SUMMARY1. The ability of two human subjects to produce sweat was measured before and after immersion for up to 4 hr in water at 32-36°C (soak).2. The ability to produce sweat declined about 4 times as rapidly when the subject was soaked at 360 C as at 32°C.3. The rate of decline characteristic of soaking at 360 C was shown by subjects exercising in water at 350 C, but not at rest at 350 C. The difference appeared to be related to the presence or absence of moderate sweating (300 g/hr) during the soak. At higher rates there was no further increase in the rate of decline.4. Soaking at 390 C for 5 min, after which the water temperature was reduced to 330 C, caused a decline consistent with the supposition that while the subject was sweating the rate of decline was the same as that at 360 C and for the rest of the time the same as that at 320 C.5. It is concluded that the rate of decline is increased if the sweat ducts are perfused, and some possible mechanisms are discussed.
The relationship to be expected between ambient humidity and skin temperature in the steady state, when other conditions are fixed, has been examined theoretically by Woodcock, Powers & Breckenridge (1956), who found it necessary to use two different methods of calculation according to whether the skin was completely wet with sweat or not.In the case of completely wet skin, the rate of evaporation is determined by the air movement and the vapour pressure gradient between the skin surface and the air. For given circumstances, therefore, it is possible to calculate the skin temperature necessary for thermal equilibrium. Similar predictions of equilibrium skin temperature for circumstances in which the skin is completely wet have been put forward by Machle & Hatch (1947) and by Brunt (1947). These predictions are supported by published work relating to the determination of the hottest environments tolerable for long periods of time, if it be assumed that skin temperature is a primary limiting factor. They have also been confirmed in the case of resting men by a method depending on the examination of changes in skin temperature before equilibrium is reached (Kerslake & Waddell, 1958).If the skin is not completely wetted by sweat, the rate of evaporative heat loss depends almost entirely on the rate of sweat production, and is found to be closely controlled at the level demanded by the ambient heat load and the metabolic rate. The only direct effect of ambient humidity in these circumstances is on the evaporative heat loss from the lungs and on the rate of diffusion of water through the skin. Since both these effects are small compared with the total heat exchanges in warm environment, it is to be expected that the rate of sweat production will be little affected by ambient humidity until the skin becomes completely wet. Robinson, Turrell & Gerking (1945) found a linear relationship between skin temperature and sweat rate when the
The exchange of heat between an object and the surrounding air may be considered to take place across a boundary air layer whose thickness depends on the ambient air movement and on the shape, size and surface characteristics of the object. If water vapour is being exchanged between the object and the air, this diffusion must also take place through the boundary air layer, and it might be expected that the rates of exchange of heat and of water vapour should both be related in the same way to the thickness of the boundary air layer. Thus if a certain change in boundary air layer thickness doubled the heat exchange by convection for a given temperature difference, it might be supposed that the water-vapour exchange for a given vapour pressure difference would also be doubled. This proposition involves the assumption that the diffusivities of heat and of water vapour in air are the same, and it happens that they are in fact nearly equal. Discrepancies are to be expected when the temperature difference is great or when the vapour pressures considered are comparable with the atmospheric pressure, but these effects are likely to be small in most conditions encountered in 'hot climate' physiology (Jakob, 1949).The assumption of a constant ratio between the coefficients for heat exchange by evaporation and by convection is implicit in Brunt's analysis of human heat balance (Brunt, 1947), and has been stated axiomatically in that of Woodcock, Powers & Breckenridge (1956), who give the ratio as 2-0, when the units used are kcal/m2.hr.mm Hg and kcal/m2.hr.°C respectively. Observations on three inert bodies, a thermometer bulb, a sphere of 6 in. diameter and a full-sized dummy man, have confirmed this value (Kerslake & Waddell, 1957). The present paper describes the results of an investigation of the ratio in human subjects.
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