Quantifying the impact of heat on human physical work capacity; part IV: interactions between work duration and heat stress severity.

High workplace temperatures negatively impact physical work capacity (PWC). Although PWC loss models with heat based on 1-h exposures are available, it is unclear if further adjustments are required to accommodate repeated work/rest cycles over the course of a full work shift. Therefore, we examined the impact of heat stress exposure on human PWC during a simulated work shift consisting of six 1-h work-rest cycles. Nine healthy males completed six 50-min work bouts, separated by 10-min rest intervals and an extended lunch break, on four separate occasions: once in a cool environment (15 °C/50% RH) and in three different air temperature and relative humidity combinations (moderate, 35 °C/50% RH; hot, 40 °C/50% RH; and very hot, 40 °C/70%). To mimic moderate to heavy workload, work was performed on a treadmill at a fixed heart rate of 130 beats·min-1. During each work bout, PWC was quantified as the kilojoules expended above resting levels. Over the shift, work output per cycle decreased, even in the cool climate, with the biggest decrement after the lunch break and meal consumption. Expressing PWC relative to that achieved in the cool environment for the same work duration, there was an additional 5(± 4)%, 7(± 6)%, and 16(± 7)% decrease in PWC when work was performed across a full work shift for the moderate, hot, and very hot condition respectively, compared with 1-h projections. Empirical models to predict PWC based on the level of heat stress (Wet-Bulb Globe Temperature, Universal Thermal Climate Index, Psychrometric Wet-Bulb Temperature, Humidex, and Heat Index) and the number of work cycles performed are presented.

Age comparison of changes in local warm and cold sensitivity due to whole body cooling.

PURPOSE: This study investigated the influence of whole body cooling on local thermal sensitivity to warm (40°C) and cold (20°C) stimuli in 10 young (age: 24 ± 2 yrs) and 10 older males (age: 69 ± 4 yrs). METHODS: Local warm and cold sensitivity was assessed at eight body regions using a 25 cm2 pressure controlled thermal probe after 40 min of whole body exposure to a thermoneutral (NEUT: 25°C/40% RH) and a cold (COLD: 12°C/50% RH) environment. Gastrointestinal temperature (Tgi), mean and local skin temperature, heart rate, whole body thermal sensation and comfort, and skin blood flow were also measured. RESULTS: Whole body cooling blunted local cold sensitivity but warm sensitivity was maintained in both age groups. Furthermore, a significant age-related decline (from young to older group) in sensitivity to a warm stimulus was observed in both NEUT and COLD conditions. Older males also had a greater ΔTgi compared to the young but had similar thermal sensation and comfort responses. CONCLUSION: The observed interaction effect of local cold stimulation and whole body cooling may be related to both stimuli triggering similar TRP channels, whereas the lack of interaction between local warm stimuli and whole body cooling may be related to these two stimuli triggering different TRP channels. The findings reiterate the potential thermoregulatory risks (e.g. cold injury and hypothermia) associated with ageing, even with such short exposure times.

Body mapping of regional sweat distribution in young and older males.

PURPOSE: Given the pressing impact of global warming and its detrimental effect on the health of older populations, understanding age-related changes in thermoregulatory function is essential. Age differences in regional sweat distribution have been observed previously, but given the typically small measurement areas assessed, the development of whole body sweat maps for older individuals is required. Therefore, this study investigated age-related differences in regional sweat distribution in a hot environment (32 °C/50%RH) in young and older adults, using a body mapping approach. METHODS: Technical absorbent pads were applied to the skin of 14 young (age 24 ± 2 years) and 14 older (68 ± 5 years) males to measure regional sweat rate (RSR) at rest (30 min) and during exercise (30 min), at a fixed heat production (200 W m-2). Gastrointestinal (Tgi) and skin temperature (Tsk), heart rate, thermal sensation, and thermal comfort were also measured. RESULTS: Whole body sweat maps showed that despite equal heat production, healthy older males had significantly lower gross sweat loss (GSL) than the young and significantly lower RSR at almost all body regions at rest and at the hands, legs, ankles, and feet during exercise. The lower sweat loss in the older group coincided with a greater increase in Tgi and a consistently higher Tsk at the legs, despite subjectively feeling slightly cooler than younger individuals. CONCLUSION: These findings support the evidence of age-related deterioration in both autonomic and subjective responses in the heat and highlight the lower extremities as the most affected body region.

