XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 497 



and by individuals. The latest recommendation of the British X- 

 Ray and Radium Protection Committee (1943) is identical with that 

 given above, which was essentially a British recommendation in origin. 



Assuming a five day work week, the maximum permissible ex- 

 posure is 0.2 r. per day. In the 1936 report of the United States Ad- 

 visory Committee on X-Ray and Radium Protection and permissible 

 daily exposure is reduced to 0.1 r. per day and this figure has been in 

 force in United States atomic energy projects. In the Canadian 

 atomic energy project the figure has been further reduced to 0.05 r. 

 per day. 



The following are the maximum permissible doses of other ionizing 

 radiations received in any one day, which have been adopted at one 

 American atomic energy station as being, to the best of available 

 knowledge, consistent with a daily exposure of 0. 1 r. of X or 7 radia- 

 tion. 



Alpha particle 0.01 r.e.p. 



Beta particle 0.1 r.e.p. 



Ganima ray 0.1 r.e.p. 



Fast neutron . 02 r.e.p. 



Thermal neutron . 05 r.e.p. 



The r.e.p. (roentgen equivalent physical) may be defined as the dose of 

 radiation that will result in dissipation of 84 ergs per gram of tissue 

 (Sect. Dl). These figures are given only as a guide. They have no 

 international authority and their validity would probably be ques- 

 tioned by some of those who have studied the subject most deeply. 

 All that can safely be said is that any revision is likely to be in the 

 direction of a further reduction in the allowed exposures. 



5. Ionizing Radiations as Carcinogenic Agents 



Recent investigations (4) have provided abundant evidence of 

 the power of ionizing radiations to produce tumors. Single exposures 

 to the skin of rats of about 4000 r. of radiation from P^^ were found 

 to induce a multiplicity of skin tumors including almost every known 

 pathological type varying from papillomas to the most malignant 

 kind of carcinoma (6). The incidence was less in mice and not sig- 

 nificant in guinea pigs and rabbits. Repeated small daily exposures 

 did not produce tumors in any of the experimental animals l)ut it is 

 well known that in man chronic exposure of the skin to /3 radiation 

 is carcinogenic. Whole body exposure to both 7 rays and neutrons 



