Table 6— DISTRIBUTION OF Sr*® IN HUMAN FEMORA (upe/g Ca) Reference HB 38 HB 57 Age (years) Distal epiphyseal plate Distal sub-epiphyseal 14 0.17 9 0.25 0.20 0.34 0.20 0.27 bone Centre of shaft Proximal sub-epiphyseal bone 0.11 0.22 8 DOSE RATE FROM RADIOSTRONTIUM In Table 7 the average dose rate to bone from the Sr*” burden in children in 1956 is com- pared with that due to natural radioactivity (Spiers, 1956). Table 7—-DOSE RATE FROM.Sr®® AND NATURAL RADIOACTIVITY TO BONE Source of Radiation Dose rate, mrem/yr Natural radiation External sources Radium in bone §2 39 Total 121 sr®*® in children under 5 in 1956 Median level (0.70 5.U.) Maximum level (1.55 8.0.) crea fo 2 4 The highest Sr*® activity recorded in 1956 gives an average dose to bone which is one-tenth of that due to the natural radium, when allowance is madefor the relative biological efficiency of the alpha rays,* and one thirtieth of the total natural dose to bone from internal and external sources. 9 MAXIMUM PERMISSIBLE BODY BURDEN OF Sr*® The maximum permissible body burden for occupational workers is one microcurie (ICRP, 1955) which is approximately equivalent to 1000 S.U. The Medical Research Council Committee (1956) have proposed a limit of one-tenth of the occupational body burden for the general population, and state also that “immediate consideration would be required if the concentration in human bones showed signs of rising greatly beyond one-hundredth of that corresponding to the maximum permissible occupational level.” The highest Sr*° activity in human bone found in our series to the end of 1956 is 1.55 S.U. which is “he of the maximum permissible for the general population, and Y of the level above which “immediate consideration would be required.” It has been suggested that a dose rate 10 times the human occupational level would be ac- ceptable for animals (Chamberlain, Loutit, Scott Russell and Martin, 1956). The Sr®° maximum permissible level for sheep would then be 10,000 S.U. The highest levels recorded in Britain are at Cmmystwyth, when 183 S.U. was found in October, 1955 (Bryant et al., 1956) and 160 S.U. in July 1956 (Table 2). *RBE of alpha rays is 10 (ICRP, 1955). 221