Independent and combined impact of hypoxia and acute inorganic nitrate ingestion on thermoregulatory responses to the cold.

PURPOSE: This study assessed the impact of normobaric hypoxia and acute nitrate ingestion on shivering thermogenesis, cutaneous vascular control, and thermometrics in response to cold stress. METHOD: Eleven male volunteers underwent passive cooling at 10 °C air temperature across four conditions: (1) normoxia with placebo ingestion, (2) hypoxia (0.130 FiO2) with placebo ingestion, (3) normoxia with 13 mmol nitrate ingestion, and (4) hypoxia with nitrate ingestion. Physiological metrics were assessed as a rate of change over 45 min to determine heat loss, and at the point of shivering onset to determine the thermogenic thermoeffector threshold. RESULT: Independently, hypoxia expedited shivering onset time (p = 0.05) due to a faster cooling rate as opposed to a change in central thermoeffector thresholds. Specifically, compared to normoxia, hypoxia increased skin blood flow (p = 0.02), leading to an increased core-cooling rate (p = 0.04) and delta change in rectal temperature (p = 0.03) over 45 min, yet the same rectal temperature at shivering onset (p = 0.9). Independently, nitrate ingestion delayed shivering onset time (p = 0.01), mediated by a change in central thermoeffector thresholds, independent of changes in peripheral heat exchange. Specifically, compared to placebo ingestion, no difference was observed in skin blood flow (p = 0.5), core-cooling rate (p = 0.5), or delta change in rectal temperature (p = 0.7) over 45 min, while nitrate reduced rectal temperature at shivering onset (p = 0.04). No interaction was observed between hypoxia and nitrate ingestion. CONCLUSION: These data improve our understanding of how hypoxia and nitric oxide modulate cold thermoregulation.

Reliability and validity of methods in the assessment of cold-induced shivering thermogenesis.

PURPOSE: To compare two analytical methods for the estimation of the shivering onset inflection point, segmental regression and visual inspection of data, and to assess the test-retest reliability and validity of four metrics of shivering measurement; oxygen uptake (V̇O2), electromyography (EMG), mechanomyography (MMG) and bedside shivering assessment scale (BSAS). METHODS: Ten volunteers attended three identical experimental sessions involving passive deep-body cooling via cold water immersion at 10 °C. V̇O2, EMG, and MMG were continuously assessed, while the time elapsed at each BSAS stage was recorded. Metrics were graphed as a function of time and rectal temperature (Tre). Inflection points for intermittent and constant shivering were visually identified for every graph and compared to segmental regression. RESULTS: Excellent agreement was seen between segmental regression and visual inspection (ICC, 0.92). All measurement metrics presented good-to-excellent test-retest reliability (ICC's > 0.75 and 0.90 respectively), with the exception of visual identification of intermittent shivering for V̇O2 measurement (ICC, 0.73) and segmental regression for EMG measurement (ICC, 0.74). In the assessment of signal-to-noise ratio (SNR), EMG showed the largest SNR at the point of shivering onset followed by MMG and finally V̇O2. CONCLUSIONS: Segmental regression provides a successful analytical method for identifying shivering onset. Good-to-excellent reliability can be seen across V̇O2, EMG, MMG, and BSAS, yet given the observed lag times, SNRs, along with known advantages/disadvantaged of each metric, it is recommended that no single metric is used in isolation. An integrative, real-time measure of shivering is proposed.

The biophysical and physiological basis for mitigated elevations in heart rate with electric fan use in extreme heat and humidity.

