The role of oxygen-increased respirator in humans ascending to high altitude

ABSTRACT: BACKGROUND: Acute mountain sickness is common for people who live in low altitude areas ascending to the high altitude. Many instruments have been developed to treat mild cases of AMS. However, long-lasting and portable anti-hypoxia equipment for individual is not yet available. METHODS: Oxygen-increased respirator (OIR) has been designed to reduce the risk of acute mountain sickness in acute exposure to low air pressure. It can increase the density of oxygen by increasing total atmospheric pressure in a mask. Male subjects were screened, and eighty-eight were qualified to perform the experiments. The subjects were divided into 5 groups and were involved in some of the tests at 4 different altitudes (Group 1, 2: 3700 m Group 3,4,5: 4000 m, 4700 m, 5380 m) with and without OIR. These tests include heart rate, saturation of peripheral oxygen (SpO2), malondialdehyde (MDA), superoxide dismutase (SOD), blood lactate (BLA) and PWC (physical work capacity) -170. RESULTS: The results showed that higher SpO2, lower heart rate (except during exercise) and better recovery of heart rate were observed from all the subjects 'with OIR' compared with 'without OIR' (P < 0.05). Moreover, compared with 'without OIR', subjects 'with OIR' in Group 1 had lower concentrations of MDA and BLA, and a higher concentration of SOD (P < 0.05), while subjects 'with OIR' in Group 2 showed better physical capacity (measured by the PWC-170) (P < 0.05). The additonal experiment conducted in a hypobaric chamber (simulating 4,000 m) showed that the partial pressure of oxygen in blood and arterial oxygen saturation were higher 'with OIR' than 'without OIR' (P < 0.05). CONCLUSIONS: We suggested that OIR may play a useful role in protecting people ascending to high altitude before acclimatization.     

Effects of daily and intermittent exposures on heat acclimation of women

Twelve women, who differed in physical condition and body size, were heat acclimated utilizing either a daily or intermittent (every 3rd day) exposure pattern in an environmental chamber. The women walked for 100 min at 5.2 km/h up a 2.5% grade on a motor-driven treadmill Climatic chamber conditions were 46.5°C T_a, 24.5°C T_wb ± 0.5°C. Although individual acclimation varied, significant reduction in heat strain was observed in all subjects, e.g., the ability to complete the assigned task with increasing ease, a decrease in working heart rate, a decrease in rectal temperature rise, a decrease in mean skin temperature, an increase in sweat rate, an increase in evaporative rate, and a decrease in heat storage. The pattern of heat exposures, daily or every third day, had no discernible effect on the rate of heat acclimation. The highly conditioned subjects showed less physiological strain, particularly during the first few heat exposures, and maintained some relative advantage throughout the series of 10 exposures. Body size, in the range studied, appeared to exert little influence on the amount of thermal strain.     

Prediction of human core body temperature using non-invasive measurement methods

The measurement of core body temperature is an efficient method for monitoring heat stress amongst workers in hot conditions. However, invasive measurement of core body temperature (e.g. rectal, intestinal, oesophageal temperature) is impractical for such applications. Therefore, the aim of this study was to define relevant non-invasive measures to predict core body temperature under various conditions. We conducted two human subject studies with different experimental protocols, different environmental temperatures (10 °C, 30 °C) and different subjects. In both studies the same non-invasive measurement methods (skin temperature, skin heat flux, heart rate) were applied. A principle component analysis was conducted to extract independent factors, which were then used in a linear regression model. We identified six parameters (three skin temperatures, two skin heat fluxes and heart rate), which were included for the calculation of two factors. The predictive value of these factors for core body temperature was evaluated by a multiple regression analysis. The calculated root mean square deviation (rmsd) was in the range from 0.28 °C to 0.34 °C for all environmental conditions. These errors are similar to previous models using non-invasive measures to predict core body temperature. The results from this study illustrate that multiple physiological parameters (e.g. skin temperature and skin heat fluxes) are needed to predict core body temperature. In addition, the physiological measurements chosen in this study and the algorithm defined in this work are potentially applicable as real-time core body temperature monitoring to assess health risk in broad range of working conditions.     

