The severe shade conditions of deserts may now be viewed in the light of these 

 values. In the table, the desert effective temperatures are given for two levels of air 

 speed, fairly still air (30ft/min.) and air at 300 ft/min. (4m.p.h.), at two levels of 

 humidity, namely 20 and 30%, for individuals wearing light clothing. 



It will be seen that up to dry bulb temperatures as high as 105°F desert condi- 

 tions yield effective temperatures not far from those which appear to be just tol- 

 erable for individuals acclimatized to hot climates. These figures, admittedly only 

 approximate, do enable one to evaluate more objectively the limits of tolerance to be 

 expected in desert conditions and in fact, despite the greatly increased heat flux the 

 body must cope with (see below) these limits are probably higher than usually thought 

 to be the case. No studies of indigenous people, however, appear to have been made 

 from this as from other points of view. The nature of acclimatization is also by no 

 means understood though, as we shall see, there are certain physiological changes in 

 the body which proceed in parallel with increased tolerance. 



3. Capacity for Work 



As indicated in the table, the desert on many days in the summer affords con- 

 ditions, even in the shade, more trying than the upper limit of the 'comfort zone* of 

 80 — 82°F effective temperature of acclimatized individuals. The heat load on the 

 inactive indoor individual is derived by convection and radiation from the surroun- 

 dings. Out of doors the direct and indirect solar heat load will be added to these. 

 Nevertheless it can be shown that there yet remains a fair margin to the body's capa- 

 city to maintain homeothermy even after dealing with the heat gain from the exterior. 

 This is the margin available for coping with the heat production of muscular work. 

 To understand how great this margin is likelyto be it is only necessary to consider 

 the maximum rate of cooling which the body is physiologically capable of developing. 

 In the desert conditions under consideration this is entirely dependent on evaporation 

 of water from the skin surface (that from the lungs adding only about 5 — 10%). It is 

 possible to predict the maximum cooling capacity of the body for any particular set of 

 desert conditions where the total surface area of the skin surface is regarded as ef- 

 fectively wetted and where a skin temperature of not higher than say, 97 or 98°F is 

 assumed. In fact, a physical body shaped like the human body, kept wet at this sur- 

 face temperature, could lose heat at the rate of about 500 or 600 Kcals/hr up to air 

 speeds of, say, 5m.p.h. In very severe desert temperatures (E. T. of 90°, air move- 

 ment 300 ft) the convective heat load impinging on the body might be of the order of 

 100 Kcals/hr, the radiation heat load on a man in the standing position might be about 

 150 Kcals/hr, so that 250 Kcals/hr remain for metabolism and work but this margin 

 would drop off rapidly as the air movement fell. 



195 



