Physiological and perceptual responses to cyclic heat stress variations.

The effect of the time presentation of a given external heat load was examined on five subjects exercising at a constant work load (50 W). The subjects, dressed in briefs, were exposed to cyclic variations for 120 min in air temperatures between 51 degrees C and 23 degrees C, under three different schedules involving heat pulses of 10-min, 20-min and 30-min duration, respectively. The strain induced by each of these conditions was compared in terms of both physiological and perceptual criteria. Results showed that between conditions, there were significant differences in skin temperature levels but not in core temperature levels, body heat storage, or body weight loss. Perception of effort and thermal sensation ratings both exhibited similar changes in all three conditions. Due to the time constant of the sweating response, sweating rates and skin wettednesses at the end of the heat pulses were lower for 10-min heat pulses than for those of 20- and 30-min duration, and these differences were perceived by the subjects. Lower perceived skin wettedness ratings are thus suggested as the main factor to explain why all subjects rated the 10-min heating-cooling cycle as the least strenuous and uncomfortable condition. It is concluded that under the conditions of this study, perceptual criteria associated with physiological criteria represent a useful means of discriminating slight differences in strain.

Contribution of skin thermal sensitivities of large body areas to sweating response.

The thermal sensitivity of different parts of the body was investigated by heating large areas of the body surface while the mean skin temperature calculated from Hardy and DuBois ' formula (1938) was always kept constant. The right arm sweating responses recorded under a local thermal clamp were related to changes in segmental skin temperatures of the different parts of the body. The results show that: 1) the various local peripheral signals are projected into integrating structures in the central nervous system; 2) the thermal sensitivity is greater for the head-and-trunk area in comparison with other parts of the body. For resting nude subjects, the formula of Hardy and DuBois remains a pertinent way for evaluating the role of skin thermal signals in the central drive for sweating. The peripheral contribution to the central sweating drive depends only on the skin temperature change and on the size of the stimulated area.

REM sleep and ambient temperature in man.

Five young adult males slept two consecutive nights under each of the five ambient temperatures chosen within the usual range: 13 degrees C, 16 degrees C, 19 degrees C, 22 degrees C, and 25 degrees C. Bedding and other ambient parameters were kept constant under all five ambient temperature conditions. The average REM cycle length significantly decreased when the ambient temperature increased from 13 degrees C to 25 degrees C. Other REM sleep characteristics such as total duration of REM sleep, average REM period, and REM sleep latency did not significantly differ from one ambient temperature condition to another.

Sweating and sweat decline of resting men in hot humid environments.

Time courses of the rates of sweating, drippage and evaporation were studied in hot humid environments. Resting subjects wearing only briefs were exposed to humid conditions, before, during and after humid heat acclimation, so that different levels of skin wettedness could be studied on the entire body. In addition, local sweat rate was measured on the right upper limb, which was enclosed in a highly ventilated arm-chamber. Thus, the arm remained drier than the rest of the body surface. The results confirm that sweating efficiency is related to the skin wettedness level, and that the decline in intensity of sweating is linked to maximal inefficient sweat drippage before the onset of hidromeiosis. Comparison of general and local sweat decreases confirms that hidromeiosis originates from skin hydration. However it is likely that some factor related to blood content acts on the hidromeiotic process, at least after humid heat acclimation.

Modifications of sweating responses to thermal transients following heat acclimation.

The sweating response was studied before and after passive humid heat acclimation in four resting male subjects who were exposed to slow thermal transients increasing air and wall temperatures from 28 degrees C to 45 degrees C. The slopes of the ambient temperature increases were +0.19 degrees C . min-1; +0.16 degrees C . min-1 or +0.14 degrees C . min-1. Dew-point temperature and air velocity were kept constant (17.5 degrees C; 0.3 m . s-1). Continuous measurements were made of oesophageal temperature, mean skin temperature, whole-body sweat loss and of right upper limb sweating responses. The local sweating response was measured from an arm chamber under a local thermal clamp (Tsk,1 = 38 degrees C). The results confirmed the fact that heat acclimation to humid heat induces a shortening in the time lag of sweat onset and increases the local sweating rates while internal temperature changes are reduced. These modifications are interpreted as a non-linearity in the response of the central controller, involving both a change in the central gain and an upward resetting of the "local sweating rate-body temperature" curves, without any shifting of the hypothalamic set-point temperature as it is currently described. However, a modification of local sweat gland activity occurring with heat acclimation cannot be ruled out.

Thermophysiological responses to humid heat: sex differences.

1. Thermophysiological responses of four men and four pre- and postovulatory women were compared in humid heat conditions. Responses of pre- and postovulatory women are similar except for body temperature levels, which were significantly higher after ovulation. 2. Pronounced sex-related differences were observed in sweating rate and in body temperature variations. For the same evaporation, the sweat rate in men was higher than in women; as a consequence of this, the dripping rate was larger in men and thus the sweat decline was more important. Body temperature increases were larger in men in function of time and therefore temperature regulation in women was considered to be more efficient.

