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

Abstract

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/m²) 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.