II 



The Absorption of Electro- 

 magnetic and Ultrasonic Energy 



I. Role of Nonionizing Radiation 



Both electromagnetic and ultrasonic energy may be absorbed by tissues 

 and cells without any specific damage at the cellular or molecular level. 

 The energy absorbed is converted to heat. If the power absorbed 

 becomes sufficiently large, the cells and their protein constituents heat 

 up to such a high temperature that they are irreversibly altered. The 

 result is identical to the changes produced by the direct application of heat. 

 Many types of irradiation do produce specific types of cellular damage. 

 In the previous chapter, the action of ionizing radiation was considered. 

 These ionizing radiations include electromagnetic radiation, provided 

 the photon energy is sufficiently high. Photons of X-ray and y-ray 

 wavelengths produce ionizations or break bonds within biological cells. 

 Photons of ultraviolet wavelengths excite reactive states in proteins and 

 nucleic acids. In the present chapter, only electromagnetic energy of 

 much longer wavelengths will be considered ; this includes a broad band 

 from the microwave region to d-c electrical currents. 



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