Hot-dry Environment Research Articles

Amplification of nocturnal oscillations in PRA and aldosterone during continuous heat exposure

To assess the effect of continuous heat exposure on the nocturnal patterns of renin, aldosterone, adrenocorticotropic hormone (ACTH), and cortisol, six young men were exposed to thermoneutral environment for 5 days, followed by a 5-day acclimation period in a hot dry environment (35 degrees C). Blood was collected at 10-min intervals during the second night at thermoneutrality (N0) and during the first (N1) and the last (N5) nights of heat exposure. Polygraphic recordings of sleep were scored according to established criteria. Continuous heat exposure led to progressive decreases in the 24-h urinary volume and in Na excretion, whereas urinary osmolality increased. After 5 days of uninterrupted heat, significant increases were found in plasma volume (P less than 0.05), osmolality (P less than 0.01), plasma Na (P less than 0.01), and protein levels (P less than 0.05). Sweat gland output increased during the first 3 days and then declined without any concomitant increases in body temperature. Compared with N0, there were no differences in plasma renin activity (PRA) and aldosterone (PA) profiles during N1 at 35 degrees C. However, during N5 the mean PRA and PA levels were significantly (P less than 0.05) enhanced, and their nocturnal oscillations were amplified (P less than 0.05). This amplification occurred mainly in the second part of the night when regular rapid-eye-movement and non-rapid-eye-movement sleep cycles were observed, leading to a general upward trend in the nocturnal profiles. The relationship between the nocturnal PRA oscillations and the sleep cycles was not modified. ACTH and cortisol patterns were not affected by continuous heat exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermoregulatory responses of middle-aged and young men during dry-heat acclimation.

Thermoregulatory responses during heat acclimation were compared between nine young (mean age 21.2 yr) and nine middle-aged men (mean age 46.4 yr) who were matched (P greater than 0.05) for body weight, surface area, surface area-to-weight ratio, percent body fat, and maximal aerobic power. After evaluation in a comfortable environment (22 degrees C, 50% relative humidity), the men were heat acclimated by treadmill walking (1.56 m/s, 5% grade) for two 50-min exercise bouts separated by 10 min of rest for 10 consecutive days in a hot dry (49 degrees C ambient temperature, 20% relative humidity) environment. During the first day of heat exposure performance time was 27 min longer (P less than 0.05) for the middle-aged men, whereas final rectal and skin temperatures and heart rate were lower, and final total body sweat loss was higher (P less than 0.05) compared with the young men. These thermoregulatory advantages for the middle-aged men persisted for the first few days of exercise-heat acclimation (P less than 0.05). After acclimation no thermoregulatory or performance time differences were observed between groups (P greater than 0.05). Sweating sensitivity, esophageal temperature at sweating onset, and the sweating onset time did not differ (P greater than 0.05) between groups either pre- or postacclimatization. Plasma osmolality and sodium concentration were slightly lower for the young men both pre- and postacclimatization; however, both groups had a similar percent change in plasma volume from rest to exercise during these tests.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Responses of older and younger women to exercise in dry and humid heat without fluid replacement

Eight older (52 to 62 yr old) post-menopausal and 8 younger (20 to 30 yr old) women were matched with respect to body size, fatness, and maximal oxygen consumption, heat-acclimated, and then exercised at 35 to 40% maximal oxygen consumption in a warm-humid (37 degrees C, 60% relative humidity) environment. Results were compared with similar data from a hot-dry (48 degrees C, 15% relative humidity) environment. No fluid replacement was provided during either of these sessions. In each environment, the older women stored more heat as evidenced by a higher rectal temperature response, but there were no inter-group differences in skin temperature or percent maximal heart rate. Four of the older women were unable to complete either the hot-dry or the warm-humid exposure, although they completed the full 2 h during acclimation sessions when water was provided ad libitum. In the hot-dry environment, the younger women's whole body and local sweat rates were significantly higher than those of the older women; in the warm-humid environment, there was no age-related difference in sweat rate. When local skin temperature and wettedness were artificially elevated, both groups exhibited the same pattern and rate of sweating. Percent decrease in plasma volume was greater for the older women in both conditions, but significantly so only in the warm-humid environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Milk Production Characteristics of the Kenana Breed of Bos indicus Cattle in Sudan

