after a long latent period. The bone doses have always been large, the minimum being 1500 r in one of the reported cases. The fallout equilibrium concentration of Sr*° in humanbones of 20 pc per gram of Ca, when it is attained, will deliver 0.056 rad per year, on the basis of a uniform skeletal distribu- tion. The equilibrium concentration will obtain in individuals who have been exposed to fallout throughout life. Therefore, the sr® has been assimilated gradually and, since chemically strontium is very similar to calcium, a uniform distribution throughout the skeleton may well be expected. Certainly it should be more uniform than in the case of the radium dial painters who started ingesting the material in adult life. Furthermore, a spotty distribution of the Sr*? itself would produce a more uniform dose distribution in bone than in the case of radium, because of the much greater ranges of the beta rays of Sr? + yas comparedto the ranges of the alpha rays of radium. Accordingly, much higher dose rates than 0.056 rad per year, due to local Sr*° concentrations, are very unlikely. It is difficult to see, therefore, how local doses of the magnitude required to produce bone damage, and subsequently bone sarcoma, can pos- sibly be reached in a lifetime. In 70 years the accumulated uniform distribution dose would be 3.9 rads, without allowing for radioactive decay of the moreor less “fixed” sr*® initially incorporated in the skeleton, which would make it even lower. Any reasonable allowance for non-uniform distribution cannot make the dose large. If the mechanism for the induction of bone sarcomais as outlined above, it is evident that proportional extrapolation to very low dose rates is hardly justified. Therefore, a dose rate in bone of 0.056 rad per year, which is about one half of the natural background level, may well be expected to produce no bone sarcomasat all. On any plausible basis, even the absolute number for the population of the United States cannot be large, since the annual deaths from bone sar- coma are approximately 2000. 4.5 Life Shortening A statistical shortening of life has been obtained experimentally by exposing animals continuously or intermittently throughout life at daily rates in excess of 0.5 r. In some experiments exposure at the rate of 0.1 r per day seemed to prolong the average life. Since the number of animals used in these experiments has been too small to produce statistically significant results at this radiation level, no definite conclusions can be drawn from low level exposure experiments. Estimates of the life shortening in man quoted in the literature, ranging from 5 to 20 days per roentgen, have been derived from theoretical relationships between dose andlife shortening based on animal experiments in which relatively large doses or dose rates were used. These estimates give a greater appearance of accuracy than is warranted by the basic data. A recent survey conducted by Shields Warren shows that the average age at death for ra- diologists who died between 1930 and 1954 was 60.5 years as compared to 65.7 years for other physicians having no known contact with radiation. The doses received by these radiologists in the exercise of their profession are estimated to vary from rather low values to about 1000 r. Dublin and Spiegelman, on the basis of a shorter period of study (1933 to 1942), found no life shortening for radiologists. Hardin B. Jones, in a study of a group of radiologists and on the basis of a reanalysis of certain of Warren’s data, finds that they have the same death rate risk as the general population at ages under 60, but over 60 the death rate is about twice as high as expected. It should be noted that the higher incidence of leukemia among radiologists cannot lower appreciably the average age at death, because the disease is very rare, even among radiologists. The question is whether the not inconsiderable doses of radiation formerly accumulated by radiologists over a long period of time may have caused an acceleration of the aging process. A categorical answer to this question cannot be given at this time, but it may be assumed that the higher levels of radiation to which some of these radiologists were exposed may have caused an appreciable life shortening in a statistical sense. The estimated accumulated dose from fallout gamma rays (0.1 r in 30 years) is so small in comparison to the occupational exposure of radiologists in the past fifty years, that this effect if it exists at all at very low dose rates, can only be extremely small, a few days at worst. 274