Physiological tolerance to uncompensable heat stress: effects of exercise intensity, protective clothing, and climate

1994 ◽  
Vol 77 (1) ◽  
pp. 216-222 ◽  
Author(s):  
S. J. Montain ◽  
M. N. Sawka ◽  
B. S. Cadarette ◽  
M. D. Quigley ◽  
J. M. McKay
Keyword(s):  
Heat Stress ◽  
Relative Humidity ◽  
Core Temperature ◽  
Exercise Intensity ◽  
Protective Clothing ◽  
Heat Exposure ◽  
Heat Strain ◽  
Published Data ◽  
Physiological Tolerance ◽  
Uncompensable Heat Stress

This study determined the influence of exercise intensity, protective clothing level, and climate on physiological tolerance to uncompensable heat stress. It also compared the relationship between core temperature and the incidence of exhaustion from heat strain for persons wearing protective clothing to previously published data of unclothed persons during uncompensable heat stress. Seven heat-acclimated men attempted 180-min treadmill walks at metabolic rates of approximately 425 and 600 W while wearing full (clo = 1.5) or partial (clo = 1.3) protective clothing in both a desert (43 degrees C dry bulb, 20% relative humidity, wind 2.2 m/s) and tropical (35 degrees C dry bulb, 50% relative humidity, wind 2.2 m/s) climate. During these trials, the evaporative cooling required to maintain thermal balance exceeded the maximal evaporative capacity of the environment and core temperature continued to rise until exhaustion from heat strain occurred. Our findings concerning exhaustion from heat strain are 1) full encapsulation in protective clothing reduces physiological tolerance as core temperature at exhaustion was lower (P < 0.05) in fully than in partially clothed persons, 2) partial encapsulation results in physiological tolerance similar to that reported for unclothed persons, 3) raising metabolic rate from 400 to 600 W does not alter physiological tolerance when subjects are fully clothed, and 4) physiological tolerance is similar when subjects are wearing protective clothing in desert and tropical climates having the same wet bulb globe thermometer. These findings can improve occupational safety guidelines for human heat exposure, as they provide further evidence that the incidence of exhaustion from heat strain can be predicted from core temperature.

Heat strain models applicable for protective clothing systems: comparison of core temperature response

1997 ◽  
Vol 83 (3) ◽  
pp. 1017-1032 ◽  
Author(s):  
R. R. Gonzalez ◽  
T. M. McLellan ◽  
W. R. Withey ◽  
S. K. Chang ◽  
K. B. Pandolf
Keyword(s):  
Heat Stress ◽  
Wind Speed ◽  
Relative Humidity ◽  
Ambient Temperature ◽  
Core Temperature ◽  
Protective Clothing ◽  
Temperature Response ◽  
The United States ◽  
Heat Strain ◽  
Intermittent Work

Gonzalez, R. R., T. M. McLellan, W. R. Withey, S. K. Chang, and K. B. Pandolf. Heat strain models applicable for protective clothing systems: comparison of core temperature response. J. Appl. Physiol. 83(3): 1017–1032, 1997.—Core temperature (Tc) output comparisons were analyzed from thermal models applicable to persons wearing protective clothing. The two models evaluated were the United States (US) Army Research Institute of Environmental Medicine (USARIEM) heat strain experimental model and the United Kingdom (UK) Loughborough (LUT25) model. Data were derived from collaborative heat-acclimation studies conducted by three organizations and included an intermittent-work protocol (Canada) and a continuous-exercise/heat stress protocol (UK and US). Volunteers from the US and the UK were exposed to a standard exercise/heat stress protocol (ambient temperature 35°C/50% relative humidity, wind speed 1 m/s, level treadmill speed 1.34 m/s). Canadian Forces volunteers did an intermittent-work protocol (15 min moderate work/15 min rest at ambient temperature of 40°C/30% relative humidity, wind speed ≈0.4 m/s). Each model reliably predicted Tc responses (within the margin of error determined by 1 root mean square deviation) during work in the heat with protective clothing. Models that are analytically similar to the classic Stolwijk-Hardy model serve as robust operational tools for prediction of physiological heat strain when modified to incorporate clothing heat-exchange factors.

Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

1998 ◽  
Vol 84 (5) ◽  
pp. 1731-1739 ◽  
Author(s):  
Stephen S. Cheung ◽  
Tom M. McLellan
Keyword(s):  
Heart Rate ◽  
Heat Stress ◽  
Aerobic Fitness ◽  
Protective Clothing ◽  
Sweat Rate ◽  
Heat Acclimation ◽  
Short Term ◽  
Chemical Protective Clothing ◽  
Uncompensable Heat Stress ◽  
C 30

—The purpose of the present study was to determine the separate and combined effects of aerobic fitness, short-term heat acclimation, and hypohydration on tolerance during light exercise while wearing nuclear, biological, and chemical protective clothing in the heat (40°C, 30% relative humidity). Men who were moderately fit [(MF); <50 ml ⋅ kg−1 ⋅ min−1maximal O2 consumption; n = 7] and highly fit [(HF); >55 ml ⋅ kg−1 ⋅ min−1maximal O2 consumption; n = 8] were tested while they were euhydrated or hypohydrated by ∼2.5% of body mass through exercise and fluid restriction the day preceding the trials. Tests were conducted before and after 2 wk of daily heat acclimation (1-h treadmill exercise at 40°C, 30% relative humidity, while wearing the nuclear, biological, and chemical protective clothing). Heat acclimation increased sweat rate and decreased skin temperature and rectal temperature (Tre) in HF subjects but had no effect on tolerance time (TT). MF subjects increased sweat rate but did not alter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased Tre and heart rate and decreased the respiratory exchange ratio and the TT regardless of acclimation state. Overall, the rate of rise of skin temperature was less, while ΔTre, the rate of rise of Tre, and the TT were greater in HF than in MF subjects. It was concluded that exercise-heat tolerance in this uncompensable heat-stress environment is not influenced by short-term heat acclimation but is significantly improved by long-term aerobic fitness.

A Field Evaluation of Construction Workers’ Activity, Hydration Status, and Heat Strain in the Extreme Summer Heat of Saudi Arabia

2020 ◽  
Vol 64 (5) ◽  
pp. 522-535 ◽  
Author(s):  
Mohammed Al-Bouwarthan ◽  
Margaret M Quinn ◽  
David Kriebel ◽  
David H Wegman
Keyword(s):  
Heart Rate ◽  
Saudi Arabia ◽  
Heat Stress ◽  
Heat Exposure ◽  
Heat Strain ◽  
Construction Workers ◽  
Hydration Status ◽  
Summer Heat ◽  
Cardiovascular Strain ◽  
Indoor And Outdoor

Abstract Objectives Assess the impact of summer heat exposure (June–September) on residential construction workers in Al-Ahsa, Saudi Arabia by evaluating (i) heart rate (HR) responses, hydration status, and physical workload among workers in indoor and outdoor construction settings, (ii) factors related to physiological responses to work in hot conditions, and (iii) how well wet-bulb globe temperature-based occupational exposure limits (WBGTOELs) predict measures of heat strain. Methods Twenty-three construction workers (plasterers, tilers, and laborers) contributed 260 person-days of monitoring. Workload energy expenditure, HR, fluid intake, and pre- and postshift urine specific gravity (USG) were measured. Indoor and outdoor heat exposures (WBGT) were measured continuously and a WBGTOEL was calculated. The effects of heat exposure and workload on heart rate reserve (HRR), a measure of cardiovascular strain, were examined with linear mixed models. A metric called ‘heat stress exceedance’ (HSE) was constructed to summarize whether the environmental heat exposure (WBGT) exceeded the heat stress exposure limit (WBGTOEL). The sensitivity and specificity of the HSE as a predictor of cardiovascular strain (HRR ≥30%) were determined. Results The WBGTOEL was exceeded frequently, on 63 person-days indoors (44%) and 91(78%) outdoors. High-risk HRR occurred on 26 and 36 person-days indoors and outdoors, respectively. The HSE metric showed higher sensitivity for HRR ≥30% outdoors (89%) than indoors (58%) and greater specificity indoors (59%) than outdoors (27%). Workload intensity was generally moderate, with light intensity work more common outdoors. The ability to self-pace work was associated with a lower frequency of HRR ≥30%. USG concentrations indicated that workers began and ended their shifts dehydrated (USG ≥1.020). Conclusions Construction work where WBGTOEL is commonly exceeded poses health risks. The ability of workers to self-pace may help reduce risks.

