Thermal comfort during work

Summary

Different methods of quantifying thermal alliesthesia were used to examine the circadian variations of thermal comfort. At eight different times of the 24 h day, four subjects were exposed to a constant room temperature of 25°C. After 30 min rest, the subjects performed 50 W bicycle ergometer work for 45 min. A 5 s temperature stimulus of 15°, 20°, 25°, 30°, 35°, and 38°C was applied on the hand, forehead, and back of the neck. For each stimulus the subjects voted their degree of thermal comfort on the subjective thermal comfort scale, ranging from +2 for very pleasant to −2 for very unpleasant. The most pleasant temperature on the back of the neck was chosen by voluntary control. This procedure was performed at the start and repeated every 15 min throughout the exposure time. A 5:5 × 2.7 cm² Peltier thermode was used to give the temperature stimuli. Voluntary control voting was carried out using the temperature control knob without looking at the temperature scale of the thermode.

The results suggest that during rest in a state of thermal neutrality the core temperature is about 0.06°C lower than the thermoregulatory set point (Cabanac et al. 1976; Strempel et al. 1976). This deviation (T_rect - T_set) from the set point is known in man-made servo-systems as the load error (Benzinger 1979). During rest in an atmosphere of thermal comfort, the “load error” in the human thermostat is found to be negative (Cabanac et al. 1976; Strempel et al. 1976; Benzinger 1979), i.e., T_set > T_rect, where T_rect is the prevailing core temperature and T_set, is the thermoregulatory set point. The small deviation from the set point is viewed as being necessary to sustain the response and to keep the temperature in equilibrium between production and loss of heat (Benzinger 1979). Thermal comfort limits during work rise from the early morning to the afternoon and then fall slowly towards the minimum level in the morning. The circadian variations of the core temperature load error associated with onset of sweating have a phase shift of 180° (12 h) with the alliesthesial reactions. Using our results and the results from Cabanac et al. (1976), it has been possible to write a circadian thermoregulatory set function to estimate the set point at any point of day time.