28 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



Despite the existing gaps in our knowledge, it is abundantly clear that radiation is by 

 far the best understood environmental hazard. The increasing contamination of the atmos- 

 phere with potential carcinogens, the widespread use of many new and powerful drugs in 

 medicine and chemical agents in industry, emphasize the need for vigilance over the entire 

 environment. Only with regard to radiation has there been determination to minimize the risk 

 at almost any cost. 



Members of this group and of its subpanels, while recommending various points of de- 

 parture for greater consideration and further research, were in no case of the opinion that any 

 sort of "crash program" would be desirable or profitable. 



The time period during which persons may be overexposed to radiation will have much 

 influence on the overall effects. For example, total body irradiation in a relatively short period 

 of time, as occurred in Japanese atomic bomb casualties and in a few accidental exposures in 

 atomic energy plants, caused early clinical effects reflecting mainly injury to the blood-forming 

 tissues and intestinal tract, which have great powers of recovery, as well as leukemia and other 

 delayed effects in various organs. 



Where, on the other hand, exposure has been suffered at a relatively low level from time 

 to time over a period of years, a variety of injurious effects may be encountered, such as leu- 

 kemia and skin cancer. Among those who have adhered to present permissible dose levels, 

 none of these effects have been detected as yet. 



Shortening of life span may result from exposure to radiation not only as a consequence 

 of damage to a specific tissue, as seen in the development of skin cancer and leukemia, but 

 also as a result of such general factors as lowered immunity, damage to connective tissue, or 

 "premature aging." There is some evidence in animals that radiation effects contributing to 

 shortening of life span may depend upon genetic constitution and on the age and physical or 

 clinical status at the time of exposure. In general, for given dose rates, the survival time is 

 shorter the more radiation energy absorbed. Life shortening is generally less, however, for a 

 given total dose absorbed over a long period of time as compared with a short period of time. 

 Life shortening in man has not been demonstrated following small doses of radiation. 



Statistical studies of mortality of U. S. physicians, comparing radiologists with other 

 physicians or with the general male population, indicate that occupational exposure of U. S. 

 radiologists may have caused an increase in mortality in past decades. Since the increase is 

 of borderline significance, it is not yet possible to make quantitative determination of life 

 shortening. A study of British radiologists suggests no increase in mortality rates among them. 



A life shortening effect in man as a consequence of substantial total body irradiation can 

 be estimated reasonably on the basis of animal experimentation and on the basis that such ex- 

 posures increase the incidence of leukemia in human populations. However, there are as yet 

 no data for man that provide a satisfactory basis for quantitative estimation of the overall life 

 shortening effect, the existence of a dose threshold, or of the dependence of the effect on dose 

 and dose fractionation. 



The lethal dose for partial body irradiation exceeds in general, that for the whole body. 

 A small volume of tissue may receive many thousand roentgens without death resulting. This 

 permits doses much greater than the lethal level for total body radiation to be employed in 

 radiation therapy. 



Radiation may have its prominent effects in particular parts of the body when it is applied 

 locally, and this may take place in two ways. First, an external source may be so handled as 



