Deep body core temperatures in industrial workers under thermal stress

To date, no field study has continuously monitored the deep body core temperatures of industrial workers. A program to continuously measure deep body core temperatures in 36 industrial workers working 10-, 12-, and 12.5-hour day and nightshifts in a hot, deep, underground mine in the Tropics was conducted. No heat illness occurred in these workers during the study. Miniaturized radio-transponders ("pills") taken orally were used to measure temperature during the transit time in the gastrointestinal tract. Commonly recommended limits for industrial hyperthermia are 38.0 degrees C, or an increase of +1 degree C. The results showed that miners regularly exceeded these limits in terms of maximum deep body core temperature (average, 38.3 degrees C; standard deviation, 0.4 degree C), maximum temperature rise (1.4 degrees C, 0.4 degree C), and maximum heat storage (431 kJ, 163 kJ) without reporting any symptoms of heat illness. A significant component of the observed elevated core temperatures was attributable to the normal circadian rhythm, which was measured at 0.9 degree C (standard deviation, 0.2 degree C). Evidence was found that workers "self-pace" when under thermal stress.