Critical Assessment of the Recommended Alert Limit Curves for Occupational Heat Exposure

Background: Occupational heat stress recommendations aim to achieve thermal equilibrium and keep core temperature (T_c) below 38.0°C. We assessed the recommended alert limit curves when: (1) work–rest ratios are adjusted based on wet-bulb globe temperature (WBGT) at a fixed rate of metabolic heat production (H_prod) and (2) H_prod is adjusted based on WBGT at a fixed work–rest ratio. We tested the hypothesis that adhering to occupational heat stress recommendations results in thermal equilibrium and prevents T_C from exceeding 38.0°C. Methods: Unacclimated adults completed 4-hour exposures at a fixed WBGT, H_prod, and work–rest ratio. There were six iterations of compliant trials (n = 70 observations; A: 24.1°C, 431 W, 60:0 min; B: 26.6°C, 461 W, 45:15 min; C: 28.4°C, 462 W, 30:30 min; D: 29.7, 453 W, 15:45 min; E: 27.3°C, 412 W, 30:30 min; F: 31.6°C, 290 W, 30:30 min) and two iterations of noncompliant trials (n = 24 observations; G: 31.6°C, 413 W, 30:30 min; H: 36.1°C, 453 W, 15:45 min). Results: Mean and peak T_C across the compliant trials were 37.6°C ± 0.3°C and 37.9°C ± 0.3°C. Thus, 65/70 (93%) and 44/70 (63%) trials did not exceed a mean or peak core T_C of 38.0°C. Mean and peak T_C across the noncompliant trials exceeded 38.0°C in all trials. The rate of heat gain differed between compliant and noncompliant trials (0.08°C ± 0.07°C/h vs. 0.41°C ± 0.34°C/h; p < 0.0001) but on average thermal equilibrium was < 0.1°C/h in the compliant trials. Conclusion: Compliance with the occupational heat stress recommendations resulted in thermal equilibrium and mitigated the development of excessive heat strain. Trial Registration: ClinicalTrials.gov: NCT04767347.