Modeling heat stress and heat strain in protective clothing

2014 ◽  
pp. 416-434 ◽  
Author(s):  
P. Bishop ◽  
K. Crew ◽  
J. Wingo ◽  
A. Nawaiseh
Keyword(s):  
Heat Stress ◽  
Protective Clothing ◽  
Heat Strain

348 INFLUENCE OF EXERCISE INTENSITY AND PROTECTIVE CLOTHING ON EXHAUSTION FROM HEAT STRAIN

1993 ◽  
Vol 25 (Supplement) ◽  
pp. S62
Author(s):  
S. J. Montain ◽  
B. S. Cadarette ◽  
J. M. McKay ◽  
M. D. Quigley
Keyword(s):  
Exercise Intensity ◽  
Protective Clothing ◽  
Heat Strain

Core temperature and sweating onset in humans acclimated to heat given at a fixed daily time

1999 ◽  
Vol 276 (4) ◽  
pp. R1095-R1101 ◽  
Author(s):  
Osamu Shido ◽  
Naotoshi Sugimoto ◽  
Minoru Tanabe ◽  
Sotaro Sakurada
Keyword(s):  
Relative Humidity ◽  
Core Temperature ◽  
Heat Exposure ◽  
Heat Acclimation ◽  
Fixed Time ◽  
Temperature Level ◽  
Before And After ◽  
Thermoregulatory Function ◽  
Daily Time ◽  
Made In

The thermoregulatory functions of rats acclimated to heat given daily at a fixed time are altered, especially during the period in which they were previously exposed to heat. In this study, we investigated the existence of similar phenomena in humans. Volunteers were exposed to an ambient temperature (Ta) of 46°C and a relative humidity of 20% for 4 h (1400–1800) for 9–10 consecutive days. In the first experiment, the rectal temperatures (Tre) of six subjects were measured over 24 h at a Ta of 27°C with and without heat acclimation. Heat acclimation significantly lowered Tre only between 1400 and 1800. In the second experiment, six subjects rested in a chair at a Ta of 28°C and a relative humidity of 40% with both legs immersed in warm water (42°C) for 30 min. The Tre and sweating rates at the forearm and chest were measured. Measurements were made in the morning (0900–1100) and afternoon (1500–1700) on the same day before and after heat acclimation. Heat acclimation shortened the sweating latency and decreased the threshold Tre for sweating. However, these changes were significant only in the afternoon. The results suggest that repeated heat exposure in humans, limited to a fixed time daily, alters the core temperature level and thermoregulatory function, especially during the period in which the subjects had previously been exposed to heat.

Low doses of melatonin and diurnal effects on thermoregulation and tolerance to uncompensable heat stress

1999 ◽  
Vol 87 (1) ◽  
pp. 308-316 ◽  
Author(s):  
Tom M. McLellan ◽  
Greg A. Gannon ◽  
Jiri Zamecnik ◽  
Valerie Gil ◽  
Greg M. Brown
Keyword(s):  
Heat Stress ◽  
Protective Clothing ◽  
Low Dose ◽  
Heart Rate Response ◽  
Heat Storage ◽  
Stress Exposure ◽  
Double Blind ◽  
Early Afternoon ◽  
Hypothermic Effect ◽  
Uncompensable Heat Stress

This study examined whether the reported hypothermic effect of melatonin ingestion increased tolerance to exercise at 40°C, for trials conducted either in the morning or afternoon, while subjects were wearing protective clothing. Nine men performed four randomly ordered trials; two each in the morning (0930) and afternoon (1330) after the double-blind ingestion of either two placebo capsules or two 1-mg capsules of melatonin. Despite significant elevations in plasma melatonin to over 1,000 ng/ml 1 h after the ingestion of the first 1-mg dose, rectal temperature (Tre) was unchanged before or during the heat-stress exposure. Also, all other indexes of temperature regulation and the heart rate response during the uncompensable heat stress were unaffected by the ingestion of melatonin. Initial Tre was increased during the afternoon (37.1 ± 0.2°C), compared with the morning (36.8 ± 0.2°C) exposures, and these differences remained throughout the uncompensable heat stress, such that final Tre was also increased for the afternoon (39.2 ± 0.2°C) vs. the morning (39.0 ± 0.3°C) trials. Tolerance times and heat storage were not different among the exposures at ∼110 min and 16 kJ/kg, respectively. It was concluded that this low dose of melatonin had no impact on tolerance to uncompensable heat stress and that trials conducted in the early afternoon were associated with an increased Tre tolerated at exhaustion that offset the circadian influence on resting Tre and thus maintained tolerance times similar to those of trials conducted in the morning.

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