Insulation and body temperature of prepubescent children wearing a thermal swimsuit during moderate-intensity water exercise

This study investigated thermal swimsuits (TSS) effects on body temperature and thermal insulation of prepubescent children during moderate-intensity water exercise. Nine prepubescent children (11.0+/-0.7 yrs) were immersed in water (23 degrees C) and pedalled on an underwater cycle-ergometer for 30 min with TSS or normal swimsuits (NSS). The rectal temperature (Tre) was maintained slightly higher with TSS than with NSS. The total insulation (Itotal) was significantly higher with TSS. The DeltaTre, Deltamean body temperature (Tb), and tissue insulation (Itissue) in the NSS condition were correlated with % body fat, which indicated that the insulation layer of subjects with low body fat was thinner than that of obese subjects, and tended to decrease body temperature. Wearing TSS increased Itotal, thereby reducing heat loss from subjects' skin to the water. Consequently, subjects with TSS were able to maintain higher body temperatures. In addition, TSS is especially advantageous for subjects with low body fat to compensate for the smaller Itissue.     

Studies on heat tolerance of subtropical natives after migration to a temperate zone

Anthropometric measurements and observations of physiological responses to heat were made in the autumn on 20 young male Japanese who were born and reared in the main islands of Japan (group M), 20 young male Japanese who were born and reared in Okinawa, subtropical zone, and who have lived in the main islands of Japan, temperate zone, for less than three years (group O). In winter the same tests were made on 25 subjects in group M and 17 subjects in group O. Group O showed lower skinfold thickness and less body fat content than group M. Both groups showed lower skinfold thickness and less body fat content in autumn than in winter. Group O was characterized by lower sweat rate and lower Na concentration in sweat for a given rise in body temperature. In both groups, greater sweat loss, lower Na concentration in sweat and smaller rise in body temperature were observed in autumn than in winter. Group O showed higher heat tolerance than group M when assessed by our numerical index for the assessment of heat tolerance. It was assumed that capacity of non-evaporative heat dissipatïon for group O was superior to that for group M, and the efficiency of sweat for cooling body in group O was better than that in group M. Differences in anthropometrical characteristics and physiological responses to heat between the two groups might reflect more advanced heat acclimatization of subjects in group O when compared with those in group M.     

Human tolerance to heat strain during exercise: influence of hydration.

This study determined whether 1) exhaustion from heat strain occurs at the same body temperatures during exercise in the heat when subjects are euhydrated as when they are hypohydrated, 2) aerobic fitness influences the body temperature at which exhaustion from heat strain occurs, and 3) curves could be developed to estimate exhaustion rates at a given level of physiological strain. Seventeen heat-acclimated men [maximal oxygen uptake (VO2max) from 45 to 65 ml.kg-1.min-1] attempted two heat stress tests (HSTs): one when euhydrated and one when hypohydrated by 8% of total body water. The HSTs consisted of 180 min of rest and treadmill walking (45% VO2max) in a hot-dry (ambient temperature 49 degrees C, relative humidity 20%) environment. The required evaporative cooling (Ereq) exceeded the maximal evaporative cooling capacity of the environment (Emax); thus thermal equilibrium could not be achieved and 27 of 34 HSTs ended by exhaustion from heat strain. Our findings concerning exhaustion from heat strain are 1) hypohydration reduced the core temperature that could be tolerated; 2) aerobic fitness, per se, did not influence the magnitude of heat strain that could be tolerated; 3) curves can be developed to estimate exhaustion rates for a given level of physiological strain; and 4) exhaustion was rarely associated with a core temperature up to 38 degrees C, and it always occurred before a temperature of 40 degrees C was achieved. These findings are applicable to heat-acclimated individuals performing moderate-intensity exercise under conditions where Ereq approximates or exceeds Emax and who have high skin temperatures.     

Alteration of the acute toxicity and various pharmacologic effects of streptomycin sulfate by calcium 4'-phosphopantothenate

The effect of calcium 4'-phosphopantothenate (CPP) on acute toxicity of streptomycin and the decrease by the antibiotic of the muscle working capacity, "holes" reflex, body temperature and oxygen intake was studied on 258 albino mice weighing 22-26 g. Medical calcium pantothenate (CPA) was used for control purposes. CPP is an antagonist of streptomycin sulfate. In a dose of 1/10 or 1/5 of the LD50 injected intraperitoneally CPP lowered acute toxicity of streptomycin and prevented its effect in a dose of 0.11--1.1 g/kg injected subcutaneously on the muscle working capacity, "holes" reflex and body temperature. The spectrum index of the CPP antitoxic effect was equal to 22.5. By its acute toxicity CPP (LD50 1.18 +/- 0.07 g/kg) did not differ from CPA (LD50 1.25 +/- 0.08 g/kg). The efficacy of CPP, by its antitoxic spectrum, was 1.8 times higher than that of CPA. CPA lowered the streptomycin effect on the "holes" reflex and body temperature, while CPP prevented it. Both the drugs did not influence the decrease in the oxygen consumption induced by streptomycin.     

