268 Applied Biophysics 



plete knowledge of the ill effects which such radiations can 

 produce. It is further necessary to know what quantity of each 

 type of radiation a person can receive continuously without 

 suffering any ill effects. This quantity is called the "tolerance 

 dose." A subsequent task in formulating the scheme is to try 

 to express the particular tolerance dose in terms of a specifiable 

 and reproducible biological standard, which in turn can, for 

 preference, be measured in terms of a physical unit. 



Of the present protective schemes, it can be said that they 

 are built on as sound a basis as existing knowledge of the ill 

 effects of various radiations permits. As more evidence regard- 

 ing blood changes and genetic effects comes to light, it may be 

 necessary to amend the present estimated tolerance doses and, 

 consequently, the protective schemes themselves. 



As regards the effects of X-rays, clinical observations in 

 different countries led to various estimates of the tolerance dose 

 in terms of a somewhat uncertain surface biological effect, 

 namely, the erythema. An average value of the figures pub- 

 lished between 1925 and 1928 indicated that a person could 



tolerate a dose in 3 davs correspondino[ to of the amount 



1,000 



of radiation re([uired to produce an erythema. Meanwhile, 

 work had been in progress with a view to establishing a physical 

 unit for the measurement of quantities of X-radiation. In 1928, 

 the rontgen (r) was accepted internationally as the unit of 

 X-ray quantity. Shortly before this, Kiistner ^^ circulated a 

 questionnaire to a number of institutions which were using deep- 

 therapy apparatus (which, at the time, operated mainly at 200 

 kilovolts), asking them to state the amount of radiation which 

 produced an erythema. The average of the values given to 

 Kiistner, when translated into rontgens, was 600 r. The tolerance 



dose thus corresponds to rontgens in 3 days, or 0.2 r per 



^ 1,000 " ^ ^ 



day. This value is at present accepted as the basis of the recom- 

 mendations of the International and British Committees. On 

 the other hand, the American Advisory Committee on X-ray 