Central and peripheral inputs in sweating regulation during thermal transients.

Eight nude resting men were exposed to consecutive heating-cooling cycles of air and wall temperatures varying from 28 to 45 degrees C in a sawtooth pattern using one of the following slopes: +/- 3.40, +/- 2.27, +/- 1.70, +/- 1.42, or +/- 1.13 degrees C . min-1. Ambient vapor pressure and air velocity were kept constant at 20.0 mbar and 0.9 m . s-1, respectively. Continuous measurements were made of rectal, esophageal, and mean skin temperatures. Local upper limb sweating response was measured from an arm chamber under a local thermal clamp. The results point out the insufficiency of an explantation based on a simple additive function of core and skin temperatures for describing the sweating regulation. During transient thermal loads, a multiplicative interaction of mean skin and core temperatures must also be taken into account for describing the central drive for local sweating response. The interindividual differences observed in the sweating regulation mechanism seem to be linked to a nonlinearity in the response of the thermoregulatory system.

Effect of hidromeiosis on sweat drippage during acclimation to humid heat.

Sweat rate and the rate of change in sweat drippage were studied during the acclimation of eight healthy male subject during exposure to heat during 10 consecutive days. During acclimation to hot humid conditions, the increase in total body sweat rate results in an increase in the rate of sweat drippage. We found, however, that on each day the drippage rate markedly decreased with time after the 1st h of heat exposure. This hidromeiosis was investigated as a function of the heat exposure time. No shortening of the onset time of hidromeiosis occurred with acclimation. With repeated heat exposures, the initial sweat rates in response to stress increased, and the subsequent decline became larger with higher sweat rates at the time of onset of hidromeiosis. Hidromeiosis appears to be a function of the degree of skin wettedness reached in the various local skin areas which determine the overall body skin wettedness upon which evaporative adjustments depend. Thus, the observed overshoot in total sweat rate as indicated by sweat drippage, and the subsequent hidromeiosis, result from initial oversweating in the poorly ventilated areas of skin. This sweat decline seems to be due to a reduction in output of the active sweat glands rather than to a reduction in active sweat gland number.

Influence of air velocity and heat acclimation on human skin wettedness and sweating efficiency.

Before and after heat acclimation, four male resting subjects were exposed to humid heat that caused levels of skin wettedness ranging from 50 to 100%. The physical experimental conditions were chosen so that the same skin wettedness was attained with modification of only the ambient water vapor pressure, at two wind speeds (0.6 and 0.9 m . s-1). The esophageal temperature (Tes), mean skin temperature (Tsk), sweating rate (msw), and dripping sweat rate (mdr) were recorded; the amounts of local drippage in the same thermal conditions before and after acclimation were also determined. The relationship between the evaporative efficiency of sweating (eta sw) and the skin wettedness (w) is reported, as is the influence of the subject's acclimation to humid heat on adjustments of skin wettedness. The effects of the air velocity on the coefficient of evaporation and on sweating efficiency are discussed. Beneficial increases in evaporation were achievable by increasing skin wettedness only when there was a consistent drippage, which differed from one body area to another and from one subject to another. The relation of drift in body temperature to skin wettedness changed with the acclimation of the subjects.

Effect of rate of change in skin temperature on local sweating rate.

To evaluate the relative contributions of positive and negative variations of mean skin temperature (+/- dTsk/dt) on thermoregulatory responses, male resting nude subjects were exposed to rapid or slow alterations in air and wall temperatures (28--45 degrees C; Pa = 20.0 mbar). Rates of heating-cooling cycles were equal to dTa/dt = +/- 3.40, 1.13, 0.57, 0.38, or 0.19 degrees C/min. Continuous measurements were made of rectal, oral, ear, and mean skin temperatures and of arm sweating (dew-point hygrometer method). During all exposures the local skin temperature was kept constant (Tsl = 39 degrees C). The results showed that peripheral inputs are a major factor in thermoregulatory processes. Cutaneous receptors produce a positive and a negative rate component within the central thermal integrator. A higher rate threshold was observed for the positive rate component than for the negative one.

Human skin wettedness and evaporative efficiency of sweating.

Rates of evaporation and sweating were recorded for three acclimatized male subjects in hot humid conditions, the ambient parameters of which were set so that the various imposed evaporative rates required the same skin wettedness at different levels of sweating. Rectal and skin temperatures were measured. Results showed that during steady state occurring during the 2nd h of exposure each subject reached the required evaporative rate by means of increases in skin wettedness regardless of the level of sweating; the sweat evaporative efficiency, defined as the ratio between evaporative rate and sweat rate, decreased as skin wettedness increased, in a range between 0.74 and 1.0 Sweat efficiency fell to 0.67 for fully wet skin. The body temperatures did not increase with time if skin wettedness was less than unity. Evaporative heat transfer coefficient (he), maximum evaporative capacity, and wettedness were estimated on the basis of the observed decrease of sweat efficiency. The relationship between skin wettedness and sweat efficiency was interpreted as a combined effect of differences in local he as well as in local sweat rates.

