496 L. H. G R A Y 



would only ionize such a cell about once a week. Bearing in mind 

 that some changes in the living cell, including the production of gene 

 mutations, are almost certainly produced by all types of ionizing radia- 

 tion, and as regards each individual radiation in strict proportion to the 

 administered dose, whatever the duration of exposure, it remains a 

 matter of the greatest interest for future research to establish whether 

 or not these infrequent ionizations of the cell have played some part 

 in the course of organic evolution. 



4. Ionizing Radiations as a Health Hazard 



In recent years, particularly in America, many investigations 

 have been directed to discovering what changes are produced in ani- 

 mals exposed for long periods of time to very low dose rates of radia- 

 tion of various kinds, in order to ascertain the hazards to which those 

 working with ionizing radiations may be exposed. The lowest dose 

 rates so far investigated have been about 100 times as great as those 

 referred to in the last paragraph. y rays, jS rays, neutrons, and a 

 particles were used in some of these investigations (4) . The various 

 species of animals examined showed characteristic types of response. 

 In most cases some significant departure from the normal, e.g., de- 

 crease in life span, decrease in testicular weight, increase in body 

 weight, and hematological change, was observed in animals exposed 

 throughout life to 2.2 r. per day of 7 radiation. Carcinogenic effects 

 referred to in the next section (A5) were observed at still lower dose 

 rates. 



It is of great importance to those engaged in research with ioniz- 

 ing radiations to know what is the maximum intensity of radiation 

 to which they may be exposed without incurring serious risk of injury 

 to health. The only recommendations of international standing re- 

 garding the maximum permissible daily exposure to personnel refers 

 to X and y radiation. It is that adopted by the International Con- 

 gress of Radiology in 1928 and is to the following effect: "The evi- 

 dence at present available suggests that under satisfactory working 

 conditions a person in normal health can tolerate with impunity ex- 

 posure to X rays and radium 7 rays to an extent of about one inter- 

 national roentgen (r.) per week." 



The evidence referred to in the recommendation has been dis- 

 cussed by Henshaw {5), who outlines the confused subsequent history 

 of the subject arising out of pronouncements by national committees 