Electric fan use in extreme heat wave conditions has been thought to be disadvantageous because it might accelerate heat gain to the body via convection. However, it has been recently shown that fan use delays increases in heart rate even at high temperatures (42 °C) in young adults. We here assess the biophysical and physiological mechanisms underlying the apparently beneficial effects of fan use. Eight males (24 ± 3 y; 80.7 ± 11.7 kg; 2.0 ± 0.1 m2) rested at either 36 °C or 42 °C, with (F) or without (NF) electric fan use (4.2 m/s) for 120 min while humidity increased every 7.5 min by 0.3 kPa from a baseline value of 1.6 kPa. Heart rate (HR), local sweat rate (LSR), cutaneous vascular conductance (CVC), core and mean skin temperatures, and the combined convective/radiative heat loss (C+R), evaporative heat balance requirements (Ereq) and maximum evaporative potential (Emax) were assessed. C+R was greater with fan use at 36 °C (F 8 ± 6, NF 2 ± 2 W/m2; P = 0.04) and more negative (greater dry heat gain) with fan use at 42 °C (F -78 ± 4, NF -27 ± 2 W/m2; P < 0.01). Consequently, Ereq was lower at 36 °C (F 38 ± 16, NF 45 ± 3 W/m2; P = 0.04) and greater at 42 °C (F 125 ± 1, NF 74 ± 3 W/m2; P < 0.01) with fan use. However, fan use resulted in a greater Emax at baseline humidity at both 36 °C (F 343 ± 10, NF 153 ± 5 W/m2; P < 0.01) and 42 °C (F 376 ± 13, NF 161 ± 4 W/m2; P < 0.01) and throughout the incremental increases in humidity. Within the humidity range that a rise in HR was prevented by fan use but not without a fan, LSR was higher in NF at both 36 °C (P = 0.04) and 42 °C (P = 0.05), and skin temperature was higher in NF at 42 °C (P = 0.05), but no differences in CVC or core temperatures were observed (all P > 0.05). These results suggest that the delayed increase in heart rate with fan use during extreme heat and humidity is associated with improved evaporative efficiency.

External muscle heating during warm-up does not provide added performance benefit above external heating in the recovery period alone.

PURPOSE: Having previously shown the use of passive external heating between warm-up completion and sprint cycling to have had a positive effect on muscle temperature (T m) and maximal sprint performance, we sought to determine whether adding passive heating during active warm up was of further benefit. METHODS: Ten trained male cyclists completed a standardised 15 min sprint based warm-up on a cycle ergometer, followed by 30 min passive recovery before completing a 30 s maximal sprint test. Warm up was completed either with or without additional external passive heating. During recovery, external passive leg heating was used in both standard warm-up (CONHOT) and heated warm-up (HOTHOT) conditions, for control, a standard tracksuit was worn (CON). RESULTS: T m declined exponentially during CON, CONHOT and HOTHOT reduced the exponential decline during recovery. Peak (11.1 %, 1561 ± 258 W and 1542 ± 223 W), relative (10.6 % 21.0 ± 2.2 W kg(-1) and 20.9 ± 1.8 W kg(-1)) and mean (4.1 %, 734 ± 126 W and 729 ± 125 W) power were all improved with CONHOT and HOTHOT, respectively compared to CON (1,397 ± 239 W; 18.9 ± 3.0 W kg(-1) and 701 ± 109 W). There was no additional benefit of HOTHOT on T m or sprint performance compared to CONHOT. CONCLUSION: External heating during an active warm up does not provide additional physiological or performance benefit. As noted previously, external heating is capable of reducing the rate of decline in T m after an active warm-up, improving subsequent sprint cycling performance.

Aerobic fitness as a parameter of importance for labour loss in the heat.

