292 



TEMPERATUKE AND HUMIDITY 



secretion above minimal values. At the 

 lower end of this zone, evaporation is mini- 

 mal and vasomotor regulation of skin insula- 

 tion is the primary regulatory process. 

 Near the higher region of the zone, moderate 

 evaporation combines with vasomotor regu- 

 lation. Ease of regulation is the feature of 

 this region. This area of unstressed equi- 

 librium would, of course, be shifted up or 

 down the operative temperature scale for a 

 subject of different body build or in different 

 positions, or for a clothed subject, or one 

 performing physical work. 



The mean values for a group of subjects 

 (17) indicate thermal equilibrium between 

 28° and 33°C, marked negative heat change, 

 and strong skin vasoconstriction, appearing 

 below 28°, and dominant sweat secretion 

 above 33°C. With clothed subjects, on the 

 other hand, the area of equilibrium is wider, 

 as might be expected, and it occurs at a lower 

 environmental temperature. Negative heat 

 change began in this case only below 25°C 

 and active sweat secretion only above 29°C. 

 With a subject performing active physical 

 work (45), more variable results were ob- 

 tained. The highest positive figure for heat 

 change was 83 Calories (with no other posi- 

 tive value over 31); the highest negative 

 value was —47 Calories, although operative 

 temperatures went as low as 12.4°C. These 

 are heat changes per man (not per square 

 meter), and the activity of the working sub- 

 ject corresponded to metabolic rates of over 

 300 Calories. Figures for evaporative heat 

 loss were also more variable with active 

 work, and they indicated onset of active 

 sweat secretion at 19°-21°C operative tem- 

 perature. The mean evaporative heat loss 

 for two subjects was 48 Calories at tempera- 

 tures below 19°C and 152 Calories above 

 21°C. 



Thus, the area of thermal equihbrium may 

 vary from 28°-33°C for the nude subject at 

 rest to 19°-21°C for the same subject per- 

 forming active work. 



Zone of Body Cooling 



At operative temperatures below 28°C in 

 Fig. 5 is an area known as the Zone of Body 

 Cooling. As one proceeds do^Tiward to suc- 

 cessively lower environmental temperatures,^ 

 heat loss due to radiation plus convection in- 

 creases progressively, from 46 Calories to ob- 

 served values of 181 and 188 Calories in 

 these experiments. In the same range, heat 

 loss due to evaporation decreases only from 

 42 to 22 Calories. The balance is taken up 

 by heat change in the body tissues, which 

 rises from —6 Calories at equilibrium to 

 — 116 Calories at operative temperatures of 

 18° and 18.5°C. The line representing nega- 

 tive heat change is essentially parallel to the 

 line representing heat loss by convection plus 

 radiation. 



Zone of Evaporative Regulation 



Above operative temperatures of 33°C (for 

 the nude resting subject) lies the Zone of 

 Evaporative Regulation. Here, as one 

 passes to higher and higher environmental 

 temperatures, heat loss due to convection 

 plus radiation decreases and, above an opera- 

 tive temperature of 35°C, changes to a pro- 

 gressively increasing heat gain. While 46 

 Calories were lost by this avenue in the zone 

 of easy equilibrium, 102 Calories were gained 

 from an environment at an operative tem- 

 perature of 41 °C. However, increased heat 

 loss from evaporation balances this situation 

 with precision, rising from 42 Calories at 

 equilibrium to 192 Calories at 41°C opera- 

 tive temperature. As a result of this adap- 

 tive process, there is no appreciable increase 

 in positive heat change. The value of this- 

 factor was 6 Calories for equihbrium and 4 

 Calories for environments at 40°C and over. 



In general, the results of the Pierce Labo- 

 ratory experiments correspond with reason- 

 able closeness to the findings of the earlier 

 workers of the Laboratory of the American 

 Society of Heating and Ventilating Engi- 

 neers at Pittsburgh. The Heating, Venti- 

 lating and Air-Conditioning Guide of the 



