Thermal Exposure Limit in a Simulated Refuge Alternative

Federal standards for refuge alternatives (RAs) mandate that they not exceed a Steadman apparent temperature (AT) of 95 degrees F (35 degrees C) at an assumed metabolic rate of 325 W, a limit that appears to be arbitrary. Occupants in an RA spend most of their time at rest (a metabolic rate less than 325 W), and thermal equilibrium can likely be maintained at an AT > 35 degrees C. The purpose of this study was to examine the upper limit of sustainable heat stress during 4- and 8-h exposures at rest. Five men underwent five 4-h trials (phase 1: AT range 39 to 49 degrees C, 90% rh) and five 8-h trials (phase 2: AT range 46 to 56 degrees C, 90% rh) in a semi-recumbent position. Descriptive statistics for gastrointestinal temperature (T-gi), heart rate (HR), systolic and diastolic blood pressure (SBP and DBP), sweat rate, fluid intake, urine color, urine specific gravity (USG), and changes in body mass are reported. Mean T-gi for all trials ranged from 36.9 to 37.3 degrees C, and mean HR for all trials ranged from 61 to 75 bpm. To determine the effects of increased heat exposure, differences in heat strain between 4- and 8-h trials were examined via repeated measures analysis of variance (ANOVA). As AT increased, there were no significant changes in Delta T-gi and Delta HR from 1 to 4 h in the 4-h and 8-h trials. During the 8-h trials, as AT increased, there was a difference in Delta T-gi between 4 and 8 h. In conclusion, there was support for sustainable exposures resulting in no significant increase in physiological strain at an AT greater than 35 degrees C AT, with a sustainable limit (no increases in T-gi due to increases in AT) below 46 degrees C AT. Although the participants were not representative of coal miners, the results provide support for sustainable exposures greater than 35 degrees C AT at an average metabolic rate of 155 W.