OBJECTIVES: To derive an empirical model for the impact of aerobic fitness (maximal oxygen consumption; V̇O2max in mL∙kg-1∙min-1) on physical work capacity (PWC) in the heat. DESIGN: Prospective, repeated measures. METHODS: Total work completed during 1 h of treadmill walking at a fixed heart rate of 130 b∙min-1 was assessed in 19 young adult males across a variety of warm and hot climate types, characterised by wet-bulb globe temperatures (WBGT) ranging from 12 to 40 °C. For data presentation and obtaining initial parameter estimates for modelling, participants were grouped into low (n = 6, 74 trials), moderate (n = 8, 76 trials), and high (n = 5, 29 trials) fitness, with group mean V̇O2max 42, 52, and 64 mL∙kg-1∙min-1, respectively. For the heated conditions (WBGT 18 to 40 °C), we calculated PWC% by expressing total energy expenditure (kJ above resting) in each trial relative to that achieved in a cool reference condition (WBGT = 12 °C = 100% PWC). RESULTS: The relative reduction in energy expenditure (PWC%) caused by heat was significantly smaller by up to 16% for the fit participants compared to those with lower aerobic capacity. V̇O2max also modulated the relationship between sweat rate and body temperature changes to increasing WBGT. Including individual V̇O2max data in the PWC prediction model increased the predicting power by 4%. CONCLUSIONS: Incorporating individual V̇O2max improved the predictive power of the heat stress index WBGT for Physical Work Capacity in the heat. The largest impact of V̇O2max on PWC was observed at a WBGT between 25 and 35 °C.

Individual Responses to Heat Stress: Implications for Hyperthermia and Physical Work Capacity.

BACKGROUND: Extreme heat events are increasing in frequency, severity, and duration. It is well known that heat stress can have a negative impact on occupational health and productivity, particularly during physical work. However, there are no up-to-date reviews on how vulnerability to heat changes as a function of individual characteristics in relation to the risk of hyperthermia and work capacity loss. The objective of this narrative review is to examine the role of individual characteristics on the human heat stress response, specifically in relation to hyperthermia risk and productivity loss in hot workplaces. Finally, we aim to generate practical guidance for industrial hygienists considering our findings. Factors included in the analysis were body mass, body surface area to mass ratio, body fat, aerobic fitness and training, heat adaptation, aging, sex, and chronic health conditions. FINDINGS: We found the relevance of any factor to be dynamic, based on the work-type (fixed pace or relative to fitness level), work intensity (low, moderate, or heavy work), climate type (humidity, clothing vapor resistance), and variable of interest (risk of hyperthermia or likelihood of productivity loss). Heat adaptation, high aerobic fitness, and having a large body mass are the most protective factors during heat exposure. Primary detrimental factors include low fitness, low body mass, and lack of heat adaptation. Aging beyond 50 years, being female, and diabetes are less impactful negative factors, since their independent effect is quite small in well matched participants. Skin surface area to mass ratio, body composition, hypertension, and cardiovascular disease are not strong independent predictors of the heat stress response. CONCLUSION: Understanding how individual factors impact responses to heat stress is necessary for the prediction of heat wave impacts on occupational health and work capacity. The recommendations provided in this report could be utilized to help curtail hyperthermia risk and productivity losses induced by heat.

Occupational Heat Stress and Practical Cooling Solutions for Healthcare and Industry Workers During the COVID-19 Pandemic.

Treatment and management of severe acute respiratory syndrome coronavirus-2, which causes coronavirus disease (COVID-19), requires increased adoption of personal protective equipment (PPE) to be worn by workers in healthcare and industry. In warm occupational settings, the added burden of PPE threatens worker health and productivity, a major lesson learned during the West-African Ebola outbreak which ultimately constrained disease control. In this paper, we comment on the link between COVID-19 PPE and occupational heat strain, cooling solutions available to mitigate occupational heat stress, and practical considerations surrounding their effectiveness and feasibility. While the choice of cooling solution depends on the context of the work and what is practical, mitigating occupational heat stress benefits workers in the healthcare and industrial sectors during the COVID-19 disease outbreak.

The Threshold Ambient Temperature for the Use of Precooling to Improve Cycling Time-Trial Performance.