Thermoregulatory adjustments during continuous heat exposure

Body temperature regulation was studied in 6 male subjects during an acclimation procedure involving uninterrupted heat exposure for 5 successive days and nights in a hot dry environment (ambient temperature = 35 degrees C, dew-point temperature = 7 degrees C; air velocity = 0.2 m.s-1). Data were obtained at rest and during exercise (relative mechanical workload = 35% VO2max). At rest, hourly measurements were made of oesophageal and 4 local skin temperatures, to allow the calculation of mean skin temperature, and of body motility and heart rate. During the working periods these measurements were made at 5 min intervals. Hourly whole-body weight loss was measured at rest on a sensitive platform scale while in the working condition just before starting and immediately after completing the bicycle exercise. The results show that, in both exercise and at rest, the successive heat exposures increased the sweat gland output during the first 3 days. Afterwards, sweat rate decreased without any corresponding change in body temperature. For the fixed workload, the sweat rate decline was associated with a decrease in circulatory strain. Adjustments in both sweating and circulatory mechanisms occur in the first 3 days of continuous heat exposure. The overall sweat rate decline could involve a redistribution of the regional sweating rates which enhances the sweat gland activities of skin areas with maximal evaporative efficiencies.     

Hats for the newborn infant.

The efficacy of a Gamgee-lined hat in reducing the rate of fall in rectal temperature of infants during the first 30 minutes of life was studied. The trial, which included 211 infants, was randomised, prospective, and controlled. One hundred and seven infants were exposed to overhead radiantheaters. Of these, only 30 had normal deliveries, so the analysis was confined to the infants who were not subjected to radiant heat, and in this group no conclusions were drawn about the efficacy of a hat or a radiant heater. In the 104 infants not subjected to radiant heat, body weight, initial rectal temperature, the application of hat, the environmental temperature, and the duration of exposure while naked were all found to influence measureably the rate of fall in rectal temperature during the first 30 minutes. Gamgee-lined hats should be routinely used to minimise heat loss, especially in small infants exposed at birth, during surgical operations, and during investigations necessitating prolonged exposure.     

Effects of color temperature of fluorescent lamps on body temperature regulation in a moderately cold environment

A study on the effects of different color temperatures of fluorescent lamps on skin and rectal temperatures in a moderately cold environment involving (i) changes in skin temperature of 7 male subjects exposed to an ambient temperature ranging from 28 degrees C to 18 degrees C (experiment I) and (ii) changes in skin and rectal temperatures and metabolic heat production of 11 male subjects exposed to ambient temperature of 15 degrees C for 90 min (Experiment II) was conducted. In Experiment I, the reduction of mean skin temperature from the control value was significantly greater under 3000 K than under 5000 K or 7500 K lighting. In Experiment II, the reductions in mean skin temperature and rectal temperature were respectively greater and smaller under 3000 K than those under 5000 K or 7500 K lighting. However, metabolic heat production was not affected by color temperature conditions. The relationships between morphological and physiological parameters revealed that no significant relation of rectal temperature to body surface area per unit body weight was found only under 3000 K. Furthermore, while the mean skin temperature was independent on the mean skinfold thickness under 3000 K, a significant negative correlation between the rectal and mean skin temperatures was observed. Therefore, body heat loss might be suppressed effectively by increasing the vasoconstrictor tone under a color temperature of 3000 K, and the body shell was dependent only on morphological factors under 5000 K and 7500 K lighting.     

