Do one-hour exposures provide a valid assessment of physiological heat strain?

Abstract

For time- and cost-efficient heat stress assessment procedures at workplaces or in experimental studies, short-time measurement periods (e.g. 1 h) are sometimes employed in lieu of whole shift observations assuming that the short time period will provide valid figures of equilibrium physiological responses. We studied the influence of exposure duration on physiological heat strain considering the modifying effects of clothing and heat acclimation using a database of 564 climatic chamber exposures performed by 28 young males under heat stress conditions with widely varying air temperature and humidity levels. We compared heart rates, rectal and mean skin temperatures, and sweat rates recorded after 1 h with the values averaged over the third hour of exposure representing steady-state. One-hour measurements agreed with equilibrium values for rather low strain levels only, with heart rates below 100 bpm and rectal temperatures below 37.2 °C. On average, one-hour values underestimated all heat strain parameters. This underestimation error was only moderately influenced by clothing and heat acclimation status, but increased significantly with air temperature and humidity, reaching considerable magnitude under hot-humid conditions associated with elevated heat strain. Regression analyses of the prediction error depending on the equilibrium response revealed that underestimation increased with equilibrium strain level. This correlation was strongest for heart rate and core temperature, and was shown to potentially cause a misclassification of hazardous working conditions as safe by given heat strain criteria.Practical Relevance: The severe underestimation of heat strain due to short measurement periods, as observed under hot-humid conditions and/or when associated with high physiological strain, will immediately impact the exposed personnel, but will also inform occupational health professionals and standard writers regarding the heat stress assessment for work shifts with high activity levels or with protective clothing.

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

“…as is common in many industrial sectors) however, further investigation is warranted to confirm the long-term impacts of heat stress exposure on physical work capacity across a range of working populations. Finally, Kampmann and Bröde ( 2022 ) compared indicators of heat strain during the first and third hour of work at a constant workload, performed in a range of climates. They observed significantly lower heat strain (heart rate, rectal temperature) estimates based on the first hour data versus the third hour, and warn that using short studies to develop heat strain prediction models risks underestimating the heat strain for a full work shift, even at moderate heat loads.…”

“…Our recent PWC models (work capacity in the heat expressed as the percentage of the total work possible in a cool climate) were developed from a large dataset that captured a wide range of heat stress severities. However, the 1-h work duration of the experiments may underestimate the physiological strain and impact associated with extended heat stress exposure (Kampmann and Bröde, 2022) and, thus, the work capacity loss in industry, where typical work shifts often James W Smallcombe and Josh Foster contributed equally to the preparation of the manuscript and is co-first author. last 6-8 h. For example, data derived from extended-duration occupational heat exposures indicate that workers often struggle to fully replace thermoregulatory sweat losses with adequate water consumption during the day, particularly when sweat rates are high (Kalkowsky and Kampmann, 2006).…”

“…Given the known importance of hydration status in maintaining PWC in the heat (Cheung and McLellan, 1998;Marino et al 2004), and an associated blunting of sweat secretion proportional to the level of intracellular dehydration (Cheuvront and Kenefick, 2014), progressive hypohydration throughout the day may exacerbate PWC losses beyond what our studies observed during shorter 1-h heat stress exposures. Additionally, with longer occupational heat stress exposures-even when rest breaks are permitted-heat may gradually accumulate in the body throughout the working day (Bröde et al 2018, Kampmann andBröde. 2022) contributing to elevated cardiovascular strain and further reduced work capacity.…”

“…Our recent PWC models (work capacity in the heat expressed as the percentage of the total work possible in a cool climate) were developed from a large dataset that captured a wide range of heat stress severities. However, the 1-h work duration of the experiments may underestimate the physiological strain and impact associated with extended heat stress exposure (Kampmann and Bröde, 2022 ) and, thus, the work capacity loss in industry, where typical work shifts often last 6–8 h. For example, data derived from extended-duration occupational heat exposures indicate that workers often struggle to fully replace thermoregulatory sweat losses with adequate water consumption during the day, particularly when sweat rates are high (Kalkowsky and Kampmann, 2006 ). Given the known importance of hydration status in maintaining PWC in the heat (Cheung and McLellan, 1998 ; Marino et al 2004 ), and an associated blunting of sweat secretion proportional to the level of intracellular dehydration (Cheuvront and Kenefick, 2014 ), progressive hypohydration throughout the day may exacerbate PWC losses beyond what our studies observed during shorter 1-h heat stress exposures.…”

“…Additionally, with longer occupational heat stress exposures—even when rest breaks are permitted—heat may gradually accumulate in the body throughout the working day (Bröde et al 2018 , Kampmann and Bröde. 2022 ) contributing to elevated cardiovascular strain and further reduced work capacity.…”