Thermal convergence fails to predict heat tolerance limits.
Work under very hot, humid conditions may raise skin temperature (Tsk) to meet rectal temperature (Tre). This convergence has been said to produce imminent collapse and therefore to predict human tolerance limits for work in heat, especially for those wearing impermeable protective clothing. The purpose of this study was to examine the ability of subjects to continue work beyond the time when they experienced convergence. A total of 15 healthy adult volunteers participated in two protocols: A) Each of nine subjects performed four experiments wearing impermeable clothing and walking on an inclined treadmill at a metabolic rate averaging 450 W with dry bulb temperature (Tdb) 29 or 38 degrees C; B) Each of nine subjects performed eight experiments wearing heavy, semipermeable clothing under a range of conditions involving Tdb = 22-40 degrees C, relative humidity 15-88%, and work load = 200-500 W. Convergence occurred in 42 cases; in the majority (60%) the subject continued walking until Tre rose to the criterion temperature 39 degrees C, working for 10-45 min after convergence. No subject approached collapse, nor did convergence have any special effect on the rate of rise of temperature or heart rate. Thus, while convergence marks severe, time-limited heat stress, it does not accurately predict tolerance limits in highly motivated subjects. Termination of work at convergence would unnecessarily truncate heat stress exposures, depriving investigators of valuable data and industrial users of a large portion of their safe work time.
Nunneley
S A
SA
USAF Armstrong Laboratory, Brooks AFB, San Antonio, TX 78235.
Antuñano
M J
MJ
Bomalaski
S H
SH
D000328:Adult; D001831:Body Temperature; D006339:Heart Rate; D006359:Heat Exhaustion / Q000175:diagnosis*; D006801:Humans; D006813:Humidity; D010539:Permeability; D005082:Physical Exertion; D011481:Protective Clothing; D012007:Rectum; D012881:Skin Temperature