Effects of artificially-induced anaemia on sudomotor and cutaneous blood flow responses to heat stress

The influence of artificially induced anaemia on thermal strain was evaluated in trained males. Heat stress trials (38.6°C, water vapour pressure 2.74 kPa) performed at the same absolute work rates [20 min of seated rest, 20 min of cycling at 30% peak aerobic power (V˙O_2peak), and 20 min cycling at 45% V˙O_2peak] were completed before (HST1) and 3–5 days after 3 units of whole blood were withdrawn (HST2). Mild anaemia did not elevate thermal strain between trials, with auditory canal temperatures terminating at 38.5°C [(0.16), HST1] and 38.6°C [(0.13), HST2; P > 0.05]. Given that blood withdrawal reduced aerobic power by 16%, this observation deviates from the close association often observed between core temperature and relative exercise intensity. During HST2, the absolute and integrated forearm sweat rate (m˙ _sw) exceeded control levels during exercise (P < 0.05), while a suppression of forehead m˙ _sw occurred (P < 0.05). These observations are consistent with a possible peripheral redistribution of sweat secretion. It was concluded that this level of artificially induced anaemia did not impact upon heat strain during a 60-min heat stress test. Accepted: 17 April 1997     

Effects of heat acclimatisation on work tolerance and thermoregulation in trained tropical natives

This study aimed to investigate the effects of heat acclimatisation on thermoregulatory responses and work tolerance in trained individuals residing in the tropics. Eighteen male trained soldiers, who are native to a warm and humid climate, performed a total of four heat stress tests donning the Skeletal Battle Order (SBO, 20.5 kg) and Full Battle Order (FBO, 24.7 kg) before (PRE) and after (POST) a 10-day heat acclimatisation programme. The trials were conducted in an environmental chamber (dry bulb temperature: 32 °C, relative humidity: 70%, solar radiation: 400 W/m²). Excluding the data sets of which participants fully completed the heat stress tests (210 min) before and after heat acclimatisation, work tolerance was improved from 173±30 to 201±18 min (∼21%, p<0.05, n=9) following heat acclimatisation. Following heat acclimatisation, chest skin temperature during exercise was lowered in SBO (PRE=36.7±0.3 vs. POST=36.5±0.3 °C, p<0.01) and FBO (PRE=36.8±0.4 vs. POST=36.6±0.3 °C, p<0.01). Ratings of perceived exertion were decreased with SBO and FBO (PRE=11±2; POST=10±2; p<0.05) after heat acclimatisation. Heat acclimatisation had no effects on baseline body core temperature, heart rate and sweat rate across trials (p>0.05). A heat acclimatisation programme improves work tolerance with minimal effects on thermoregulation in trained tropical natives.     

The role of solar and UV radiation in environmental stress assessment

The purpose of this study was to evaluate the role of ultraviolet (UV) radiation in environmental assessment. Analysis of two meteorological databases, from Israel and New Zealand, used in this study revealed that the UV radiation component in the environmental stress assessment was not significant. However, because of the health hazards, an independent UV index should be implemented and used for preventing acute and chronic injuries. A prediction of UV radiation from solar radiation measurement would be a great benefit for such an index.     

Development and validity of a universal empirical equation to predict skin surface temperature on thermal manikins

Clothing evaporative resistance is an important input in thermal comfort models. Thermal manikin tests give the most accurate and reliable evaporative resistance values for clothing. The calculation methods of clothing evaporative resistance require the sweating skin surface temperature (i.e., options 1 and 2). However, prevailing calculation methods of clothing evaporative resistance (i.e., options 3 and 4) are based on the controlled nude manikin surface temperature due to the sensory measurement difficulty. In order to overcome the difficulty of attaching temperature sensors to the wet skin surface and to enhance the calculation accuracy on evaporative resistance, we conducted an intensive skin study on a thermal manikin ‘Tore’. The relationship among the nude manikin surface temperature, the total heat loss and the wet skin surface temperature in three ambient conditions was investigated. A universal empirical equation to predict the wet skin surface temperature of a sweating thermal manikin was developed and validated on the manikin dressed in six different clothing ensembles. The skin surface temperature prediction equation in an ambient temperature range between 25.0 and 34.0 °C is T_sk=34.0–0.0132HL. It is demonstrated that the universal empirical equation is a good alternative to predicting the wet skin surface temperature and facilitates calculating the evaporative resistance of permeable clothing ensembles. Further studies on the validation of the empirical equation on different thermal manikins are needed however.     

Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3 degrees C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35 degrees C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34 degrees C air) before 35-min cycle exercise: 20 min at approximately 65% VO2peak then a 15-min work-performance trial. At exercise onset, mean core (Tc, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. Tc: CON 38.4+/-0.2 (+/-S.E.), LW 37.9+/-0.1, and LC 37.8+/-0.1 degrees C; HR: 177+/-3, 163+/-3 and 167+/-3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5+/-1.6) and LW (13.6+/-1.0 ml.100 ml tissue(-1) x min(-1)). Subsequent power output was higher in LW (2.95+/-0.24) and LC (2.91+/-0.25) than in CON (2.52+/-0.28 W kg(-1), P=0.00, N=8), yet final Tc remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.     

Firefighter feedback during active cooling: A useful tool for heat stress management?

Monitoring an individual's thermic state in the workplace requires reliable feedback of their core temperature. However, core temperature measurement technology is expensive, invasive and often impractical in operational environments, warranting investigation of surrogate measures which could be used to predict core temperature. This study examines an alternative measure of an individual's thermic state, thermal sensation, which presents a more manageable and practical solution for Australian firefighters operating on the fireground. Across three environmental conditions (cold, warm, hot & humid), 49 Australian volunteer firefighters performed a 20-min fire suppression activity, immediately followed by 20 min of active cooling using hand and forearm immersion techniques. Core temperature (T_c) and thermal sensation (TS) were measured across the rehabilitation period at five minute intervals. Despite the decline in T_c and TS throughout the rehabilitation period, there was little similarity in the magnitude or rate of decline between each measure in any of the ambient conditions. Moderate to strong correlations existed between T_c and TS in the cool (0.41, p<0.05) and hot & humid (0.57, p<0.05) conditions, however this was resultant in strong correlation during the earlier stages of rehabilitation (first five minutes), which were not evident in the latter stages. Linear regression revealed TS to be a poor predictor of T_c in all conditions (SEE=0.45–0.54 °C) with a strong trend for TS to over-predict T_c (77–80% of the time). There is minimal evidence to suggest that ratings of thermal sensation, which represent a psychophysical assessment of an individual's thermal comfort, are an accurate reflection of the response of an individual's core temperature. Ratings of thermal sensation can be highly variable amongst individuals, likely moderated by local skin temperature. In account of these findings, fire managers require a more reliable source of information to guide decisions of heat stress management.     

Changes in exercise and post-exercise core temperature under different clothing conditions

 This study evaluates the effect of different levels of insulation on esophageal (T _es) and rectal (T _re) temperature responses during and following moderate exercise. Seven subjects completed three 18-min bouts of treadmill exercise (75% VO_2max, 22°C ambient temperature) followed by 30 min of recovery wearing either: (1) jogging shoes, T-shirt and shorts (athletic clothing); (2) single-knit commercial coveralls worn over the athletic clothing (coveralls); or (3) a Canadian Armed Forces nuclear, bacteriological and chemical warfare protective overgarment with hood, worn over the athletic clothing (NBCW overgarment). T _es was similar at the start of exercise for each condition and baseline T _re was ∼0.4°C higher than T _es. The hourly equivalent rate of increase in T _es during the final 5 min of exercise was 1.8°C, 3.0°C and 4.2°C for athletic clothing, coveralls and NBCW overgarment respectively (P<0.05). End-exercise T _es was significantly different between conditions [37.7°C (SEM 0.1°C), 38.2°C (SEM 0.2°C and 38.5°C (SEM 0.2°C) for athletic clothing, coveralls and NBCW overgarment respectively)] (P<0.05). No comparable difference in the rate of temperature increase for T _re was demonstrated, except that end-exercise T _re for the NBCW overgarment condition was significantly greater (0.5°C) than that for the athletic clothing condition. There was a drop in T _es during the initial minutes of recovery to sustained plateaus which were significantly (P<0.05) elevated above pre-exercise resting values by 0.6°C, 0.8°C and 1.0°C, for athletic clothing, coveralls, and NBCW overgarment, respectively. Post-exercise T _re decreased very gradually from end-exercise values during the 30-min recovery. Only the NBCW overgarment condition T _re was significantly elevated (0.3°C) above the athletic clothing condition (P<0.05). In conclusion, T _es is far more sensitive in reflecting the heat stress of different levels of insulation during exercise and post-exercise than T _re. Physiological mechanisms are discussed as possible explanations for the differences in response. Received: 30 June 1998 / Accepted: 19 February 1999     

