HEALTH PHYSICS AND RADIATION PROTECTION 89 



tion commonly encountered and also in terms of biological response, 

 Tables 3-2 and 3-3 present some illustrative values. 



Table 3-2. Approximate Radiation Dosages Commonly Encountered 



Source Dosage 



Cosmic and natural radioactivity 0. 0008 r/day 



Back of luminous-dial watch 02 r/day 



Diagnostic X ray, chest . 05 r per film 



Fluoroscopy 0.3 r/sec 



Dental film 5 r 



Diagnostic X ray, pregnancy, lateral 9 r 



X-ray shoe-fitting machine 10-15r per fitting 



[From Frederick P. Cowan, Everyday Radiation, Phys. Today, 5: 10-16 (1952); 

 C. F. Behrens, Permissible Dosage and Considerations of Calculated Risk, Naval 

 Medical Research Institute, NP-354(j, July 2, 1951; Harold H. Plough, Radiation 

 Tolerances and Genetic Effects, Nucleonics, 10: l()-20 (1952).] 



External Hazards. Maximum Permissible Levels. There is need to 

 know the amount of radiation that may be received without harmful 

 consequence by human beings during their daily work. The term tol- 

 erance, which is often used, implies that there is a radiation level that 

 can be tolerated with complete safety. Since there is as yet no substan- 

 tial evidence to support this concept of a threshold response to radiation, 

 the adjectives maximum permissible are commonly used to indicate a 

 reluctance to guarantee freedom from damage. When reliable clinical 

 or experimental data are available, the practice has been to set the permis- 

 sible levels about ten times lower than those known to have produced 

 minimal effects. References (4, 6, 19, 26, 30 to 32, 35 to 38) may be 

 consulted for a general discussion of the philosophy of the tolerance 

 concept. 



On the basis of extensive experience of radiologists and technicians in 

 work with X rays and radium therapy, as well as more limited animal 

 experimentation, the U.S. National Committee on Radiation Protection 

 has recommended the tolerance dose, or maximum permissible dose, of X 

 and gamma rays to be 0.3 r/week. This applies to absorption over the 

 whole body. In the case of exposure of the hands and forearms to X, 

 gamma, and beta radiation, the maximum permissible dose is recom- 

 mended as 1.5 r/week or its energy equivalent (4). It is known that a 

 given dose of radiation will produce more damage if delivered over a 

 shorter period of time. Consequently it is not good practice to accept 

 routinely high dosages which are then balanced by an appropriate radia- 

 tion-free period. (See note added in proof, page 113.) 



Spalding et al. (39) have recently surveyed typical radiation dosages as 

 received by X-ray technicians and isotope users employing less than 



