The influence of a moderate temperature drift on thermal physiology and perception

Effects of temperature cycles on human thermal comfort in built environment under summer conditions

Dynamic temperature control strategies are feasible for enhancing energy flexibility and reducing energy consumption in buildings. However, guidelines for designing such dynamic thermal environments are lacking. In this study, 30 participants were recruited to undergo four experimental cycles formed by combining two temperature ranges (25-28 °C and 26-29 °C) and two temperature change rates (3 °C/h and 6 °C/h). Variations in the subjective perception and physiological responses with time were recorded throughout the experiments. The participants reported cooler thermal sensation and better thermal comfort for the same temperature during the ramp-down phase than during the ramp-up phase, which was more pronounced at faster temperature changes. The limits on temperature variations in the current standards underestimate the thermal acceptability of people. Although the temperature cycles exceeded the limits in the standards, sustained thermal comfort and high thermal acceptability were achieved when the temperature changed within 25-28 °C. At a rapid 6 °C/h change rate, the thermal sensation briefly deviated from the comfort zone when Top changed within 26-29 °C, suggesting that the limits should be set relative to the temperature change span. The comfortable temperature ranges for change rates of 3 °C/h and 6 °C/h in summer conditions were 22.8-28.7 °C and 22.8-28.4 °C, respectively, which are broader than the recommended indoor temperature range for summer in the Chinese standard. These findings indicate the potential of temperature variations to extend the thermal comfort zones while consuming less energy without requiring additional cooling devices.

Effect of time-of-day on human dynamic thermal perception

Implementing heating and cooling set-point temperature modulations in buildings can promote energy savings and boost energy flexibility. However, time and time-of-day requirements in current indoor climate regulations are either overly simplified or ignored completely. A better understanding of how human thermal responses vary throughout the day is useful to effectively design and operate energy-flexible buildings. To date, only a handful of studies have looked at diurnal changes in thermal perception and mostly near steady-state neutrality without controlling for light exposure. This is the first experimental investigation aimed at understanding how the time of the day influences physiological and subjective human sensory responses to a localized dynamic thermal stimulus under constant light rich in long wavelengths (red). Results indicated that humans responded physiologically differently depending on the time of the day with a higher rate of change in the skin temperature in the evening compared with the afternoon. Furthermore, the increase of thermal sensation during the warming skin temperature transients was found to be greater in the evening. No differences were observed under steady-state thermal conditions. This evidence suggests that accounting for the time of the day is important when dynamically operating buildings, such as during demand-response programs.

Exploring a more reasonable temperature exposure calculation method based on individual exposure survey and city-scale heat exposure impact assessment

The inability to quantify the difference between ambient temperature (AT) and personal exposure temperature (PET) is a common limitation in environmental health research. The actual exposure variability is underestimated when we used measurements from fixed monitoring stations to estimate PET. The study aims to explore a more reasonable temperature exposure calculation method to relate PET to AT and links heat exposure to adverse health events. We measured hourly PET of 129 participants from July 8th to July 13th, 2021 in Xinyi City, China. The linear mixed-effects model was used to build the relationship between hourly PET and AT in rural and town. Several calculation methods that can capture the intensity, frequency and duration of daily exposure were used to calculate the daily PET and AT and establish the relationship between the two factors. A generalized linear model was used to establish the relationship between city-scale AT indicators and health endpoints from January 1st, 2013 to December 31st, 2015 in Shanghai, China. The result showed that the hourly PET was significantly related to AT, wind speed, air pressure, precipitation, outside time, and air-conditioning use. Among several daily temperature indicators, we found that AT_DHAT (Degree Hours Above Threshold (27.4 °C)) was tight with the PET_DHAT in different regions (R² > 0.99). DHAT strengthened the relationship between daily AT and health endpoint in the urban-scale heat-related health impact study, especially in respiratory diseases. The method proposed in this study can improve the accuracy of future epidemiological studies on the effects of heat exposure.

Gender differences in thermal responses to temperature ramps in moderate environments

Some studies revealed that steady-uniform thermal environments are not the optimal environmental state to ensure thermal comfort, and temperature ramps offer potential advantages over traditional air conditioning methods. Moreover, when exposed to the same environmental conditions, gender differences in thermal responses were often observed, but the gender differences in the ramped conditions and causal relationships remain unclear. Therefore, an experimental research was conducted in a chamber by controlling the rates and directions to study the gender differences in thermal responses to temperature ramps. Three temperature ramps conditions (A: 26 °C-24 °C-26 °C; B: 26 °C-28 °C-26 °C; C: 26 °C-30 °C-26 °C) were investigated with 60 healthy participants (30 females and 30 males) recruited. The main conclusions indicated that women are more sensitive to temperature ramp-down environments than those of their male counterparts. Direction of temperature ramps had a significant effect on human responses in cool environments but no effect was observed in warm environments. Moreover, there was no significant differences in subjective responses between genders in a 2 °C ramp-up environment from 26 °C to 28 °C. Due to psychological differences, men have a wider range of temperature acceptability than women. Furthermore, the relationships between thermal sensation and thermal comfort, thermal sensation and thermal acceptability were also established, indicating that thermal sensation had significant impacts on other psychological responses. This paper has reference value for related researchers and designers to take temperature ramps and gender differences account in the design of indoor thermal environments, which benefits to improve thermal comfort, health and energy efficiency.

