ADAPTATIONS OF HUMAN BODY TO VARYING THERMAL CONDITIONS 



299 



the subject secreting 900 grams at 43 °C. 

 Vertical lines dra-wTi dowTiward on the graph 

 would indicate these limits, and similar 

 limits are set for the resting nude in moving 

 air, for the subject producing 500 grams at 

 46°C, for the subject producing 900 grams 

 at 51 °C, and for the subject producing 900 

 grams at 56°C. At any point below the 

 slope of the graph and to the left of the 

 appropriate vertical line, the subject can 

 maintain thermal balance. Above the slope 

 and to the right of the vertical line, adapta- 

 tion is no longer possible on the basis of these 

 assumptions. 



The general topic of adjustment to ex- 

 treme heat and cold has been the subject of 

 many excellent studies within the past few 

 years. The relations noted above have been 

 investigated for a number of different condi- 

 tions of exercise, clothing, and type of cli- 

 matic exposure. Many of these studies 

 grew out of mihtary interests of the recent 

 war period. This has a bearing on the use of 

 such concepts as limit and tolerance in rela- 

 tion to human climatic endurance. Neither 

 term is susceptible of exact definition, since 

 human motivation inevitably enters in the 

 practical testing of such concepts. In gen- 

 eral, the limits suggested by our work as 

 reported in the foregoing discussions are con- 

 servative. Interested readers will find in a 

 recent book by Adolph (1) an extended dis- 

 cussion of the many factors involved in the 

 problems of limit and tolerance in relation to 

 heat stress. Taylor and Marbarger (38) 

 have extended this work to include a special 

 treatment of tolerance times in environments 

 in which ultimate adjustment is not possible. 



When the upper hmits of evaporative 

 regulation are reached, disastrous conse- 

 quences ensue. With our clothed subjects, 

 profound discomfort was manifest with high 

 relative humidity (70-80 percent) even at 

 atmospheres between 37° and 39°C, and 

 some subjects showed nausea and other sub- 

 jective symptoms so severe that the experi- 

 ments had to be discontinued. Under such 

 conditions, skin temperatures and rectal 



temperatures begin to rise sharply and, what 

 is even more serious, metabohsm rises also, 

 thus creating a vicious spiral. 



The data obtained in our Laboratory cor- 

 respond well with the findings of McConnell, 

 Houghten, and Yaglou (30) that saturated 

 air at 32.2°C is the "upper limit of man's 

 ability to compensate for atmospheric condi- 

 tions" in still air. They are also in accord 

 with the reports of Cadman and Haldane (7). 

 Cadman states that at 29.4°C wet-bulb tem- 

 perature, the body temperature invariably 

 rises, while at 33.9° wet-bulb "one is in a 

 terrible state"; Haldane states that at 31°- 

 32°C wet-bulb "in fairly still air, the body 

 temperature begins to rise, even in the case 

 of persons stripped to the waist and doing 

 no work; and when air is saturated this rise 

 continues until symptoms of heat stroke 

 arise." These limits may be tolerated and 

 exceeded by highly motivated, physically fit, 

 and well-acclimatized subjects. However, 

 no situation is easily conceivable in which 

 the physiological and psychological cost of 

 adjustment can be considered permissible as 

 a standard practice for personnel entrusted 

 with serious duties. 



These effects of heat stress are not, of 

 course, instantaneous. The Pittsburgh in- 

 vestigators found exceptional subjects who 

 could endure such extreme conditions as 

 70°C with a relative humidity of 15 percent 

 for half an hour. The body temperature 

 under such conditions rose several degrees. 

 The pulse rate seemed to be the best measure 

 of sensations, 135 beats per minute corre- 

 sponding to marked discomfort, and pulse 

 rates exceeding 150 being almost unbearable. 

 Restlessness, irritability, headache, palpita- 

 tion of the heart, soreness of the eyes, a 

 severe oppression of the chest, dizziness, and 

 confusion, were among the sensations experi- 

 enced ; and weakness and a dragged-out feel- 

 ing persisted for some time after such experi- 

 ments had closed. 



Under such conditions of very active 

 sweating, the body, of course, suffers from 

 water loss and, in even more serious degree, 