PURPOSE: Cycling time-trial performance can be compromised by moderate to high ambient temperatures. It has become commonplace to implement precooling prior to competition to alleviate this performance decline. However, little is known about the ambient temperature threshold above which precooling becomes an effective strategy for enhancing endurance performance. The aim of this study was to investigate the effect of precooling in different environmental temperatures on time-trial (TT) performance. METHODS: Trained cyclists completed 2 TTs with (COLD) and without (CON) precooling using an ensemble of  ice vest and sleeves in ambient temperatures of 24°C, 27°C, and 35°C. RESULTS: TT performance was faster following COLD in both 35°C (6.2%) and 27°C (2.6%; both Ps < .05) but not 24°C (1.2%). Magnitude-based inferential statistics indicate that COLD was very likely beneficial to performance in 35°C, likely beneficial in 27°C, and possibly beneficial in 24°C. Mean power was 2.4%, 2.5%, and 5.6% higher following COLD and considered to be likely beneficial in 24°C and very likely beneficial in 27°C and 35°C. COLD reduced mean skin temperature throughout the warm-up and into the TT in all ambient temperatures (P < .05). Sweat loss was lower following COLD in 24°C and 27°C but not 35°C. There was no effect of COLD on gastrointestinal temperature at any point. CONCLUSIONS: Precooling with an ice vest and sleeves is likely to have a positive effect on TT performance at temperatures above 24°C, with a clear relationship between ambient temperature and the magnitude of effect of precooling.

Subjective responses to display bezel characteristics.

High quality flat panel computer displays (FPDs) with high resolution screens are now commonplace, and black, grey, white, beige and silver surrounds ('bezels'), matt or glossy, are in widespread use. It has been suggested that bezels with high reflectance, or with a high gloss, could cause eyestrain, and we have investigated this issue. Twenty office workers (unaware of the study purpose) used six different FPDs, for a week each, at their own desk. These displays were identical apart from the bezel colour (black, white or silver) and shininess (matt or glossy). Participants completed questionnaires about their visual comfort at the end of each week, and were fully debriefed in lunch-time focus groups at the end of the study. For the white and the silver bezels, the glossiness of the bezel was not an issue of concern. The participants were significantly less content with the glossy black surround than with the matt black surround, and in general the glossy black bezel was the least-liked of all those used. With the possible exception of this surround, there was no evidence of significantly increased visual discomfort, indicative of eyestrain, as a result of high or low bezel reflectance, or of high glossiness.

Should electric fans be used during a heat wave?

Heat waves continue to claim lives, with the elderly and poor at greatest risk. A simple and cost-effective intervention is an electric fan, but public health agencies warn against their use despite no evidence refuting their efficacy in heat waves. A conceptual human heat balance model can be used to estimate the evaporative requirement for heat balance, the potential for evaporative heat loss from the skin, and the predicted sweat rate, with and without an electrical fan during heat wave conditions. Using criteria defined by the literature, it is clear that fans increase the predicted critical environmental limits for both the physiological compensation of endogenous/exogenous heat, and the onset of cardiovascular strain by an air temperature of ∼3-4 °C, irrespective of relative humidity (RH) for the young and elderly. Even above these critical limits, fans would apparently still provide marginal benefits at air temperatures as high as 51.1 °C at 10%RH for young adults and 48.1 °C at 10%RH for the elderly. Previous concerns that dehydration would be exacerbated with fan use do not seem likely, except under very hot (>40 °C) and dry (<10%RH) conditions, when predicted sweat losses are only greater with fans by a minor amount (∼20-30 mL/h). Relative to the peak outdoor environmental conditions reported during ten of the most severe heat waves in recent history, fan use would be advisable in all of these situations, even when reducing the predicted maximum sweat output for the elderly. The protective benefit of fans appears to be underestimated by current guidelines.

Reducing muscle temperature drop after warm-up improves sprint cycling performance.

PURPOSE: This study aimed to determine the effect of passive insulation versus external heating during recovery after a sprint-specific warm-up on thigh muscle temperature and subsequent maximal sprint performance. METHODS: On three separate occasions, 11 male cyclists (age = 24.7 ± 4.2 yr, height = 1.82 ± 0.72 m, body mass = 77.9 ± 9.8 kg; mean ± SD) completed a standardized 15-min intermittent warm-up on a cycle ergometer, followed by a 30-min passive recovery period before completing a 30-s maximal sprint test. Muscle temperature was measured in the vastus lateralis at 1, 2, and 3 cm depth before and after the warm-up and immediately before the sprint test. Absolute and relative peak power output was determined and blood lactate concentration was measured immediately after exercise. During the recovery period, participants wore a tracksuit top and (i) standard tracksuit pants (CONT), (ii) insulated athletic pants (INS), or (iii) insulated athletic pants with integrated electric heating elements (HEAT). RESULTS: Warm-up increased Tm by approximately 2.5 °C at all depths, with no differences between conditions. During recovery, Tm remained elevated in HEAT compared with INS and CONT at all depths (P < 0.001). Both peak and relative power output were elevated by 9.6% and 9.1%, respectively, in HEAT compared with CONT (both P < 0.05). The increase in blood lactate concentration was greater (P < 0.05) after sprint in HEAT (6.3 ± 1.8 mmol·L(-1)) but not INS (4.0 ± 1.8 mmol·L(-1)) versus CONT (4.1 ± 1.9 mmol·L(-1)). CONCLUSIONS: Passive heating of the thighs between warm-up completion and performance execution using pants incorporating electrically heated pads can attenuate the decline in Tm and improve sprint cycling performance.

