Three thermal comfort experiments with human participants in climate chambers were designed, prepared and run. These experiments were dedicated to studying warm and cold cyclical temperature variations at different times of the day during both summer and winter. These studies included over 200 participants (about half females and half males) aged between 20 and 60 years old. The experiments aimed to better understand how the psycho-physiological phenomena of thermal alliesthesia and thermal habituation affect human thermal perception under dynamic cyclical variations of indoor air temperature. They also aimed to quantify the impact of interindividual and, in particular, sex differences in the unfolding of these two phenomena. During the experiments, the environmental conditions in the test rooms were finely controlled and monitored, including air temperature, mean radiant temperature, relative humidity, air velocity, light (spectrum and illuminance), and CO2 concentration. Skin temperature was measured with both thermocouples and a thermal camera. Participants were asked to repeatedly fill out a questionnaire to assess their thermal sensation, thermal comfort/pleasure and thermal preference.
The analysis of the collected experimental data revealed interesting insights into the impact of interindividual differences on human dynamic thermal perception. In particular, females were found to have greater skin temperature variations during cooling transients than males and, correspondingly, experienced stronger thermal sensation overshoot responses. These differences in perceptual responses could be fully explained in terms of skin temperature differences since the relationship between the thermal sensation vote and the skin temperature was found to be independent of sex. We tested whether other factors influence the interindividual variability of the dynamic sensory response and found that the participant's Body Mass Index (BMI) and age affect the thermal sensation overshoot response to cutaneous thermal transients by diminishing it as the BMI increases and the age decreases. We hypothesize that the decrease in thermal sensitivity with the BMI could be due to a higher thickness of subcutaneous fat associated with a higher BMI. While the observed increase of thermal sensitivity with age up to 50-60 years old could be explained in terms of menopausal hormonal changes. These experimental results suggest that we need to account for interindividual differences not only in thermophysiological models but also in physiological-based thermal perception models.
The collected experimental data were also used to build a novel model that can predict the Whole-Body Dynamic Thermal Sensation and Thermal Comfort under uniform and transient thermal conditions. The model is based on physiological signals (body core temperature, mean skin temperature and its rate of change, skin wetness) simulated with Gagge’s two-node thermophysiological model. The model has three specific features:
• It correctly accounts for the impact of high relative humidity at high air temperatures by including skin wettedness as an input.
• It correctly accounts for the effect of exercise. The relation between thermal sensation and mean skin temperature when exercising is not the same as that observed during rest since exercise reduces thermal sensitivity.
• It correctly models the dynamic sensory phenomena of “thermal overshoot” and “thermal alliesthesia” a function of both the rate of change in the skin temperature and the mean skin temperature.
The validation is performed against more than 60 thermal exposures to steady-state and transient conditions and shows that the novel model has better predictive performances than the classical Fanger’s PMV/PPD model, especially for dynamic conditions far from neutrality.