Effect of primary hypohydration on physical work capacity

Physical work capacity (PWC₁₈₀) was assessed with different levels of hypohydration in 25 heat-acclimatized male volunteers in hot dry (45°C DB, 30% RH) and hot humid (39°C DB, 60% RH) conditions equated to a heat stress level of 34°C on the WBGT scale. Heat acclimatization was carried out by exposing the subjects for 8 consecutive days in a climatic chamber with moderate work for two 50 min work cycles and 10 min intervening rest pauses. Acclimatization resulted in significant decreases in heart rate (27 bpm), oral temperature (0.8°C), mean skin temperature (1.2°C) and a significant increase in sweating rate (120 g h^–1 m^–2). Day-to-day variations in body hypohydration levels during heat acclimatization were not significantly different, although water intake was found to increase significantly from day 3 onwards when the subjects were in ad lib water intake state. The heat acclimatized subjects were then hypohydrated to varying degrees, viz. 1%, 2% and 3% body weight deficit, with moderate work in heat in the climatic chamber and after successful recovery from the effects of thermal stress and exercise; their physical work capacity was assessed individually. Physical work capacity was found to decrease significantly with hypohydration as compared to controls. The decrease was of the order of 9%, 11% and 22% in the hot dry condition and 6%, 8% and 20% in the hot humid condition with hypohydration levels of 1%, 2% and 3% respectively. The decrease was more pronounced during 3% hypohydration level under both heat stress conditions. This decrease was in spite of significant increases in maximal ventilation. However, the PWC₁₈₀ under the two heat stress conditions, when compared, did not reveal any significant difference. It was concluded that the heat stress vehicle did not adversely affect the physical work capacity. On the other hand, the decreases in physical work capacity were found to be closely related to the primary hypohydration level in heat-acclimatized tropical subjects.Abbreviations WBGT wet bulb globe temperature - bam beats per minute - YSI Yellow Springs Instrument - EKG electrocardiogram     

Thermoregulatory stress during rest and exercise in heat in patients with a spinal cord injury

Twelve subjects with spinal cord injuries and four controls (all male) were exposed to heat while sitting at rest or working at each of three environmental temperatures, 30, 35 and 40 degrees C, with a relative humidity of 50%. Exercise was accomplished at a load of 50 W on a friction-braked cycle ergometer which was armcranked or pedalled. Functional electrical stimulation of the legs was provided to the subjects with quadriplegia and paraplegia to allow them to pedal a cycle ergometer. The data showed that individuals with quadriplegia had the poorest tolerance for heat. As an example, in this group, accomplishing armcrank ergometry while working at an environmental temperature of 40 degrees C resulted in an increase in aural temperature of 2 degrees C in 30 min. The aural temperature of individuals with paraplegia working for the same length of time under the same conditions rose approximately 1 degree C. There was virtually no change in the aural temperature in the control subjects.     

Thermal insulation and body temperature wearing a thermal swimsuit during water immersion

This study evaluated the effects of a thermal swimsuit on body temperatures, thermoregulatory responses and thermal insulation during 60 min water immersion at rest. Ten healthy male subjects wearing either thermal swimsuits or normal swimsuits were immersed in water (26 degrees C or 29 degrees C). Esophageal temperature, skin temperatures and oxygen consumption were measured during the experiments. Metabolic heat production was calculated from oxygen consumption. Heat loss from skin to the water was calculated from the metabolic heat production and the change in mean body temperature during water immersion. Total insulation and tissue insulation were estimated by dividing the temperature difference between the esophagus and the water or the esophagus and the skin with heat loss from the skin. Esophageal temperature with a thermal swimsuit was higher than that with a normal swimsuit at the end of immersion in both water temperature conditions (p<0.05). Oxygen consumption, metabolic heat production and heat loss from the skin were less with the thermal swimsuit than with a normal swimsuit in both water temperatures (p<0.05). Total insulation with the thermal swimsuit was higher than that with a normal swimsuit due to insulation of the suit at both water temperatures (p<0.05). Tissue insulation was similar in all four conditions, but significantly higher with the thermal swimsuit in both water temperature conditions (p<0.05), perhaps due to of the attenuation of shivering during immersion with a thermal swimsuit. A thermal swimsuit can increase total insulation and reduce heat loss from the skin. Therefore, subjects with thermal swimsuits can maintain higher body temperatures than with a normal swimsuit and reduce shivering thermo-genesis.     

Changes in plasma volume during hypohydration and rehydration in subjects from the tropics

Plasma volume (PV) at different levels of hypohydration was determined using radio-iodinated serum albumin-125 in 28 heat acclimated male volunteers in hot dry condition in a climatic chamber. The heat acclimated subjects were hypohydrated to varying degrees i.e. 1%, 2%, 3% and 4% body mass deficit by moderate work in hot conditions in a climatic chamber maintained at 45 degrees C dry bulb temperature and 30% relative humidity. A rehydration study was carried out in only those subjects who were hypohydrated to 3% and 4% body mass and they were brought back to a 2% level of hypohydration by giving a calculated amount of water. A significant decrease in PV was observed at 3% and 4% hypohydration only. The magnitude of the decrease was the same in both the groups and not related to the level of hypohydration. With partial rehydration in the 3% hypohydrated group PV was restored fully, while in the 4% hypohydrated group restoration was incomplete, indicating that at this hypohydration level some of the replenished water that entered in plasma may have moved to the intracellular compartment which may have contributed more at 4% hypohydration. It is suggested that with higher levels of thermal hypohydration significant reduction in the intracellular compartment may result in accentuated physiological strain during work in the heat.     

