Changes of thermal balance induced by passive heating in resting man

Abstract

Heat acclimatization has been induced in 12 resting healthy men by 90-min exposure to 45C dry bulb and 24% relative humidity for 9 successive days. The most significant results ovserved were 1) increased sensitivity of sweating with marked quickening of sweat measured, 2) decreased rate of body heat storage associated with a lower rectal temperature at end of exposure, as follows: 14.07 plus or minus 1.58 Wtimeshtimeskg-1 before and 9.39 plus or minus 1.69 afterward for body heat storage; 37.55 plus or minus 0.15C before and 36.99 plus or minus 0.24C afterward for rectal temperature. In contrast, no significant changes were observed in the final sweat rates, mean skin temperatures, or the heat conductance between the body interior and skin surface. The quickness of the heat dissipation process caused by both increased sensitivity of sweating and lower internal body temperature is the major factor in achieving a thermal balance and a decreased body heat content after acclimatization.

Physiological factors associated with the onset of sweating.

“…Nevertheless, when the differences in subjects' thermal state prior to exercise were taken into account, the variations between subjects and even within subjects remained large. Moreover, the average "threshold" heat storage at the onset varied from one study to another: from 109 (HoUDAS et al, 1973) to 62 kJ/m2 (HENANE and BITTEL, 1975) in resting subjects, and from 64 (CHAPPUIS et a!.,1976) to 37 kJ/m2 (present study) in exercising subjects. This large range of values does not rule out the body heat content as a determining factor in the initiation of sweating but at least suggests that its threshold level may partly depend on the exposure conditions.…”

“…The large variability in heat storage at onset observed within and among subjects might be related to the shorter duration (30 min) of the resting period in a neutral environment in comparison to its duration (90 min) in other studies where variability was lower (HoUDAS et al,1973;HENANE and BITTEL, 1975). Nevertheless, when the differences in subjects' thermal state prior to exercise were taken into account, the variations between subjects and even within subjects remained large.…”

“…The importance of core temperature in the initiation of sweating may be emphasized further when comparing the present data and data reported in resting subjects exposed to similar step changes in ambient temperature. In resting subjects, average sweating delays were 9.3 min when exposed to 50°C (COLIN and HOUDAS, 1965) and 15.3 min when exposed to 45°C (HENANE and BITTEL, 1975), while the average TSk at the onset were 37.5 and 37°C, respectively. At rest, the onset of sweating occurred thus significantly later and with higher skin temperatures than in the exercising subjects of this study.…”

“…Other factors such as the subjects' maximal aerobic power, state of training, or state of heat acclimation (HENANE and BITTEL, 1975;RoBERTS et a!., 1977) may have contributed to the differences observed among studies. Although subject's posture often differed between resting and exercising exposures, this factor seems to have little influence on the onset of sweating (JOHNSON and PARK, 1981).…”

Heat and mass transfer in air-fed pressurised suits

“…Normal body temperature T b0 is about 310 K and this is achieved when there is thermal balance between the amount of heat produced by the body and losses due to a number of different causes. The heat storage q S may be expressed as (Henane et al [4]; Gonzalez et al [2]; Havenith [3])…”