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Thermal convergence fails to predict heat tolerance limits.
S A Nunneley1, M J Antuñano, S H Bomalaski
1USAF Armstrong Laboratory, Brooks AFB, San Antonio, TX 78235.
Aviation, Space, and Environmental Medicine
|October 1, 1992
Summary
Skin temperature (Tsk) and rectal temperature (Tre) convergence during heat stress does not predict collapse. Highly motivated individuals can continue working safely for extended periods after convergence, challenging its use as a heat tolerance limit.
Area of Science:
- Environmental Physiology
- Occupational Health
- Human Performance in Extreme Environments
Background:
- Working in hot, humid conditions can cause skin temperature (Tsk) to rise, potentially meeting rectal temperature (Tre).
- This Tsk-Tre convergence is often cited as a predictor of imminent collapse and a limit for human heat work tolerance, particularly with impermeable clothing.
Purpose of the Study:
- To investigate if subjects can continue working beyond the point of Tsk-Tre convergence.
- To determine if convergence accurately predicts heat tolerance limits in healthy volunteers.
Main Methods:
- Two protocols were used with 15 healthy volunteers in either impermeable or heavy semipermeable clothing.
- Subjects performed treadmill walking under various heat conditions (Tdb 22-40°C, RH 15-88%) and workloads (200-500 W).
- Skin and rectal temperatures, heart rate, and work duration were monitored.
Main Results:
- Convergence occurred in 42 instances, with 60% of subjects continuing to work for 10-45 minutes post-convergence until Tre reached 39°C.
- No subjects experienced collapse, and convergence did not significantly alter the rate of temperature or heart rate increase.
- Convergence was observed to mark severe heat stress but did not predict tolerance limits.
Conclusions:
- Tsk-Tre convergence is not a reliable indicator of imminent collapse or a precise predictor of heat tolerance limits in motivated individuals.
- Stopping work at convergence would prematurely end heat stress exposures, limiting valuable research data and safe work time in industrial settings.
- Current understanding of heat stress tolerance may need revision, as motivated individuals can exceed perceived limits indicated by convergence.