The effects of a novel personal comfort system on thermal comfort, physiology and perceived indoor environmental quality, and its health implications - Stimulating human thermoregulation without compromising thermal comfort

The classical textbook interpretation of thermal comfort is that it occurs when the thermoregulatory effort is minimized. However, stimulating human thermoregulatory systems may benefit health and increase body thermal resilience. To address this gap, we tested a novel personal comfort system (PCS) that targets only the extremities and the head, leaving the rest of the body exposed to a moderately drifting temperature (17-25°C). A randomized, cross-over study was conducted under controlled laboratory conditions, mimicking an office setting. Eighteen participants completed two scenarios, one with a PCS and another one without a PCS in 17-25°C ambient conditions. The results indicate that the PCS improved thermal comfort in 17-23°C and retained active thermoregulatory control. The torso skin temperature, underarm-finger temperature gradients, energy expenditure, substrate oxidations and physical activity were not affected by the PCS in most cases. Only slight changes in cardiovascular responses were observed between the two scenarios. Moreover, the PCS boosted pleasure and arousal. At 25°C, the PCS did not improve thermal comfort, but significantly improved air quality perceptions and mitigated eye strain. These findings suggest that human physiological thermoregulation can be stimulated without compromising thermal comfort by using a PCS that only targets the extremities in cold conditions.

Human thermal perception and time of day: A review

The circadian clock regulates diurnal variations in autonomic thermoregulatory processes such as core body temperature in humans. Thus, we might expect that similar daily fluctuations also characterize human thermal perception, the ultimate role of which is to drive thermoregulatory behaviors. In this paper, we explore this question by reviewing experimental and observational thermal comfort investigations which include the "time of day" variable. We found only 21 studies considering this factor, and not always as their primary analysis. Due to the paucity of studies and the lack of a specific focus on time-of-day effects, the results are difficult to compare and appear on the whole contradictory. However, we observe a tendency for individuals to prefer higher ambient temperatures in the early evening as compared to the rest of the day, a result in line with the physiological decrease of the core body temperature over the evening. By drawing from literature on the physiology of thermoregulation and circadian rhythms, we outline some potential explanations for the inconsistencies observed in the findings, including a potential major bias due to the intensity and spectrum of the selected light conditions, and provide recommendations for conducting future target studies in highly-controlled laboratory conditions. Such studies are strongly encouraged as confirmed variations of human thermal perceptions over the day would have enormous impact on building operations, thus on energy consumption and occupant comfort. List of abbreviations: TSV: Thermal Sensation Vote; TCV: Thermal Comfort Vote; Tpref: Preferred Temperature; TA: Indoor Air Temperature; RH: Indoor Relative Humidity; Tskin: Skin Temperature; Tty: Tympanic Temperature; Tre: Rectal Temperature; Toral: Oral Temperature.

Effects of Acute Heat and Cold Exposures at Rest or during Exercise on Subsequent Energy Intake: A Systematic Review and Meta-Analysis

The objective of this meta-analysis was to assess the effect of acute heat/cold exposure on subsequent energy intake (EI) in adults. We searched the following sources for publications on this topic: PubMed, Ovid Medline, Science Direct and SPORTDiscus. The eligibility criteria for study selection were: randomized controlled trials performed in adults (169 men and 30 women; 20–52 years old) comparing EI at one or more meals taken ad libitum, during and/or after exposure to heat/cold and thermoneutral conditions. One of several exercise sessions could be realized before or during thermal exposures. Two of the thirteen studies included examined the effect of heat (one during exercise and one during exercise and at rest), eight investigated the effect of cold (six during exercise and two at rest), and three the effect of both heat and cold (two during exercise and one at rest). The meta-analysis revealed a small increase in EI in cold conditions (g = 0.44; p = 0.019) and a small decrease in hot conditions (g = −0.39, p = 0.022) for exposure during both rest and exercise. Exposures to heat and cold altered EI in opposite ways, with heat decreasing EI and cold increasing it. The effect of exercise remains unclear.