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 38.0degreesC, or an increase of + 1degreesC. The results showed that miners regularly exceeded these limits in terms of maximum deep body core temperature (average, 38.3degreesC; standard deviation, 0.4degreesC), maximum temperature rise (1.4degreesC, 0.4degreesC), 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 0.9degreesC (standard deviation, 0.2degreesC). Evidence was found that workers "self-pace" when under thermal stress.
