Defining Sex‐ and Age‐Specific Initial Stay Times for Continuous Moderate‐Intensity Work in Hot Environments Before Heat‐Mitigation Controls Are Implemented – Part A

Rationale Industry guidelines recommend work‐rest allocations to limit increases in core temperature (Tcore; > 38.0°C or > Δ1.0°C above resting) during work in the heat. However, the initial duration of permissible work before implementing these allocations is not specified (initial stay time [IST]) and individual factors such as sex and age affecting thermoregulation are not considered. Objectives The objective of this study was to assess whether factors of sex and age influence IST. Methods We examined the permissible work duration before reaching Tcore of 38.0°C or Δ1.0°C above resting in young (18–30 years) and older (50–69 years) males and females performing moderate‐intensity (200 W/m 2 ) simulated work at ambient conditions of 26, 29, and 32°C wet‐bulb globe temperature (WBGT). Findings IST significantly decreased as ambient conditions intensified ( p < 0.0001). Median (95% CI) durations to 38°C across groups were 59 (43–87) minutes at 26°C, 40 (34–65) minutes at 29°C, and 35 (31–46) minutes at 32°C WBGT. Likewise, median times to achieve Δ1.0°C above resting temperature significantly differed by environmental conditions ( p < 0.0001): 66 (56–128) minutes at 26°C, 68 (54–80) minutes at 29°C, and 44 (41–53) minutes at 32°C WBGT. Older females were significantly more likely to experience Tcore elevations of 38.0°C (hazard ratio [95% CI]: (3.1 [1.6, 5.8], p < 0.001) and Δ1.0°C above resting (1.9 [1.1, 3.0], p = 0.007) compared to young males. In contrast, IST for young females and older males were similar to young males. Impact Our findings underscore the impact of hotter ambient conditions on IST and identify older females as particularly susceptible during moderate‐intensity work in heat emphasizing the need for tailored occupational heat exposure guidelines.