Do the Threshold Limit Values for Work in Hot Conditions Adequately Protect Workers?

Area of Science:

• Occupational Health
• Environmental Physiology
• Heat Stress Management

Background:

• Core body temperature regulation is critical during occupational heat exposure.
• Existing heat stress guidelines, such as the ACGIH TLV, aim to prevent excessive core temperature rise (38.0°C).

Purpose of the Study:

• To evaluate core temperature responses and body heat content changes (ΔHb) under conditions simulating occupational heat stress.
• To assess the efficacy of ACGIH TLV in maintaining thermal equilibrium during varied work-rest cycles and heat exposures.

Main Methods:

• Nine males performed 120-minute cycling protocols at a fixed heat production (360 W) under different wet-bulb globe temperatures (WBGT) and work-rest (WR) allocations.
• Protocols included continuous cycling at 28.0°C WBGT and intermittent WR ratios (3:1, 1:1, 1:3) at higher WBGTs (29.0°C, 30.0°C, 31.5°C).
• Core temperature (ΔTre) and mean body temperature (ΔTb) were measured via thermometry, with ΔHb determined by direct calorimetry.

Main Results:

• Heat balance was not achieved in any protocol, with rectal temperature increasing over time (ΔTre > 0°C·min).
• Projected core temperatures indicated similar distributions above and below 38.0°C if work extended to 4 hours.
• ΔHb was similar across protocols, but ΔTb measurements differed between calorimetry and thermometry in intermittent work conditions.

Conclusions:

• The ACGIH TLV for heat stress may not ensure heat balance for workers.
• Inconsistent ΔTb and ΔHb measurements suggest current guidelines may not adequately protect against heat strain during occupational work.
• Further research is needed to refine heat stress guidelines for diverse work-rest scenarios and environmental conditions.