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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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The study of the afferent system upon which the regulation of sweat production depends is hindered in many circumstances by circulatory adjustments which may alter the thermal topography of the tissues. Thus cooling one region is known to produce reflex vasoconstriction elsewhere, which may alter the temperature of and temperature gradients within the skin of regions other than that deliberately cooled. Since the thermal parameter to which the relevant cutaneous thermoreceptors may be sensitive is unknown, appropriate correction for these effects is impossible. The apparent disagreement between the results of Kuno (1956) and Hill (1921), for example, may be reconcilable on the basis of secondary effects of this sort.The difficulty may be overcome by working under conditions in which there is no heat exchange from the general body surface. Neither the skin temperature nor the thermal gradients within the skin are then affected by alterations of the cutaneous circulation, and the effects of cooling one region on the rate of sweating from another may be examined on the assumption that the output of receptors elsewhere is unaffected by the procedure.The present paper reports the results of an investigation conducted under these conditions. METHODSThe bath. The subject lay in a rectangular metal tank 170 cm by 60 cm by 40 cm deep. This was fitted with an overflow pipe standing 37 cm above the base, which determined the normal working depth of the water. Stirring was provided by a pump situated at one side near the subject's feet. Water taken in at this point was delivered through a closed loop of copper pipe which ran round the bath about 5 cm below the surface of the water, and which was drilled at intervals to distribute the water. Supplies of hot and cold water were led through control valves to pipes terminating near the pump inlet. The temperature of the bath was held equal to the subject's mouth temperature by manual adjustment of the water supplies.The subject's head was above the water and was enclosed in an insulated box fed with saturated air at the same temperature as the bath water. False walls of aluminium foil were hung inside the box so that the temperature of the surfaces surrounding the subject's head might be assumed to be close to that of the bath. No direct measurement of the rate

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SUMMARY1. The relation between the rate of sweat production, S, and the rate of weight loss, W, has been examined under conditions in which the rate of evaporation was small.2. 9 could be found from TV provided that a film of liquid was maintained over the skin surface. This could be achieved initially by immersing the subject in water containing detergent. Thereafter the film was maintained so long as the rate of weight loss exceeded about 10 g/min.3. When the rate of weight loss was changing and the rate ofevaporation was constant, o could be calculated as + 2*5 W.4. When the subject was constantly sprayed with water while being weighed, the correction for W became negligible. In this case there was no lower limit to the sweat rate which could be measured, but spraying considerably reduced the accuracy of the measurement.5. The output of sweat from a ventilated capsule on the forearm correlated well with estimates of central sweating drive based on weight measurements corrected for hidromeiosis.