276 



TEMPERATURE AND HUMIDITY 



for men. In comparison with the benefits 

 conferred by variations in clothing, we may 

 conclude that no human adjustment of 

 basal or resting heat production is of major 

 importance as a practical long-term protec- 

 tion against either extremes of temperature. 

 A quantitative study of the total metab- 

 ohsm of submarine crews engaged in typi- 

 cal routines would yield a reasonable 

 estimate of the average daily heat produc- 

 tion. In the absence of such studies, 

 considerations described above lead to the 

 conclusion that this figure is in the range 

 from 3000 to 4000 kg.cal. per day per man. 



TABLE V 



DiFFEBENTIAL THERMAL BEHAVIOR OF HeATED 



Mannikin and Human Subjects When 



Exposed to Temperatures of 



25°, 30°, AND 35°C 



Human Temperature Regulation 



In the preceding section it has been shown 

 that rest and work are systematically 

 associated with heat production. The 

 energy for this process is derived from food 

 combustion within the body. Hence, it is 

 possible to consider that human occupancy 

 of confined spaces is analogous to occupancy 

 of the space by multiple heat engines, 

 consuming fuel, and producing heat and 

 motion. A characteristic difference between 

 the mechanical and the biological process, 

 however, lies in the low temperature at 

 which human combustion proceeds (near 



37°C) and the elaborate and sensitive mech- 

 anism within the human body which holds its 

 internal operating temperature near to this 

 fixed point, despite large variations in heat 

 production, and wide swings in external 

 temperature. 



The reality of this thermal regulation and 

 its efficiency may be illustrated by reference 

 to Fig. 1 and Table V. Fig. 1 shows a 

 copper replica of the human body. When 

 filled with water and supplied with internal 

 heat equivalent to 0.75 met, the gradient of 

 temperature from the internal water mass 

 through the skin to the outside air in a room 

 at 30°C is given in Table V. By regulation 

 of the fluid circulation rate the gradients of 

 heat loss estabUshed may be made quite 

 similar to those found in a resting human 

 subject at the comfortable and thermally 

 neutral condition of 30°C. When the 

 temperature of the environment is raised or 

 lowered by 5°C, every step in the tempera- 

 ture gradient of the heated mannikin reflects 

 in an exact manner the temperature change 

 in the environment. If the human body 

 responded to a change of db5°C in the 

 environment in the same manner, the in- 

 dividual would be prostrated with heat at 

 35°C and rigid with cold, perhaps un- 

 conscious, at the 25°C condition. However, 

 as one may see in Table V, this is not at all 

 the case. The human data of this table 

 indicate that the deep internal temperature 

 is quite constant over this 10°C range of 

 environmental temperature. If we add to 

 this primary thermoregulatory ability of the 

 nude body the additional range conferred by 

 voluntary choice of clothing and regulation 

 of heat production through activity, the 

 range of external temperature over which 

 body temperature may be maintained near 

 37°C for limited periods extends at least 

 from -40°C to -f 50°C. There is, however, 

 a substantial physiological cost attached to 

 such survival adjustments, and maximum 

 human efficiency is preserved only in a 

 narrow range from approximately 15° to 

 28°C for the clothed subject. 