A comparison between the technical absorbent and ventilated capsule methods for measuring local sweat rate.

This study assessed the accuracy of the technical absorbent (TA) method for measuring local sweat rate (LSR) relative to the well-established ventilated capsule (VC) method during steady-state and nonsteady-state sweating using large and small sample surface areas on the forearm and midback. Forty participants (38 males and two females) cycled at 60% peak oxygen consumption for 75 min in either a temperate [22.3 ± 0.9°C, 32 ± 17% relative humidity (RH)] or warm (32.5 ± 0.8°C, 29 ± 7% RH) environment. Simultaneous bilateral comparisons of 5-min LSR measurements using the TA and VC methods were performed for the back and forearm after 10, 30, 50, and 70 min. LSR values, measured using the TA method, were highly correlated with the VC method at all time points, irrespective of sample surface area and body region (all P < 0.001). On average, ≈ 79% of the variability observed in LSR measured with the VC method was described by the TA method. The mean difference in absolute LSR using the TA method (TA-VC with 95% confidence intervals) was -0.23 [-0.30,-0.16], -0.11 [-0.21,0.00], -0.03 [-0.14,+0.08], and +0.02 [-0.07,+0.11] mg · cm(-2) · min(-1) after 10, 30, 50, and 70 min of exercise, respectively. Duplicate LSR measurements within each method during steady-state sweating were highly correlated (TA: r = 0.96, P < 0.001, n = 20; VC: r = 0.97, P < 0.001, n = 20) with a mean bias of +0.07 ± 0.14 and +0.01 ± 0.10 mg · cm(-2) · min(-1) for TA and VC methods, respectively. The mean smallest detectable difference in LSR was 0.12 and 0.05 mg · min(-1) · cm(-2) for TA and VC methods, respectively. These data support the TA method as a reliable alternative for measuring the rate of sweat appearance on the skin surface.

The effects of solar radiation on thermal comfort.

The aim of this study was to investigate the relationship between simulated solar radiation and thermal comfort. Three studies investigated the effects of (1) the intensity of direct simulated solar radiation, (2) spectral content of simulated solar radiation and (3) glazing type on human thermal sensation responses. Eight male subjects were exposed in each of the three studies. In Study 1, subjects were exposed to four levels of simulated solar radiation: 0, 200, 400 and 600 Wm(-2). In Study 2, subjects were exposed to simulated solar radiation with four different spectral contents, each with a total intensity of 400 Wm(-2) on the subject. In Study 3, subjects were exposed through glass to radiation caused by 1,000 Wm(-2) of simulated solar radiation on the exterior surface of four different glazing types. The environment was otherwise thermally neutral where there was no direct radiation, predicted mean vote (PMV)=0+/-0.5, [International Standards Organisation (ISO) standard 7730]. Ratings of thermal sensation, comfort, stickiness and preference and measures of mean skin temperature (t(sk)) were taken. Increase in the total intensity of simulated solar radiation rather than the specific wavelength of the radiation is the critical factor affecting thermal comfort. Thermal sensation votes showed that there was a sensation scale increase of 1 scale unit for each increase of direct radiation of around 200 Wm(-2). The specific spectral content of the radiation has no direct effect on thermal sensation. The results contribute to models for determining the effects of solar radiation on thermal comfort in vehicles, buildings and outdoors.