Data were analyzed on the Kenana, a Bos indicus breed of cattle indigenous to northern Sudan. Cattle were kept at Um Banein in a hot dry tropical environment 13°.04 latitude north at an altitude of 435m. Analyzed were lactation yield (1597kg), lactation length (264), calving interval (530), and annual lactation yield (1225kg). Between 1966 and 1980 all these traits except lactation length were significantly affected by lactation number and year. None was significantly influenced by the season in which lactation started. Kenana have considerable potential in the Sudan environment as a dairy breed, but further selection and an open nucleus system of breeding to introduce additional genetic material is required to express this potential.

Heat tolerance, thermoregulation and ageing.

Older individuals, regardless of how one classifies 'old', are the most rapidly growing portion of the population. Statistics from heat waves and other morbidity-mortality data strongly suggest that older persons are at greater risk of developing life-threatening manifestations of heat stress such as heat stroke. Most laboratory studies have found that ageing is associated with decreased heat tolerance and alterations in thermoregulatory effector responses. What is not so clear is the role of ageing per se in this decreased heat tolerance (as opposed to some concomitant functional decline, e.g. decreasing VO2max, which accompanies ageing across a population). Studies have shown decrements in resting heat tolerance and responses to thermal transients in older adults. With exercise in warm environments, most research has focused on the sweating response and it appears that whether or not sweating rate declines in the elderly is dependent upon the environment studied. In hot dry environments, older persons (of both sexes) consistently secrete sweat at a lower rate than their younger counterparts. However, as humidity of the environment increases, this difference tends to disappear. A key issue which deserves further attention is that of hydration in the elderly, both in terms of body water content and compartmentalisation, and of skin water content. Finally, ageing is often accompanied by other conditions (e.g. hypertension, diabetes, cardiovascular insufficiencies, long term therapeutic drug modalities) which further affect heat tolerance and thermoregulation.

Skin blood flow during incremental exercise in a thermoneutral and a hot dry environment.

Eight physically fit men performed two incremental bicycle ergometer tests, one in an ambient temperature of 25 degrees C and the other at 40 degrees C. Oesophageal temperature (Tes) increased continuously throughout the tests up to 38.0 and 38.3 degrees C, respectively. In both environments, forearm blood flow (plethysmography) was linearly related to Tes above the Tes threshold for vasodilation, but at the heaviest work loads this relationship was clearly attenuated and therefore indicated skin vasoconstriction, which tended to be more pronounced at 25 degrees C. During recovery at 25 degrees C, in some subjects the forearm blood flow increased above the levels observed at the end of the graded exercise in spite of a decreasing Tes. Skin blood flow, measured by laser Doppler flow meter at the shoulder, was quantitatively different but, on average, seemed to reveal the same response pattern as the forearm blood flow. In spite of the higher level of skin blood flow in the heat, blood lactate accumulation did not differ between the two environments. The present results suggest that there is competition between skin vasoconstriction and vasodilation at heavy work rates, the former having precedence in a thermoneutral environment to increase muscle perfusion. During short-term graded exercise in a hot environment, skin vasoconstriction with other circulatory adjustments seems to be able to maintain adequate muscle perfusion at heavy work levels, but probably not during maximum exercise.

