Indicators to assess physiological heat strain – Part 3: Multi-country field evaluation and consensus recommendations

Abstract

In a series of three companion papers published in this Journal, we identify and validate the available thermal stress indicators (TSIs). In this third paper, we conducted field experiments across nine countries to evaluate the efficacy of 61 meteorology-based TSIs for assessing the physiological strain experienced by individuals working in the heat. We monitored 372 experi-enced and acclimatized workers during 893 full work shifts. We continuously assessed core body temperature, mean skin temperature, and heart rate data together with pre/post urine specific gravity and color. The TSIs were evaluated against 17 published criteria covering physiological parameters, practicality, cost effectiveness, and health guidance issues. Simple meteorological parameters explained only a fraction of the variance in physiological heat strain (R 2 = 0.016 to 0.427; p < 0.001), reflecting the importance of adopting more sophisticated TSIs. Nearly all TSIs correlated with mean skin temperature (98%), mean body temperature (97%), and heart rate (92%), while 66% of TSIs correlated with the magnitude of dehydration and 59% correlated with core body temperature (r = 0.031 to 0.602; p < 0.05). When evaluated against the 17 published criteria, the TSIs scored from 4.7 to 55.4% (max score = 100%). The indoor (55.4%) and outdoor (55.1%) Wet-Bulb Globe Temperature and the Universal Thermal Climate Index (51.7%) scored higher compared to other TSIs (4.7 to 42.0%). Therefore, these three TSIs have the highest potential to assess the physiological strain experienced by individuals working in the heat.

Potential Impacts of Different Occupational Outdoor Heat Exposure Thresholds among Washington State Crop and Construction Workers and Implications for Other Jurisdictions

“…There are several considerations for decision-making about heat metrics. WBGT, which in a recent analysis has been reported to have a high potential to reflect physiological strain experienced by workers [ 47 ], is the basis for WA heat rule trigger temperatures [ 27 ] but is not used as an outward-facing metric in existing U.S. heat rules, as it may not be practical to assess in all workplace settings. Though certain groups of workers, such as agricultural workers, may transit between the Western states of CA, OR, and WA for work, these states currently use different metrics (air temperature in CA and WA versus HI in OR) in their heat rules.…”

“…These differences in humidity within a state illustrate the challenges of defining outdoor heat exposure metrics for statewide policies. HI is a simplified metric that accounts for the impact of humidity on worker exposure, yet HI does not take into account solar radiation, makes assumptions about wind level, and may not optimally reflect physiological strain experienced by workers [ 47 ]. Considerations of the effect of humidity on HRI risk given the local climate, employment patterns across space and time, the tradeoffs of assessing real-time and forecasted heat exposure metrics, and alignment with nearby jurisdictions where workers may also work should be considered in decisions about heat metrics.…”

Cardiovascular Stress and Characteristics of Cold-Induced Vasodilation in Women and Men during Cold-Water Immersion: A Randomized Control Study

“…Each of the three experimental sessions was performed inside a 32.5 m 3 environmental chamber (HGX22G/190-4, Bock GmbH, Frickenhausen,,Germany) under different conditions: cool (wet-bulb globe temperature: 10.8 °C; air temperature: 16 ± 1°C; relative humidity: 45 ± 5%; air velocity: 0.2 m/s; solar radiation: 0 W/m 2 ), thermoneutral (wet-bulb globe temperature: 17.2 °C; air temperature: 23 ± 1 °C; relative humidity: 45 ± 5%; air velocity: 0.2 m/s; solar radiation: 0 W/m 2 ), and hot (wet-bulb globe temperature: 27.2 °C; air temperature: 34 ± 1 °C; 45 ± 5% relative humidity; air velocity: 0.2 m/s; solar radiation: 0 W/m 2 ) environments. The overall thermal stress experienced by the participants in our study was expressed by means of wet-bulb globe temperature which was found to be the most efficacious thermal-stress indicator for assessing the physiological strain [ 38 , 39 , 40 ]. The present study is a randomized control trial.…”

The effect of the participatory heat education and awareness tools (HEAT) intervention on agricultural worker physiological heat strain: results from a parallel, comparison, group randomized study

“…Though it would be ideal to have participation on the exact same dates by all companies and workers, we were able to account for time-varying variability in factors such as heat exposure, and also accounted for company, in our analyses. Fifth, we used HI rather than more complex environmental metrics that also account for solar radiation and wind speed and that correlate better with heat strain [59]. However, we were able to assess physiologic heat strain directly, using estimated core body temperature, in our analyses.…”