Effects of thermal, personal and behavioural factors on the physiological strain, thermal comfort and productivity of Australian shearers in hot weather.

Multiple-regression analyses were used to evaluate the separate and combined effects of factors that are commonly expected to influence strain and productivity in a hot workplace. Forty-three men were studied throughout 54 man-days of shearing sheep and pressing wool bales, in air temperatures 19-41 degrees C and Wet-bulb Globe Temperature index (WBGT) 16-29 degrees C; 43% of the observations of WBGT exceeded 26.7 degrees C, the Threshold Limit Value (TLV) for the subjects' work rate of 400 W. Subjects were men of age 18-59 years, fat-free mass 44-77 kg, and body fat content 11-26%, who had drunk an estimated 0-207 g alcohol the previous evening. Afternoon mean values of rectal temperature (Tre) exceeded 38.0 degrees C (maximum 38.4 degrees C) in 4 of the 15 observations made when WBGT > TLV, and in none of those made when WBGT < TLV. Over the 10 h work day the subjects sweated 2.4-9.9 kg, but they replaced their sweat losses so successfully that warmer weather and heavier sweating were not accompanied by significantly greater dehydration. Surprisingly, the fatter men felt cooler, and those who had drunk more alcohol the previous evening had lower Tre and tended to be more productive. Age was not associated with any measured response. All factors together explained barely half the observed variation in Tre and thermal comfort, and almost none of the variation in productivity. The findings highlight the uncertainty inherent in attempts to define safe limits for occupational heat stress; they show how such uncertainty could restrict the usefulness in the shearing industry of the current heat-stress guidelines; and they demonstrate the effectiveness of the behavioural responses that permit shearers to perform sustained strenuous work in a hot environment without excessive physiological strain.