288 



TEMPERATURE AND HUMIDITY 



mal factors of an environment by means of 

 physical instruments alone and thereby pre- 

 dict in every case the value of the evapora- 

 tive heat loss of a man. This is due to the 

 fact that the wetted area of the skin is a 

 variable physiological factor, the behavior of 

 which is not kno-uTi for all environmental 

 conditions. For the conditions in which the 

 wetted area is known, the evaporative heat 

 loss can be predicted from the measurement 

 of air temperature and velocity, relative 

 humidity, skin temperature, and the char- 

 acteristic dimension of the body for convec- 

 tion. The cooling effected by evaporation 

 may be computed from the weight of water 

 evaporated and the latent heat equivalent 

 of water at the temperature of evaporation. 



The Adaptations of the Human Body to 

 Varying Thermal Conditions 



Our discussion of the characteristics of 

 human heat production and of the physical 

 nature of the processes involved in heat in- 

 terchange with the environment has pre- 

 pared the way for a description and analysis 

 of the thermal behavior of man when exposed 

 to various ambient conditions. In this sec- 

 tion material dealing with the unclothed 

 body is stressed. Similar material for the 

 clothed body is available in the reports of 

 Winslow, Herrington, and Gagge (44), and 

 Gagge, Winslow, and Herrington (17). 

 However, since in undersea operations heat 

 stress is less well understood and perhaps 

 more critical than cold stress, the reactions 

 of subjects wearing only underwear, lowers, 

 or supporters are of greater value than data 

 on subjects fully clothed in civilian dress. 



Combined Iniiuence of Various 

 Environmental Factors 



Numerous attempts have been made in 

 the past to simplify the measurement of the 

 various environmental factors influencing 

 human comfort. In England, the Dufton 

 Eupatheoscope (12) and the Vernon Globe 

 Thermometer (4), and in this country an 

 instrument called the Thermo-Integrator 



(39) were devised in the hope of recording a 

 single figure which would indicate the com- 

 bined influence of air temperature, air move- 

 ment, relative humidity, and mean radiant 

 temperature upon human comfort. All 

 these attempts have been abandoned as the 

 various investigator discovered that the 

 physiological reactions of the human body 

 vary so widely at different points in the 

 temperature scale that no single physical 

 instrument can provide a true picture of the 

 environmental influences concerned. Only 

 an independent determination of the four 

 distinct factors mentioned above can give a 

 real measure of the thermal demands of the 

 environment (35). 



The Research Laboratory of the American 

 Society of Heating and Ventilating Engineers 

 at Pittsburgh approached the subject from 

 another standpoint. Its investigators meas- 

 ured the temperature, velocity, and relative 

 humidity of the air (ignoring radiation 

 effects, since air and walls were at the same 

 temperature). They then obtained from 

 their subjects votes as to the relative comfort 

 of the conditions maintained, and prepared 

 a graph of "Effective Temperatures" repre- 

 senting (at several rates of air velocity) the 

 combination of air temperature and relative 

 humidity yielding equal sensations of com- 

 fort. These graphs have become standard 

 for practice in air conditioning, and have 

 given reasonably satisfactory results under 

 hot conditions. There is serious question, 

 however, as to their validity under environ- 

 mental conditions which are in the cool to 

 moderate range, and recent modifications 

 have been suggested which should remedy 

 the previous undue allowance for the warm- 

 ing effect of high humidities in the moderate 

 comfort range. 



At the Pierce Foundation a figure repre- 

 senting the combined influence of air tem- 

 perature and mean radiant temperature has 

 been found of value, since the physiological 

 reactions to these two factors are so similar 

 (as is not the case where evaporative re- 

 sponse to relative humidity is in question). 



