Table 8— AVERAGE NATURAL BACKGROUND RADIATION TO THE SKELETON® Total dose, to Skeletal dose rate, age T0 mrem/year rem 8 0.56 12 12 30 0.84 0.84 2.10 Source of radiation K* (internal) Ra? (internal) 12 Local gamma rays (external) 60 4.20 134 9.40 MsTh(internal) RaD (internal) Cosmic rays (external) Total 0.84 deliver about 8.5 rads* to the skeleton during a 70-year lifetime. This is comparable to the average natural background dose to the bone for the same time period and a factor of about 4 below the maximum natural background dose to which small segments of the general population may be exposed as a result of differences in altitude and natural radium content of soils and minerals. It is a factor of 40 below the lowest skeletal dose which has produced minimal SOURCE AND CONDITION OF RADIATION O08 x-ray (THERAPY) 36yc0 Ra? » wo S 4000 us 8 < Ww 400 40 fe jy O4uc Ratt O4 pe Ra ELLELLLP NO OBSERVABLE EFFECTS 77” LD NATURAL BACKGROUND |” - Y YI; 7 [NO OBSERVAS LE EFF CTS ey LEyy) yyyyy : Yy So meee Ye _og 20ype Sr9°%G Ca (AV. EQ. CONT. TESTS) LE wa ira) BONE,SARCOMA 7/7 BONE SARCOMA _ Ys ALL.SOURCES TO 70 YRS. 4.) c. 904% x “a jjNe aa tthe SIcetee. cever se ay . 0.04 § Jy Y * FIXED IN BONE ~ 25 YEARS % * CONSTANT FOR 20 YEARS, DECAYING WITH 28 YEAR HALF-LIFE TO AGE 70 Fig. 7—Estimated Sr” skeletal radiation dose in relation to human experience. nondeleterious bone changes and a factor of about 10 below the leukemia doubling dose. These data suggest that the present average maximum Sr® equilibrium level will result in a lifetime radiation dose of 1 to 2 per cent of the accepted maximum permissible level for the general population. With continued biospheric contamination indefinitely at the past 5-year rate, the average maximum radiation dose may approach about 20 per cent of the presently accepted maximum permissible level. * Eight and five-tenths rads is the calculated dose assuming incorporation to age 20 and decayto age 70 with no more incorporation. If equilibrium were maintained, the calculated skeletal dose would be about 21 rads. Since some but not all of the skeleton undergoes remodeling plus exchange, somewhere between 8 and 21 rads is probably more correct. 301