Sweating response in man during transient rises of air temperature.

Nude men were exposed to neutral environments (Ta = 28 degrees C, Pw = 20 mbar) changing to warm environments (Ta = 50 degrees C, Pw = 20 mbar). The transient period from neutral to warm environment lasted 4 min (dTA/DT = 5.50 degrees C/min) or 20 min (DTa/dt = 1.10 degrees C/min) or 40 min (dTa/dt = 0.55 degrees C/min) or 60 min (dTa/dt = 0.37 degrees C/min). Continuous measurements were made of rectal and mean skin temperatures and of body weigth loss. Sweating started before appreciable variation in rectal temperature. Onset of sweating could be explained by a peripheral proportional and rate control. Unsteady-state sweating can be predicted by summated stimulation of skin and rectal temperatures. This stimulation could be increased for some subjects by a multiplicative effect due to differences in local skin temperatures. This multiplicative effect occurred during the first transient period.

[Effects of adaptation to work in heat of rectal temperature evolution during recovery]. / Influence do l'adaptation au travail a la chaleur sur l'évolution de la température rectale au cours da la récupéation

Evolution of rectal temperature (Tre) during recovery in different air temperatures was studied following different patterns of heat load before and after adaptation to work in heat (10 consecutive days). Three subjects have been exposed, after a 30 min rest period (Ta=28degrees C, Pwa=14 mb) to 4 heat loads, each producing 1 degreeC increase in Tre in approximatively 30 min (Co:Ta= 50 degrees C, Pwa = 60 mb, W = O watt; C1; 50 degrees C, 42 mb, 50 W on bicycle ergometer; C2;39 degrees C, 38 mb, 100 W and C3: 28 degrees C, 31 mb, 150 W). After of these heat loads, subjects were allowed to recover during 2 h at Ta = 28, 22 or 16 degrees C (Pwa = 14 mb). Results show that: (a) the cooler was the Ta, the faster was the recovery time; (b) before adaptation occurs, the evolution of Tre depended on the preceding heat load pattern; (c) the more intense was the work load, the more the adaptation reduced time for subsequent recovery. The interaction obtained between adaptation and intensity of preceding work load is discussed. The evolutions of leg skin temperatures suggest that a decreased local heat conductance (of inferior limbs) is associated with a local increase in external heat exchange. Adaptation to work in heat would take the form of a local re-adjustment of internal and external heat exchanges.

[Influence of hydrothermal ambient conditions on sweat evaporation efficiency]. / Influence des conditions hygrothermiques ambiantes sur le rendement évaporatoire de la sudation thermique

Sweat efficiency is defined as the ratio between evaporative and sweat rates. The work was carried out on two resting subjects acclimatised to humid heat. Body sweat rate and rate of sweat loss by dripping were recorded separately by continuous weighing. Evaporation from the skin was obtained by the difference between the two weight loss curves. The subjects were exposed for 75 minutes to increases in humidity levels as constant air temperatures (42, 44, 46, or 48 degrees C). The amplitude of the increases was successively equal to 7.5, 15.0, 22.5 or 50.0 mb of water vapor pressure. During the 75 minutes preceding each increase the water vapor pressure of the air was maintained at 20.0 mb. 1. Sweat efficiency decreases prior to complete wetting of the skin surface. The inter-individual mean value of the wetted skin area threshold over which sweat efficiency is less than 1 is around 60%. 2. Sweat efficiency is linearly related to the reciprocal of the required wetted skin area (see article). These results are compared with those of other authors. The differences observed are explained in terms of physiological or physical variables involved in the sweat rate control or in the evaporative sweat loss. These include wetness of skin, posture, activity of subjects and the velocity of air over the skin surface.

[Experimental determination of coefficient of evaporative heat loss in still air (author's transl)]. / Détermination expérimentale du coefficient moyen d'échange de chaleur par évaporation en air calme

The authors have determined the coefficient of evaporative heat loss of the human body (he) by means of humidity steps in low air movement (Va less than or equal to 0,2 m/s). Such a determination requires a fully wetted skin and this implies therefore some loss of dripping sweat. The collection of this dripping sweat allows the determination of the total evaporation: this evaporation exists on the skin surface and around the drops during their fall from the skin to the oil pan where dripping sweat is collected. An estimation of this dripping sweat evaporation allows to assess the skin evaporation and, consequently, the evaporative coefficient he. In these experimental conditions: E = S - SNE - 0,0005 SNE (PsH2O - PaH2O) where E is the skin evaporative rate (g/h);S = total sweat rate (g/h);SNE = the nonevaporative sweat rate (g/h);PaH2O = the partial pressure of saturated water (at Ts) on skin (mb) and PaH2O the partial pressure of water vapor in ambient air (mb). The coefficient of evaporative heat loss in low air movement thus found, is 5,18 +/- 0,22 W/m2-mb.