Osmotic factors in restitution from thermal dehydration in rats

The degree of voluntary dehydration after thermal dehydration was assessed while supplying drinking water of varying NaCl concentrations. Adult male albino rats were exposed to a hot-dry environment (dry bulb temp 36 degrees C; relative humidity 20%) for 6-8 h, and dehydration of 7% body wt was induced. After dehydration the rats were divided into three groups, and distilled water and 0.45 or 0.9% NaCl solution was given ad libitum. The amount of fluid intake reached an equilibrium in 6-12 h; the 0.45 and 0.9% NaCl groups regained almost all lost water, whereas the distilled water group regained only half. During rehydration the 0.45 and 0.9% NaCl groups gained Na by approximately 430-650 mueq and lost K by 90-130 mueq, whereas the distilled water group lost Na slightly and K by 80 mueq/100 g body wt. As for the electrolyte balance during thermal dehydration, rats excreted Na and K into urine and saliva. Na loss was 200 mueq/100 g, almost all of which was derived from the interstitial space of skin and skeletal muscle, and K loss was 230 mueq/100 g, almost all of which was derived from intracellular space of skeletal muscle. Total cation loss (Na, K) during thermal dehydration, including K excretion during rehydration, was 510-560 mueq/100 g, which was almost identical to the Na gained by rats given 0.45 or 0.9% NaCl solution. These results suggest that voluntary dehydration is caused by the dilutional inhibition of drinking due to loss of electrolytes during thermal dehydration.

Effect of atropine on the exercise-heat performance of man

This paper summarizes the findings from two recent studies involving the physiological effects of atropine (0 to 4 mg, im) on soldiers performing physical exercise in hot-dry environments. Study I determined the threshold of physiological effects and the gradation of these effects with increasing dosage of atropine. Study II examined the effects of exercise-heat acclimation on the reduced physical exercise performance that occurs following atropine administration. The following new observations were made: (1) a 0.5-mg dose of atropine elevates heart rate, rectal temperature, and mean skin temperature; (2) atropine exerts its peak physiological effects approximately 70 min after intramuscular injection; (3) within the dosage levels tested, the magnitude of the elevated heart rate response is curvilinearly related to atropine dosage, whereas, the magnitude of the elevated rectal temperature response is linearly related to atropine dosage; (4) repeated administration of atropine over a number of days does not alter thermoregulatory responses; (5) heat acclimation improves exercise-heat performance of individuals under the influence of atropine by enabling a reduced rectal temperature; and (6) heat acclimation increases the sweat output of individuals under the influence of atropine; however, the absolute reduction in sweat output from atropine is the same pre- and post-heat acclimation.

Heat exchange following atropine injection before and after heat acclimation.

The effect of saline and atropine injection (2 mg, im) on eight healthy male subjects before and after heat acclimation was studied while each subject walked on a treadmill (1.34 m X s-1) in a hot-dry environment (ambient temperature = 48.4 degrees C, dew-point temperature = 20.5 degrees C). Partitional calorimetric analysis was done for the periods in which maximum sweat inhibition occurred (30 min). Mean skin temperature, rectal temperature, and heart rate were continuously observed. Evaporative loss from the skin was calculated by changes in body weight (Sauter balance); heat transfer coefficients were defined by Nishi equations. A prediction of sweat inhibition based on an analysis of heat storage and its effect on a theoretical temperature, which can be graphed on a psychrometric chart, was developed. A rational effective temperature (ET) defined as the operative temperature at the intersection of the 50% rh, which encompasses total heat exchange, was used to compare the effects of atropine before and after heat acclimation. The results show that heat acclimation reduced ET by approximately 2.5 degrees C when compared with the unacclimated state after atropine injection. Thus heat acclimation reduces the hazards of heatstroke caused by exercise in the heat with atropine injection.

Effect of Atropine on the Exercise–Heat Performance of Man

Effect of Atropine on the Exercise-Heat Performance of Man. SAWKA, M. N., LEVINE, L., KOLKA, M. A., APPLETON, B. S., JOYCE, B. E., AND PANDOLF, K. B. (1984). Fundam. Appl. Toxicol. 4, S19O–S194. This paper summarizes the findings from two recent studies involving the physiological effects of atropine (0 to 4 mg, im) on soldiers performing physical exercise in hot-dry environments. Study I determined the threshold of physiological effects and the gradation of these effects with increasing dosage of atropine. Study II examined the effects of exercise-heat acclimation on the reduced physical exercise performance that occurs following atropine administration. The following new observations were made: (1) a 0.5-mg dose of atropine elevates heart rate, rectal temperature, and mean skin temperature; (2) atropine exerts its peak physiological effects approximately 70 min after intramuscular injection; (3) within the dosage levels tested, the magnitude of the elevated heart rate response is curvilinearly related to atropine dosage, whereas, the magnitude of the elevated rectal temperature response is linearly related to atropine dosage; (4) repeated administration of atropine over a number of days does not alter thermo-regulatory responses; (5) heat acclimation improves exercise-heat performance of individuals under the influence of atropine by enabling a reduced rectal temperature; and (6) heat acclimation increases the sweat output of individuals under the influence of atropine; however, the absolute reduction in sweat output from atropine is the same pre- and post-heat acclimation.

