Abstract
Heat stress decreases human physical work capacity (PWC), but the extent to which solar radiation (SOLAR) compounds this response is not well understood. This study empirically quantified how SOLAR impacts PWC in the heat, considering wide, but controlled, variations in air temperature, humidity, and clothing coverage. We also provide correction equations so PWC can be quantified outdoors using heat stress indices that do not ordinarily account for SOLAR (including the Heat Stress Index, Humidex, and Wet-Bulb Temperature). Fourteen young adult males (7 donning a work coverall, 7 with shorts and trainers) walked for 1 h at a fixed heart rate of 130 beats∙min−1, in seven combinations of air temperature (25 to 45°C) and relative humidity (20 or 80%), with and without SOLAR (800 W/m2 from solar lamps). Cumulative energy expenditure in the heat, relative to the work achieved in a cool reference condition, was used to determine PWC%. Skin temperature was the primary determinant of PWC in the heat. In dry climates with exposed skin (0.3 Clo), SOLAR caused PWC to decrease exponentially with rising air temperature, whereas work coveralls (0.9 Clo) negated this effect. In humid conditions, the SOLAR-induced reduction in PWC was consistent and linear across all levels of air temperature and clothing conditions. Wet-Bulb Globe Temperature and the Universal Thermal Climate Index represented SOLAR correctly and did not require a correction factor. For the Heat Stress Index, Humidex, and Wet-Bulb Temperature, correction factors are provided enabling forecasting of heat effects on work productivity.
Highlights
Environmental heat exposure has a negative impact on human health and physical working capacity (PWC) (Flouris et al, 2018; Foster et al, 2021a, 2021b; Ioannou et al, 2021a), incurring significant economic damage through its impact on workplace productivity (Hübler et al, 2008; Zander et al, 2015; Hsiang et al, 2017)
We modeled the difference in physical work capacity (PWC) between the SHADE and solar radiation (SOLAR) models to form correction factors based on the solar intensity
We show that, based on the change in mean radiant temperature with SOLAR, the relative shift in the value of wetbulb globe temperature (WBGT) and universal thermal climate index (UTCI) predicts the reduction in PWC caused by SOLAR appropriately
Summary
Environmental heat exposure has a negative impact on human health and physical working capacity (PWC) (Flouris et al, 2018; Foster et al, 2021a, 2021b; Ioannou et al, 2021a), incurring significant economic damage through its impact on workplace productivity (Hübler et al, 2008; Zander et al, 2015; Hsiang et al, 2017). While models of PWC based on various climate indices have recently been developed (Dunne et al, 2013; Kjellstrom et al, 2018; Foster et al, 2021b), at present, none account for the effect of solar or general thermal radiation. In 338 trials with work paced based on heart rate (limit of 130 beats·min−1 (moderate to heavy work)), our group recently developed empirical models for PWC based on a suite of heat stress indices (Foster et al, 2021b). Those trials were conducted without added SOLAR. To use the equations for both conditions with and without SOLAR, correction factors may be needed, especially for
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