Thermal Exposure Limit for Mine Refuge Chambers

Le résumé fourni par la source

Refuge chambers (RCs) provide shelter to miners trapped during a disaster. Industry standards mandate that RCs not exceed a Steadman Apparent Temperature (AT) (also known as Heat Index) of 95°F (35°C). This limit is designed for adults performing moderate work and maintaining thermal equilibrium at a core temperature ≤ 37°C. However, occupants in a RC spend most of their time at rest, and are able to maintain thermal equilibrium at a core temperature > 37°C. Thus, the temperature limit may be unnecessarily restrictive. PURPOSE: To determine the upper limit of sustainable heat stress that is protective for most individuals during long-term heat exposure at rest. METHODS: In Phase 1, five men (age = 23.6 ± 3.2: BMI = 23.6 ± 2.5) underwent five 4-hr trials in a semi-recumbent position (mean VO2 = 0.43 L/min) at ATs of 39.4, 40.0, 42.8, 45.6 and 49.0°C, rh 90%. In phase 2, five men (age = 23.6 ± 3.2; BMI = 25.1 ± 2.2) underwent five 8-hr trials in a semi-recumbent position (mean VO2 = 0.44 L/min). The starting AT for phase 2 was 3°C below the highest AT from Phase 1, and increased in a stepwise fashion (ATs = 45.6, 50.8, 51.7, 54.4 and 56.1°C; 90% rh). In all trials, gastrointestinal temperature (Tgi) and heart rate (HR) were assessed every 15 min. Oxygen uptake was assessed at the midpoint of each trial. Fluids were offered ad libitum. Sweat rate (SR) was determined from pre- and post-body weights. Repeated measures ANOVA was used to assess differences between trials. T-tests were used to assess pre- to post-trial differences. Significance was set at 0.05. RESULTS: Maximum AT of 56.1°C had an ending HR of 76 ± 1 bpm and ending Tgi of 37.5 ± 0.3°C; well below the assumption of the Steadman AT limit. There were no differences (p>0.05) in Tgi at 45.6°C at 4 hrs in Phase 1 (Tgi = 37.0 ± 0.6°C) or Phase 2 (Tgi = 37.2 ± 0.1°C) or at 8 hrs in Phase 2 (Tgi = 37.1 ± 0.3°C). As AT increased, the difference in Tgi from 4 to 8 hrs increased (p<0.05). There was no difference in HR (p>0.05) and no between-phase difference in SR (p>0.05). CONCLUSION: As there were no differences in Tgi at 4 hrs and a greater difference from 4 to 8 hrs as AT increased, 4 hrs may not be long enough to detect a change in Tgi. The effect of climate evidenced by increasing Tgi which occurred between 43.3 and 46°C-AT provides support for sustainable exposures greater than 35°C-AT with a likely ceiling below 46°C-AT.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.