Hydration and vascular fluid shifts during exercise in the heat.

This study examined the effects of hypohydration on plasma volume and red cell volume during rest in a comfortable (20 degrees C, 40% relative humidity) and exercise in a hot-dry (49 degrees C, 20% relative humidity) environment. A group of six male and six female volunteers [matched for maximal O2 uptake (VO2 max)] completed two test sessions following a 10-day heat acclimation program. One test session was completed when subjects were euhydrated and the other when subjects were hypohydrated (-5% from base-line body wt). The test sessions consisted of rest for 30 min in a 20 degrees C antechamber, followed by two 25-min bouts of treadmill walking (approximately 30% of VO2 max) in the heat, interspersed by 10 min of rest. No significant differences were found between the genders for the examined variables. At rest, hypohydration elicited a 5% decrease in plasma volume with less than 1% change in red cell volume. During exercise, plasma volume increased by 4% when subjects were euhydrated and decreased by 4% when subjects were hypohydrated. These percent changes in plasma volume values were significantly (P less than 0.01) different between the euhydration and hypohydration tests. Although red cell volume remained fairly constant during the euhydration test, these values were significantly (P less than 0.01) lower when hypohydrated during exercise. We conclude that hydration level alters vascular fluid shifts during exercise in a hot environment; hemodilution occurs when euhydrated and hemoconcentration when hypohydrated during light intensity exercise for this group of fit men and women.

Hypohydration and heat acclimation: plasma renin and aldosterone during exercise.

This study was designed to assess the effects of hydration, acclimation, environment, and exercise on plasma levels of renin and aldosterone. Sixteen subjects exercised (1.34 m X s-1), both pre- and postacclimation, when euhydrated or hypohydrated (-5% of body wt) in a comfortable (20 degrees C, rh = 40%), hot-wet (35 degrees C, rh = 79%), or hot-dry (49 degrees C, rh = 20%) environment. Although light exercise in a thermoneutral environment had no effects on plasma levels of renin activity (PRA) or aldosterone (ALD), exercise in both hot environments resulted in significantly increased levels of both. Increments in both PRA and ALD were greater when hypohydrated, and PRA effects were significantly moderated by heat acclimation in both the euhydration and hypohydration experiments. Although PRA and ALD responses were generally correlated, acclimation did not consistently attenuate ALD increments. We concluded that hydration state, acclimation level, and environmental conditions all affected the responses of PRA and ALD to light exercise.

Differential effects of hot-humid and hot-dry environments on mental functions.

Twenty five subjects acclimatised to heat artificially were exposed to "basic effective temperatures" (BET) of 25.0 degrees, 29.6 degrees, 32.2 degrees, 33.3 degrees and 35.0 degrees C BET under conditions of both humid and dry heat. The object of the investigation was to ascertain the nature of effects of varying degrees of heat stress on mental alertness, associative learning, reasoning ability and dual-performance efficiency. A further aim was to determine the temperature levels at which impairment of psychological functions was severe enough to be of practical concern. The duration of each exposure was four hours, during which subjects performed physical exercise followed by rest every 30 min. It was found that all the psychological functions tested were adversely affected under extreme heat, and that a significant drop in various psychological functions was seen at effective temperatures of 32.2 degrees C and 33.3 degrees C in hot-humid and hot-dry conditions respectively. It is concluded that at the same effective temperatures the magnitude of the overall effect on psychological functions under humid conditions is relatively greater than that under dry conditions.