Simulation of hand cooling due to touching cold materials

A simple analytical model has been developed to simulate the cooling of the hands due to touching various types of cold material. The model consisted of a slab of tissue, covered on both sides with skin. The only active mechanism was the skin blood flow. The blood flow was controlled by body core temperature, mean skin temperature, and local hand temperature. The blood flowed along the palm before returning via the back of the hand. The control function was adapted from an earlier study, dealing with feet, but enhanced with a cold induced vasodilatation term. The palm of the hand was touching materials that were specified by conductivity and heat capacity. The hand was initially at a steady-state in a neutral environment and then suddenly grabbed the material. The resulting cooling curves have been compared to data from an experiment including six materials (foam, wood, nylon, steel, aluminium and metal at a constant temperature), three temperatures (-10, 0, and 10 degrees C), two thermal states of the body (neutral and 0.4 degrees C raised), and with and without gloves. There was a fair general agreement between the model and the experiment but the model failed to predict three specific effects: the unequal effect of equal 10 degrees C steps in cold surface temperature on the temperature of the palm of the hand, the cooling effect of nylon, and the rapid drop in back of the hand temperature. Nevertheless the overall regression was 0.88 with a standard deviation between model and experiment of about 2.5 degrees C.     

Monte Carlo simulations of individual variability and their effects on simulated heat stress using thermoregulatory modeling

This paper addresses a variable-dependence (VD) MC method developed based on a previous attempt (VI-MC method) (J. Therm. Biol. 29 (2004), 515) to be incorporated in a thermoregulatory model. Simulated individuals with anthropometrics by VI- and VD-MC methods for US Army population were compared using principal component analysis and Fisher's exact tests. The results indicated that VD-MC data represented overall body size as the primary component and body shape as the secondary component that were more realistic and similar to the measured US Army data (p>0.05) rather than VI-MC data (p<0.05). Such differences consequently affected individual thermoregulatory responses to simulated heat stress. The VD-MC method provides a more realistic representation of individual variability and thus underpins more realistic predictions of individual thermoregulatory responses.     

Interindividual differences in thermal comfort and the responses to skin cooling in young women

The present study aims to understand the effects of interindividual differences in thermal comfort on the relationship between the preferred temperature and the thermoregulatory responses to ambient cooling. Thirteen young women subjects chose the preferred ambient temperature (preferred T_a) in a climate chamber and were categorized into the H group (preferring ≥29 °C; n=6) and the M group (preferring <29 °C; n=7). The H group preferred warmer sensations than the M group (P<0.05) and the average of preferred T_a was 27.6 °C and 30.2 °C in the M group and H group, respectively. Then all subjects were exposed to temperature variations in the climate chamber. During T_a variations from 33 °C to 25 °C, the H group felt colder than the M group, although no difference was noted in the T_sk (mean skin temperature) and T_s-hand between the 2 groups. From the view of the relationship between the T_sk and thermal sensation, although the thermal sensitivity to the T_sk was almost similar in the H and M groups, the H group might have lower threshold to decreasing T_a than the M group.     

Anaerobic respiration and antioxidant responses of Corythucha ciliata (Say) adults to heat-induced oxidative stress under laboratory and field conditions

High temperature often induces oxidative stress and antioxidant response in insects. This phenomenon has been well documented under controlled laboratory conditions, but whether it happens under fluctuating field conditions is largely unknown. In this study, we used an invasive lace bug (Corythucha ciliata) as a model species to compare the effects of controlled thermal treatments (2 h at 33–43 °C with 2 °C intervals in the laboratory) and naturally fluctuating thermal conditions (08:00–14:00 at 2-h intervals (29.7–37.2 °C) on a hot summer day in a field in Shanghai, China) on lipid peroxidation (malondialdehyde (MDA) was the marker) and anaerobic respiration (lactate dehydrogenase (LDH) was the marker), as well as superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione reductase (GR). The results show that MDA concentration increased significantly in response to heat stresses with similar trend in the laboratory and field. LDH activities did not significantly vary across temperatures in the laboratory-exposed individuals, but they significantly increased by rising temperature in the field. The activities or concentrations of SOD, CAT, GSH, and GR all significantly increased with increasing temperature in the two populations. These findings indicate that high temperature induces oxidative stress, resulting in high anaerobic respiration and antioxidant defenses in C. ciliata under both the laboratory and field conditions, which likely provide a defense mechanism against oxidative damage due to the accumulation of ROS.     

