Defining Sex‐ and Age‐Specific Initial Stay Times for Continuous Heavy‐Intensity Work in Hot Environments Before Heat‐Mitigation Controls Are Implemented—Part B

Rationale Industry guidelines recommend work‐rest schedules to prevent unsafe core temperature elevations (> 38.0°C or > Δ1.0°C above baseline resting) in an “average” worker exposed to occupational heat stress. While permissible initial work durations (initial stay times [IST]) before implementing rest schedules have been suggested, existing data are limited to moderate‐intensity efforts. Objectives The objective of this study was to assess whether factors of sex and age influence IST during heavy‐intensity work. Methods We assessed work durations before reaching unsafe core temperature thresholds (i.e., > 38.0°C or > Δ1.0°C above baseline resting) among young (18–30 years) and older (50–69 years) males and females during simulated heavy‐intensity work at 26°C, 29°C, and 32°C wet‐bulb globe temperatures (WBGT). Findings The likelihood of surpassing a core temperature of 38.0°C significantly increased at 32°C WBGT (hazard ratio [95% confidence interval]: 2.0 [1.5, 2.7], p = 0.001), but not at 29°C WBGT (1.1 [0.9, 1.4], p = 0.342), compared to 26°C WBGT. Median (interquartile range) durations to reach 38.0°C across groups were 32 (23–52), 29 (23–47), and 26 (18–35) minutes at 26°C, 29°C, and 32°C WBGT, respectively. The likelihood of reaching Δ1.0°C above baseline resting significantly increased at both 32°C (3.8 [2.6, 5.4], p < 0.001) and 29°C WBGT (1.5 [1.1, 2.0], p < 0.001), with median durations being 55 (40–82), 42 (35–56), and 33 (29–42) min, respectively. Older females were more likely to reach 38.0°C (2.4 [1.3, 4.3], p = 0.004) and Δ1.0°C above baseline resting (2.5 [1.4, 4.7], p = 0.003) compared to young males. No other sex‐ or age‐related differences were detected ( p > 0.145). Impact These findings highlight the heightened vulnerability of older females during heavy‐intensity occupational heat stress, emphasizing the need for tailored guidelines to ensure equitable workforce protection.