Effect of Exposure to a Hot-Humid and a Hot-Dry Environment on Thyroid Hormone Values in Sheep

SUMMARY The free thyroxine index (FT4), free tri-iodothyronine index (FT3), thyroxine (T4)/tri-iodothyronine (T3) ratio, serum cholesterol, total plasma protein (TPP) concentration, packed cell volume (PCV), rectal temperature and feed intake were determined in six adult Letelle wethers exposed to a hot-humid, a hot-dry and a cool-dry environment. The temperature-humidity index (THI) was used to determine the combined effect of ambient temperature and humidity on the values measured. Values measured during the hot environments were compared with those measured during cool-dry exposure. During the first week of the hothumid exposure mean FT3 values increased 20·6% (P The results suggested that an increase in thyroid hormone-binding capacity increased thyroid hormone concentration during the initial phases of exposure to the hot-humid environment, whereas prolonged exposure to the same environment depressed T3 values. Exposure to a hot-but-dry environment did not change thyroid hormone concentration. The changes in feed intake and blood cholesterol values were not related to thyroid hormone concentration.

The effects on sheep urinary and plasma electrolytes of 4 weeks' exposure to different ambient temperatures and humidities

Urinary electrolyte output and plasma electrolyte concentration were determined in six Letelle wethers exposed to hot-humid, hot-dry, cool-humid and cool-dry environments. Urinary Na+ output increased during the first week of exposure to both hot environments, decreased after the third week of hot-humid exposure, and declined during the first 2 weeks of the cool-humid exposure. Changes in urinary Na+ output were accompanied by major changes in the opposite direction of urinary K+ and Cl- output. The changes in urinary electrolyte output were accompanied by lesser changes in plasma Na+, K+, and Cl- concentration. Increases in the temperature-humidity index were correlated with increased urine Na+ concentration, decreased urine K+ and Cl- concentration, decreased urine and plasma osmolality, decreased plasma Na+ concentration, and increased plasma Cl- concentration. Changes in urine volume, feed intake, total plasma protein concentration, and plasma cortisol were related to changes in urinary electrolyte output, especially after prolonged heat exposure. The results provide convincing evidence that changes in ambient temperature and humidity were related to major changes in urinary electrolyte output during the first weeks of exposure. More prolonged changes in ambient temperature and humidity were associated with major changes in urine output and feed intake which, in turn, were related to major changes in urinary electrolyte output and minor changes in plasma electrolyte concentration.

Physiological responses of men and women to humid and dry heat.

ARTICLESPhysiological responses of men and women to humid and dry heatY. Shapiro, K. B. Pandolf, B. A. Avellini, N. A. Pimental, and R. F. GoldmanY. Shapiro, K. B. Pandolf, B. A. Avellini, N. A. Pimental, and R. F. GoldmanPublished Online:01 Jul 1980https://doi.org/10.1152/jappl.1980.49.1.1MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByExercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategiesJulien D. Périard, Thijs M. H. Eijsvogels, and Hein A. M. Daanen27 August 2021 | Physiological Reviews, Vol. 101, No. 4Sex Differences in Human Thermoregulation: Relevance for 2020 and BeyondR. Yanovich,* I. Ketko,* and N. Charkoudian15 April 2020 | Physiology, Vol. 35, No. 3Sex-dependent responses to exertional heat stroke in miceChristian K. Garcia, Alex J. Mattingly, Gerard P. Robinson, Orlando Laitano, Michelle A. King, Shauna M. Dineen, Lisa R. Leon, and Thomas L. Clanton19 September 2018 | Journal of Applied Physiology, Vol. 125, No. 3Personal cooling strategies to improve thermal comfort in warm indoor environments: Comparison of a conventional desk fan and air ventilation clothingEnergy and Buildings, Vol. 174Physiological Responses to Overdressing and Exercise-Heat Stress in Trained RunnersMedicine & Science in Sports & Exercise, Vol. 50, No. 6Current knowledge on the effects of humidity on physiological and psychological responsesJournal of the Human-Environment System, Vol. 20, No. 1Extremes of human heat tolerance: life at the precipice of thermoregulatory failureJournal of Thermal Biology, Vol. 29, No. 7-8Human ACE I/D polymorphism is associated with individual differences in exercise heat toleranceYuval Heled, Daniel S. Moran, Liran Mendel, Arie Laor, Elon Pras, and Yair Shapiro1 July 2004 | Journal of Applied Physiology, Vol. 97, No. 1Can gender differences during exercise-heat stress be assessed by the physiological strain index?Daniel S. Moran, Yair Shapiro, Arie Laor, Sharona Izraeli, and Kent B. Pandolf1 June 1999 | American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, Vol. 276, No. 6The effects of heat and exercise on sweat iron lossMedicine &amp Science in Sports &amp Exercise, Vol. 28, No. 2Prediction modeling of physiological responses and human performance in the heatComputers in Biology and Medicine, Vol. 16, No. 5Heat balance and transfer in men and women exercising in hot-dry and hot-wet conditions*24 October 2007 | Ergonomics, Vol. 24, No. 5Sex differences in acclimation to a hot-dry environment†;27 March 2007 | Ergonomics, Vol. 23, No. 7 More from this issue > Volume 49Issue 1July 1980Pages 1-8 Copyright & PermissionsCopyright © 1980 the American Physiological Societyhttps://doi.org/10.1152/jappl.1980.49.1.1PubMed7399982History Published online 1 July 1980 Published in print 1 July 1980 Metrics

