The recommended Threshold Limit Values for heat exposure fail to maintain body core temperature within safe limits in older working adults

ABSTRACT

Purpose: The American Conference of Governmental and Industrial Hygienists (ACGIH®) Threshold Limit Values (TLV® guidelines) for work in the heat consist of work-rest (WR) allocations designed to ensure a stable core temperature that does not exceed 38°C. However, the TLV® guidelines have not been validated in older workers. This is an important shortcoming given that adults as young as 40 years demonstrate impairments in their ability to dissipate heat. We therefore evaluated body temperature responses in older adults during work performed in accordance to the TLV® recommended guidelines.

Methods: On three occasions, 9 healthy older (58 ± 5 years) males performed a 120-min work-simulated protocol in accordance with the TLV® guidelines for moderate-to-heavy intensity work (360 W fixed rate of heat production) in different wet-bulb globe temperatures (WBGT). The first was 120 min of continuous (CON) cycling at 28.0°C WBGT (CON[28°C]). The other two protocols were 15-min intermittent work bouts performed with different WR cycles and WBGT: (i) WR of 3:1 at 29.0°C (WR3:1[29°C]) and (ii) WR of 1:1 at 30.0°C (WR1:1[30°C]). Rectal temperature was measured continuously. The rate of change in mean body temperature was determined via thermometry (weighting coefficients: rectal, 0.9; mean skin temperature, 0.1) and direct calorimetry.

Results: Rectal temperature exceeded 38°C in all participants in CON[28°C] and WR3:1[29°C] whereas a statistically similar proportion of workers exceeded 38°C in WR1:1[30°C] (χ²; P = 0.32). The average time for rectal temperature to reach 38°C was: CON[28°C], 53 ± 7; WR3:1[29°C], 79 ± 11; and WR1:1[30°C], 100 ± 29 min. Finally, while a stable mean body temperature was not achieved in any work condition as measured by thermometry (i.e., >0°C·min⁻¹; all P<0.01), heat balance as determined by direct calorimetry was achieved in WR3:1[29°C] and WR1:1[30°C] (both P ≥ 0.08).

Conclusion: Our findings indicate that the TLV® guidelines do not prevent body core temperature from exceeding 38°C in older workers. Furthermore, a stable core temperature was not achieved within safe limits (i.e., ≤38°C) indicating that the TLV® guidelines may not adequately protect all individuals during work in hot conditions.

KEYWORDS:

• Calorimetry
• climate change
• exposure guidelines
• older adults
• thermometry
• thermoregulation

Acknowledgments

The authors would like to thank all the participants who took part in the study. This study was conducted at the Human and Environmental Physiology Research Unit of the University of Ottawa.

Funding

Funding support provided by the Electrical Power Research Institute, the Ontario Ministry of Labour, and the Natural Sciences and Engineering Research Council of Canada (All funds held by Dr. Glen P. Kenny). Dr. Glen P. Kenny is supported by a University of Ottawa Research Chair Award. Mr. Dallon Lamarche was supported by funds from the Ontario Ministry of Labour. Mr. Andrew W. D'Souza was supported by fundings from the Electrical Power Research Institute. Mr. Robert D. Meade is supported by a Natural Sciences and Engineering Research Council of Canada Alexander Graham Bell graduate scholarship (CGS-D). The provision of financial support does not in any way infer or imply endorsement of the research findings by either agency. The authors declare no conflict of interest. The results of the present study do not constitute endorsement by the Journal of Occupational & Environmental Hygiene or other funding agencies.