Whole-body heat exchange in women during constant- and variable-intensity work in the heat.

Abstract

Time-weighted averaging is used in occupational heat stress guidelines to estimate the metabolic demands of variable-intensity work. However, compared to constant-intensity work of the same time-weighted average metabolic rate, variable-intensity work may cause decrements in total heat loss (dry + evaporative heat loss) that exacerbate heat storage in women. We therefore used direct calorimetry to assess whole-body total heat loss and heat storage (metabolic heat production minus total heat loss) in women and men during constant- and variable-intensity work of equal average intensity. Ten women [mean (SD); 31 (11) years] and fourteen men [30 (8) years] completed two trials involving 90-min of constant- and variable-intensity work (cycling) eliciting an average metabolic heat production of ~ 200W/m2 in dry-heat (40°C, ~ 15% relative humidity). External work was fixed at ~ 40W/m2 for constant-intensity work, and alternated between ~ 15 and ~ 60W/m2 (5-min each) for variable-intensity work. When expressed as a time-weighted average over each work period, total heat loss did not differ between men and women (mean difference [95% CI]; 4W/m2 [-11, 20]; p = 0.572) or between constant- and variable-intensity work (1W/m2 [-3, 5]; p = 0.642). Consequently, heat storage did not differ significantly between men and women (-4W/m2 [-17, 8]; p = 0.468) or between constant- and variable-intensity work (0W/m2 [-3, 3]; p = 0.834). Neither whole-body heat loss nor heat storage was modulated by the partitioning of work intensity, indicating that time-weighted averaging is appropriate for estimating metabolic demand to assess occupational heat stress in women.