The Cumulative Effects of Three Consecutive Days of Simulated Occupational Heat Stress on Physiological Strain in Young Adults

PURPOSE: Many workers complete prolonged shifts (≥8 h) across consecutive days in the heat, yet it is unclear whether this produces cumulative increases in physiological strain that must be considered in heat safety guidance. This study quantified physiological strain across three consecutive days of simulated occupational heat stress in physically active, young adults. METHODS: Twenty participants (15 males, 5 females) were recruited to complete two 74-h trials in a randomized cross-over design trial. Each trial spanned four consecutive days. Days 1-3 involved baseline measurements (07:00-08:30h), an ~8-h work shift (08:30-16:00h) and an at home recovery (16:00-07:00h [+1 day]). Day 4 involved a final measurement period (07:00-08:00h). Work shifts involved cycles of 30 min work (~367W; ~4.3METs) and 30 min seated rest in summer clothing. Each trial differed only by environment during the work shifts: hot (35°C; 63%RH; 31°C WBGT) or control (18°C; 53%RH; 14°C WBGT). Core temperature, heart rate, mean skin temperature, fluid balance, and perceptual responses were assessed at baseline and end-of-shift. RESULTS: End-of-shift core temperature, core temperature area under the curve, heart rate, and mean skin temperature were higher in hot versus control (all p ≤. 003). However, with the exception of heart rate, which was 20 (95% CI; [12, 27]) beats·min⁻¹ lower on day 3 versus day 1 in the hot condition (p =.018), no outcomes differed across days in hot or control conditions (p ≥.182). CONCLUSIONS: Although there is still a need for larger studies with more protracted exposures in other populations. These findings indicate that there are minimal cumulative physiological effects of three consecutive days of moderate-intensity work in the heat in young, physically active adults.