been 

 constant for the same length of time; and allowing for the difference in 

 human population sampled, the radio-activity of recently deposited bone is 

 also reasonably constant. 



^^^Caesium has not received the same amount of attention so far as ^''Sr, 

 but the analyses of Booker* at the Atomic Energy Research Establishment 

 show that the levels of ^^'Cs in the dried milk from Frome have also been 

 approximately constant during this time. Since caesium has a much faster 

 turnover in the mammalian body than strontium, one can usefully investigate 

 the caesium content of the adult, who should within a relatively short space 

 of time come into equililDrium with his environment. As ^^"Cs is a nucleide 

 which emits y-rays, the body content can be assayed by measuring the total 

 output of y radiation with a whole-body counter, and with the appropriate 

 discriminator one can identify the specific y-rays of ^^'Cs. Rundo's measure- 

 ments at Harwell^ on the same individuals over the last year or so show that 

 there is little alteration in total body content of ^'^'Cs. The contamination 

 of both milk and the human subject thus seem to be at present dependent 

 also on rate of fall-out. 



15 213 



