THERMAL INTERCHANGE WITH ENVIRONMENT 



287 



of evaporative regulation, relative humidity- 

 does not change Ht. This is understand- 

 able, as Gagge points out, since the change in 

 the vapor pressure factor {Es — RHEa) is 

 compensated for by an increase in the wet 

 area of the skin. 



The effect of wind upon the wetted area 

 and upon Hv will, of course, depend more 

 upon the change in total heat loss than upon 

 the immediate alteration in Hv caused by 

 suddenly blowing air upon the body. That 

 is, if the skin is sweating with a value of w/z 

 of 50 percent in quiet air, turning on an elec- 



Hr can be measured rather easily by observ- 

 ing the weight loss of the man, and it is 

 necessary to do this under the various condi- 

 tions of environment before com can be de- 

 termined. 



As seen in Fig. 4a, evaporation in the 

 physiological zone of temperature regulation 

 is not affected by humidity because coju 

 compensates to keep Hv independent of hu- 

 midity (Fig. 4b). Gagge showed that ufj. 

 has a maximum value of about J28.5 

 kg.cal./m.Vhr./cm.Hg. for two nude sub- 

 jects and that this represents a 100 percent 



120- 

 100- 

 80- 

 60- 

 40- 

 20- 



— 

 80- 

 60- 



40- 

 20- 

 O — 

 120- 

 100- 

 80- 

 60- 

 40- 

 20- 



SUBJECT I 



Subject iv 



Subject vii 



IS 20 2S 30 3S 40 



100- 

 80 

 O 60- 

 iJ 40 

 5 20 

 l/> o 



Subject 



AIR Temperature in 



Fig. 4. 4-a (left), Evaporative heat loss in clothed subjects in relation to air temperature. 4-b, 

 (right), Percentage of maximum possible area of wetted skin surface (w) in relation to air temperature 

 (solid circles high humidity, open circles low humidity). 



trie fan will cause a large initial fall in co/x 

 with a subsequent readjustment to a new un 

 which will just let Hv balance the heat equa- 

 tion. It is for this reason that the laws of 

 vaporization relating air velocity, tempera- 

 ture, vapor pressure, etc., as determined 

 from physical analogues of the skin, are 

 valid only under conditions which are pre- 

 determined by observations made on the 

 skin. 



Formulae 12 and 12a are deceptive in 

 their simplicity, and the physiological vari- 

 able of the wetted surface should not be lost 

 sight of, since the only practical way to 

 measure coju is to measure Hv Fortunately, 



wetted skin. By measuring oin for other 

 conditions of the atmosphere, the degree of 

 wetness of the skin can be calculated by 



TF% = ^X100. 



Winslow, Herrington, and Gagge (43) have 

 done this for the nude man for high and low 

 humidities and for environmental tempera- 

 tures between 16°C and 42°C. The evapo- 

 rative loss is approximately the same 

 throughout their experimental range, regard- 

 less of the relative humidity. 



In summary, one may conclude that it is 

 not possible at present to measure the ther- 



