ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



289 



They derived a factor referred to as "Opera- 

 tive Temperature," defined as the sum of 

 the radiation constant multiplied by the 

 mean wall temperature, and of the convec- 

 tion constant multiplied by the mean air 

 temperature, divided by the sum of the two 

 respective constants. The operative tem- 

 perature is not a physical condition, since it 

 is weighted by the factors of convection area, 

 radiation area, and air movement; but a 

 measure of heat demand which allows for the 

 physiological reactions involved. When air 

 movement is moderate (25-30 feet per 

 minute), however, the convection and radia- 

 tion constants are very close to each other, 

 and operative temperature is not radically 

 different from the mean of air and mean 

 radiant temperatures. For the conditions 

 which obtain in ordinary indoor spaces, the 

 engineer can safely assume a mean between 

 air and wall temperatures as representing 

 operative temperature. When operative 

 temperature occurs in the discussion, the 

 reader may assume that the condition is sub- 

 jectively similar to the thermal condition he 

 associates with an air temperature of the 

 same value. 



The General Physical Phenomena of Heat 

 Interchange 



The thermal interchanges between the 

 body and its environment are expressed by 

 the formula: 



(13) M-EzLCd^R = ^H,ov 



M - E ± AnK{ATw) ± ^c(ATx) 



V = MI. 



All units are expressed in kg.cal. per hour. 

 M = observed rate of metabolism, E = rate 

 of coohng due to the sweat actually evapo- 

 rated, Ar = the effective radiation area for 

 a given subject in a given position, in square 

 meters, K = {-UzTs^) from the first approxi- 

 mation of Stefan's law {k being the universal 

 radiation constant), I^Tw = difference be- 

 tween skin temperature and mean radiant 

 temperature in 0°C, Ac = the convection 

 constant for a given subject in a given posi- 



tion, ATa = the difference between skin 

 temperature and air temperature in 0°C, 

 V = mean turbulent velocity of air move- 

 ment in cm. /sec, and Ai/ = change in heat 

 content due to shifts in mean body tem- 

 perature. 



The actual relative magnitude of the heat 

 interchange by various avenues may vary 

 within wide limits. It is sometimes stated 

 that radiation accounts for about two-fifths 

 of the heat loss from the body, convection 

 for two-fifths, and evaporation for one-fifth. 

 This is approximately true for a resting sub- 



TABLE VII 



Variation in Percentage of Heat Loss by 



Radiation, Convection, and Evaporation 



UNDER Different Conditions 



(from DuBois, 14) 



ject at low environmental temperature with 

 little air movement, low relative humidity, 

 and air and walls of about the same tempera- 

 ture. The mean of six of our experiments 

 with a nude subject with both air and walls 

 at 24.3°C showed that 21 percent of the 

 actual heat lost to the environment was due 

 to evaporation, 37 percent to radiation, and 

 42 percent to convection. 



Variations in atmospheric conditions will, 

 however, produce the most diverse ratios. 

 DuBois (14) reports figures given in Ta- 

 ble VII. 



Clearly no general statement as to the per- 

 centage relation of various avenues of heat 

 loss can be useful. By means of parti tional 