Responses of physically active and acclimatized men and women to exercise in a desert environment

In most studies of physiological responses of men and women to exercise in the heat, the subjects are unacclimatized and the females are unaccustomed to strenuous physical activity. In this investigation, 5 men and 6 women known to be physically active in an outdoor hot-dry environment were studied in a temperate environment (Tdb=23C, WBGT=18C) and in an outdoor desert environment (Tdb=39C, WBGT=29C) during rest and exercise at 50% Vo2max. No differences between the groups occurred in Vo2, HR or Tre in either environment. Of greatest interest was that no differences occurred in evaporative weight losses, sweating rates, or % body weight losses in the heat. It was concluded that (1) women accustomed to exercise in a desert climate are able to substantially increase their sudorific response, and (2) that acclimatized male and female subjects of similar aerobic capacity have comparable responses to rest and exercise in a desert environment.

WORK‐HEAT TOLERANCE OF DISTANCE RUNNERS*

Physical training in a cool environment by subjects not previously trained improves their work-heat tolerance, but can not replace heat acclimatization to the standard heat stress conditions employed by a variety of investigators. This is attributed to the inability of these subjects to sustain prolonged work at high metabolic rates. Thus, they are not maintaining high core body temperatures long enough to bring about an adaptive change to heat. On the other hand, the intense and prolonged (years) training of long distance runners in a temperature environment at high metabolic rates has acclimatized them for at least 4 hours of mild work (MR 160 kcal/m2-hr) in both hot dry (50/27 C db/wb) and hot wet (36.7/33.1 C db/wb) environments, but not for work at high energy expenditures (MR 540 kcal/m2-hr) in a less severe thermal stress (35/21 C db/wb). These highly trained athletes can improve their work-heat tolerance at high metabolic rates in a warm climate by training at competitive speeds in a cool environment dressed in sweat clothing or by training at near competitive speeds in the heat. In either of these situations the athlete is cautioned to consume water at frequent intervals to offset the dehydration associated with excessive sweating under these conditions.

Respiratory responses to intermittent and prolonged exercise in a hot-dry environment

Ventilatory gas exchange ratio (R), V̇O 2, ventilation (V̇ E), respiration rate (RR), rectal temperature (T re), and heart rate (HR) were determined for four acclimatized subjects during intermittent and prolonged exercise on a treadmill at 24° and 45°C (dry) as follows: 1) 8 cycles (10 min. exercise and 5 min. rest), and 2) prolonged exercise lasting for 90 min. While during intermittent and prolonged exercise, V̇O 2 and V̇ E did not differ in the heat, RR, T re, HR and the respiratory dead space were higher in the hot ambient environment. After steady-state attainment, exercise R was higher in the initial as compared to the last cycles with higher values in neutral as compared to the hot ambient condition. It was concluded that heat was more effective than time in lowering the R, probably with a greater dependence on fat oxidation in the latter exercise cycles which seemed to be more pronounced in the heat.