ON HEAT CONDUCTION; 98.6° F: A CONSTANT? 



227 



at 27° C {T a = 300°K) and if the body is uncovered, up to 100 Cal/hr could 

 be lost to the surroundings as infrared electromagnetic radiation alone. 

 For vaporization, the rate, v A , is given by 



v< = K A AJ(v/d)SP 



where A w is the wetted area of exposed skin; v is the velocity of the air; d is the 

 effective thickness of the heated layer of air on the surface of the skin;/(z>/rf) 

 describes the convection which carries the moisture away; and AP is the 

 driving "force/" i.e., the difference in vapor pressure, P, of the liquid on the 

 surface at skin temperature and that of water at the ambient temperature - 

 the latter reduced by the relative humidity, RH. The important factor is the 

 last one. Thus the liquid on the surface strives to set up an equilibrium pres- 

 sure of vapor with the atmosphere which surrounds it, but never quite suc- 

 ceeds, since the atmosphere is nearly always undersaturated (RH < 100 per 

 cent). For example, if the skin temperature is 34°C (91°F) and the RH = 

 60 percent for an ambient of 20° C (68° F), quite common conditions, 



IP = P(34°) - 0.6P(20°) = 0.04 atm 



At very high temperatures {T a > 80°F) this method is the body's escape 

 valve for excess heat. Each gram of water lost by vaporization removes 0.58 

 Cal from the skin. In the lungs, inhaled air becomes saturated and then is 



TABLE 8-11. Estimated Per Cent of Heat Loss, by Each of Four Principal Methods. 



*Assume 50 percent relative humidity. See Refs. 2 to 4, and 21 