Physiological and metabolic responses to work in heat with graded hypohydration in tropical subjects

Studies were conducted on 25 healthy male volunteers aged 20-25 years drawn randomly from the tropical regions of India. The subjects initially underwent an 8 day heat acclimatization schedule with 2 hours moderate work in a climatic chamber at 45 degrees C DB and 30% RH. These heat acclimatized subjects were then hypohydrated to varying levels of body weight deficits, i.e. 1.3 +/- 0.03, 2.3 +/- 0.04 and 3.3 +/- 0.04%, by a combination of water restriction and moderate exercise inside the hot chamber. After 2 hours rest in a thermoneutral room (25 +/- 1 degree C) the hypohydrated subjects were tested on a bicycle ergometer at a fixed submaximal work rate (40 W, 40 min) in a hot dry condition (45 degrees C DB, 30% RH, 34 degrees C WBGT). Significant increases in exercise heart rate and oral temperature were observed in hypohydrated subjects as compared to euhydration. Sweat rate increased with 1% and 2% hypohydration as compared to euhydration, but a significant decrease was observed with 3% hypohydration. Na+ & K+ concentrations in arm sweat increased with increase in the level of hypohydration. Oxygen consumption increased significantly only when hypohydration was about 2% or more. It appears that the increased physiological strain observed in tropical subjects working in heat with graded hypohydration is not solely due to reduced sweat rates.     

Water loss distribution in exercising men under hot conditions

Dynamics of sweating and water loss distribution were studied in 7 exercising men under thermoneutral conditions (Ta, 25 degrees C; Tw, 24 degrees C and RH, 54%) and during moderate heat exposure (Ta, 30 degrees C; Tw, 30 degrees C; RH, 54%). The subjects performed bicycle exercise at intensity of 50% V O2 max. Dynamics of sweating was greater after heat exposure (delay in onset of sweating 3.6 and 1.4 min, p less than 0.05; time constant 10.1 and 7.3 min, p less than 0.02). The dynamics of sweating was related to the net body heat load (r = -0.80, p less than 0.001). Sweat evaporation from the skin (Esk) was significantly higher in heat exposed exercising subjects while dripping sweat (mdrip) did not differ significantly. Water loss distribution in relation to total water loss during control exercise was as follows: (Ediff + Eres) 14.8% (Esk) 59.6%; and (mdrip) 25.6%. During exercise under heat exposure (Ediff + Eres) was 12.1%; (Esk) was 67.5%; and (mdrip) was 20.4%. It is concluded that moderate heat exposure accelerate sweating reaction but does not change significantly water loss distribution in exercising subjects. Dripping sweat seems to be an attribute of sweating not only in hot humid conditions but also under temperate temperature and air humidity.     

Differences in the heat stress associated with white sportswear and being semi-nude in exercising humans under conditions of radiant heat and wind at a wet bulb globe temperature of greater than 28 °C

This study investigated whether wearing common white sportswear can reduce heat stress more than being semi-nude during exercise of different intensities performed under radiant heat and wind conditions, such as a hot summer day. After a 20-min rest period, eight male subjects performed three 20 min sessions of cycling exercise at a load intensity of 20 % or 50 % of their peak oxygen uptake (VO_2peak) in a room maintained at a wet bulb globe temperature (WBGT) of 28.7?±?0.1 °C using two spot lights and a fan (0.8 m/s airflow). Subjects wore common white sportswear (WS) consisting of a long-sleeved shirt (45 % cotton and 55 % polyester) and short pants (100 % polyester), or only swimming pants (SP) under the semi-nude condition. The mean skin temperature \( \left(\overline{T} sk\right) \) was greater when subjects wore SP than WS under both the 20 % and 50 % exercise conditions. During the 50 % exercise, the rating of perceived exertion (RPE) and thermal sensation (TS), and the increases in esophageal temperature (ΔTes) and heart rate were significantly higher (P?<?0.001–0.05), or tended to be higher (P?<?0.07), in the WS than SP trials at the end of the third 20-min exercise session. The total sweat loss (m _sw,tot) was also significantly higher in the WS than in the SP trials (P?<?0.05). However, during the 20 % exercise, the m _sw,tot during exercise, and the ΔTes, RPE and TS at the end of the second and third sessions of exercise did not differ significant between conditions. The heat storage (S), calculated from the changes in the mean body temperature (0.9Tes + 0.1 \( \overline{T} sk \) ), was significantly lower in the WS trials than in the SP trials during the 20 min resting period before exercise session. However, S was similar between conditions during the 20 % exercise, but was greater in the WS than in the SP trials during 50 % exercise. These results suggest that, under conditions of radiant heat and wind at a WBGT greater than 28 °C, the heat stress associated with wearing common WS is similar to that of being semi-nude during light exercise, but was greater during moderate exercise, and the storage of body heat can be reduced by wearing WS during rest periods.     

