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

Abstract

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.