Do the threshold limit values for work in hot conditions adequately protect workers?

Abstract

Purpose We evaluated core temperature responses and the change in body heat content (ΔH b) during work performed according to the ACGIH threshold limit values (TLV) for heat stress, which are designed to ensure a stable core temperature that does not exceed 38.0°C. Methods Nine young males performed a 120-min work protocol consisting of cycling at a fixed rate of heat production (360 W). On the basis of the TLV, each protocol consisted of a different work-rest (WR) allocation performed in different wet-bulb globe temperatures (WBGT). The first was 120 min of continuous (CON) cycling at 28.0°C WBGT (CON[28.0°C]). The remaining three protocols were intermittent work bouts (15-min duration) performed at various WR and WBGT: (i) WR of 3:1 at 29.0°C (WR3:1[29.0°C]), (ii) WR of 1:1 at 30.0°C (WR1:1[30.0°C]), and (iii) WR of 1:3 at 31.5°C (WR1:3[31.5°C]) (total exercise time: 90, 60, and 30 min, respectively). The change in rectal (ΔT re) and mean body temperature (ΔT b) was evaluated with thermometry. ΔH b was determined via direct calorimetry and also used to calculate ΔT b. Results Although average rectal temperature did not exceed 38.0°C, heat balance was not achieved during exercise in any work protocol (i.e., rate of ΔT re > 0°C·min -1; all P values ≤ 0.02). Consequently, it was projected that if work was extended to 4 h, the distribution of participant core temperatures higher and lower than 38.0°C would be statistically similar (all P values ≥ 0.10). Furthermore, ΔH b was similar between protocols (P = 0.70). However, a greater ΔT b was observed with calorimetry relative to thermometry in WR3:1[29.0°C] (P = 0.03), WR1:1[30.0°C] (P = 0.02), and WR1:3[31.5°C] (P < 0.01) but not CON[28.0°C] (P = 0.32). Conclusion The current study demonstrated that heat balance was not achieved and ΔT b and ΔH b were inconsistent, suggesting that the TLV may not adequately protect workers during work in hot conditions. © Government of Canada, 2016.