The effects of two types of clothing on seasonal cold tolerance

This study investigated the effects of two types of clothing, leaving legs covered or uncovered, on seasonal cold tolerance in women. Experiments were carried out to compare cold tolerance at an ambient temperature (T _a) of 10° C in December between two groups of subjects, who wore either knee-length skirts (skirt group) or full-length trousers (trouser group) for 3 months from September to November. The main results are summarized as follows: rectal temperatures continued to fall for 40 min in the trouser group when the subjects were covered by a blanket, while it became stable in 30 min in the skirt group; rectal temperatures showed greater increases in the skirt group when the blanket was removed after 40 min exposure to T _a of 10° C; metabolic heat production was kept significantly lower in the skirt group when uncovered or covered by a blanket at T _a of 10° C; metabolic heat production was negatively correlated with mean skin temperature and was always higher in the trouser group when measured at the same mean skin temperature; in the uncovered condition diastolic blood pressure increased significantly in the trouser group but not in the skirt group. These results would suggest that the subjects who wore skirts for 3 months from September to November had improved their ability to tolerate the cold.     

Effects of airflow on body temperatures and sleep stages in a warm humid climate

Airflow is an effective way to increase heat loss—an ongoing process during sleep and wakefulness in daily life. However, it is unclear whether airflow stimulates cutaneous sensation and disturbs sleep or reduces the heat load and facilitates sleep. In this study, 17 male subjects wearing short pyjamas slept on a bed with a cotton blanket under two of the following conditions: (1) air temperature (Ta) 26°C, relative humidity (RH) 50%, and air velocity (V) 0.2 m s⁻¹; (2) Ta 32°C, RH 80%, V 1.7 m s⁻¹; (3) Ta 32°C; RH 80%, V 0.2 m s⁻¹ (hereafter referred to as 26/50, 32/80 with airflow, and 32/80 with still air, respectively). Electroencephalograms, electrooculograms, and mental electromyograms were obtained for all subjects. Rectal (Tre) and skin (Ts) temperatures were recorded continuously during the sleep session, and body-mass was measured before and after the sleep session. No significant differences were observed in the duration of sleep stages between subjects under the 26/50 and 32/80 with airflow conditions; however, the total duration of wakefulness decreased significantly in subjects under the 32/80 with airflow condition compared to that in subjects under the 32/80 with still air condition (P < 0.05). Tre, Tsk, Ts, and body-mass loss under the 32/80 with airflow condition were significantly higher compared to those under the 26/50 condition, and significantly lower than those under the 32/80 with still air condition (P < 0.05). An alleviated heat load due to increased airflow was considered to exist between the 32/80 with still air and the 26/50 conditions. Airflow reduces the duration of wakefulness by decreasing Tre, Tsk, Ts, and body-mass loss in a warm humid condition.     

Circadian variations in the sweating mechanism.

Sweat rates and body temperatures of human subjects were measured at 0200, 1000, and 1800 h during a heat exposure of 90 min. The latent period of sweating was not significantly altered in the evening but significantly shortened during the night. Mean body temperature corresponding to the onset of sweating was nearer to the basal body temperature during the night, while during the day the difference between these two temperatures became larger. This phenomenon seems related to the circadian cycle of vasomotor adjustment, since during the night body conductance was higher than during the day and corresponded to a state of a vasodilatation similar to that observed at the onset of sweating. During the day, this situation was reversed. During steady state, the following changes were observed: sweating rate, night less than morning less than evening; skin temperatures, night less than morning less than evening; and rectal temperature increase, morning less than evening less than night. It is hypothesized that these changes are due to either different metabolic rates or an imbalance between heat gains and losses which preserve the circadian rhythm of the body temperature, even under thermal loads.