Evaluation of a temperate-environment test of heat tolerance in prior heatstroke patients and controls

It has been reported that scores from a temperate-environment step test describe the heat-tolerance status of prior heatstroke patients (HP). This investigation evaluated the ability of this temperate-environment heat-tolerance test (HTT) to indicate altered heart rate (HR) and rectal temperature (Tre) responses of HP, after 7 days of heat acclimation. On day 1, ten male HP (61 +/- 7 days post-heatstroke) and five control subjects (C) bench-stepped (0.30 m high, 27 steps.min-1) for 15 min (25.8 degrees C dry bulb, 16.2 degrees C wet bulb). On days 2-8, subjects underwent heat acclimation (40.1 degrees C dry bulb, 23.8 degrees C wet bulb; treadmill, 90 min.day-1). Heat acclimation resulted in significant decreases in final HR (152 +/- 5 vs 130 +/- 3 beats.min-1, P less than 0.025) and final Tre (38.62 +/- 0.11 vs 38.13 +/- 0.07 degrees C, p less than 0.01) in HP. One HP but no C was defined heat intolerant, exhibiting inability to adapt to daily exercise in the heat. On day 9, HP repeated HTT, exactly as performed on day 1; mean group HTT scores did not change (day 1 = 39 +/- 6; day 9 = 48 +/- 6, P greater than 0.05). All physical characteristics and physiological responses of HP (days 1, 2, 7, 9) were statistically similar (P greater than 0.05) to those of C. In contrast to heat-acclimation data, HTT scores (score less than or equal to 30) indicated that four HP were heat intolerant on day 1 and two HP were heat intolerant on day 9.(ABSTRACT TRUNCATED AT 250 WORDS)     

Daily activity and body temperature

Body temperature varies between 36 and 39° C in states ranging from sleep to high levels of sustained exercise, but it is not known whether this continuum of body temperature is related to a continuum of activity. Calorimetric studies of sedentary days were undertaken with four levels of food intake, men doing mild sustained exercise, and men and women walking and cycling vigorously. Steady states of metabolism were followed by slow exponential changes to steady states of heat loss (Q), followed in turn by changes in rectal temperature (T _re). Regression analysis showed a continuous, curvilinear relationship between Q andT _re from the low end of the activity spectrum (50 W) to progressively higher levels of exercise (600 W). These related continua of activity and body temperature appear to be the result of heat regulation.     

Performance enhancement in rally car drivers via heat acclimation and race simulation

To investigate the combined use of an interactive racecar simulator and heat acclimation on psychomotor (driving) performance, eight rally drivers underwent 4 days of repeated heat (50 degrees C) exposure (1 h x day(-1)) during which they performed a simulated rally drive (3x12-min stages each separated by a 2-min break), after first cycling for 15 min at 125 W to induce some degree of fatigue and heat storage prior to beginning the rally. During the rally stages, a generic set of pace notes were read to the subject by a co-driver. In each simulation, sweat loss, heart rate, core (rectal) and skin temperatures were recorded and driving and psychomotor performance were assessed by recording stage times and time to complete a psychomotor test. Levels of physiological and perceived thermal strain were also recorded. Significant decreases in rally stage times (88 s; P<0.005), psychomotor test time (18 s; P<0.01), final core (0.25 degrees C; P<0.001) and skin (0.44 degrees C; P<0.005) temperatures, heart rate (16 beats x min(-1); P<0.05) and physiological (15 W x m(-2); P<0.005) and perceived thermal (3.7 units; P<0.01) strain were evident by the end of the final simulation, and a significant (P<0.05) increase in sweat sensitivity (+0.33 g x h(-1) x degrees C(-1)) was also recorded. These results suggest that both heat acclimation and race simulation can improve the psychomotor performance of rally drivers, although the relative contribution of each factor was not determined here. However, in a practical setting, these factors would not be used in isolation. After performing the acclimation and simulation protocol prior to an actual rally, drivers subjectively reported improvements in tolerating a high thermal load and in their ability to control the rally vehicle.