The management of the mine at Mount Isa, Queensland, Australia decided to enquire into the following questions with regard to men working underground in hot conditions:(a) Which of the various heat stress indices predicts most accurately the effects on workmen of the various heat stress factors which occur in the mine at Mount Isa?(b) How best should the limits of heat stress be judged at which the normal 8-hour shift should be reduced to a 6-hour shift, or at which work should be stopped?With these objects in mind, oral temperatures were measured on 86 workmen after three hours of ordinary work in the mine and also on 36 occasions on 29 volunteers after three hours of stepping on and off a stool at a work rate of 1,560 ft. lb./min. These men were studied in different environmental heat stresses over the range that occurs in the mine. Dry bulb air temperatures (D.B.), wet bulb temperatures (W.B.), velocity of air movements, and globe temperatures (G.T.) were measured in the micro-climate in which each man worked. An estimate was made of the work rate of the 86 workmen. From these estimates and measurements, the predicted 4-hourly sweat rate (P4SR) and corrected effective temperature (C.E.T.) values were determined for each heat stress condition. P4SR values varied between 0·9 and 6·5 and C.E.T. between 70° and 95°F.Correlation coefficients were calculated between oral temperatures and W.B.s, C.E.T.s, and P4SRs and are 0·51, 0·64, and 0·75 respectively. Further analysis was confined to C.E.T. and P4SR. Plots of oral temperature on P4SR for conditions where G.T. was more than 10°F. above D.B. were found to fall well below the rest of the plots, indicating that P4SR exaggerates the effect of mean radiant temperature. These data were therefore excluded from the rest of the analysis. Regression equations were calculated for oral temperature on P4SR and for oral temperature on C.E.T. for (a) men `on the job', for (i) conditions where D.B. was more than 10°F. above W.B. and (ii) for conditions where D.B. was less than 10°F. above W.B., and (b) for men `stepping'. This analysis showed that one overall regression line can be used for all three conditions for oral temperature on P4SR, but for oral temperature on C.E.T. at least two different regression lines would be needed. Also the correlation coefficients between oral temperature and P4SR were generally higher than between oral temperature and C.E.T. For the prediction of oral temperature in the mine at Mount Isa the P4SR index is to be preferred to the C.E.T. scale.These results indicate that the emphasis given to G.T. in the P4SR index is too great. A multi-variance analysis of the P4SR index shows that, in the middle of the range of heat stress conditions examined, a unit change in P4SR would be obtained by about the same change in W.B. and G.T. This is at variance with the present results and also with the experimental findings of the M.R.C. Climatic Physiology Unit at Singapore. It appears, therefore, that the P4SR index should be revised in this regard.When it came to setting limits of heat stress for a 6-hour shift and for `stop-work', it was decided to base the limit for the 6-hour shift on a 1:100 probability of men reaching an oral temperature of 100·5°F. (rectal temperature of 101·5°F.) and to base the `stop work' limit on a 1:2,000 probability of reaching an oral temperature of 101·5°F. (rectal temperature of 102·5°F.). The reasons for this choice of physiological criteria are given in full in the paper. P4SR values at which these limits are reached were determined by calculating 1:100 and 1:2,000 probability belts to the overall regression line of oral temperature on P4SR. The P4SR value at the intersection of the 1:100 probability limit and the oral temperature of 100·5°F. is 3·8 and the P4SR value at the intersection of the 1:2,000 probability limit and the oral temperature of 101·5°F. is 5·0. These then are the limits of heat stress in the mine at Mount Isa for a 6-hour shift and for `stop-work'.A simple graphical method has been developed and is in use in the mine for determining when the level of work and environmental heat stress have reached either a P4SR value of 3·8, when the shift is reduced to six hours; or, when the heat stress has reached a P4SR value of 5·0, when work is stopped.
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