Human Thermal Adaptation Research Articles

Quantifying human thermal adaptation for indoor environments through field surveys and climate chamber experiments

ABSTRACT Thermal comfort boundaries established through climate chamber experiments are often found to significantly vary in real-world conditions based on the potential for thermal adaptation. In this context, this research aims to quantify the extent of thermal adaptation using a personalized ventilation system. We hypothesize that a personalized ventilation system can enhance thermal adaptation compared to real-world settings with limited opportunity for thermal adaptation. The study is based on thermal comfort surveys in mixed-mode open-plan office spaces of a humid subtropical region (Cwa), and a controlled climate chamber with a provision for personalized ventilation. We obtained 5629 subjective thermal responses through longitudinal field surveys accompanied by environmental measurements. In a climate chamber, 16 acclimatized volunteers were subjected to 140 different set-points with air temperature (Ta), relative humidity (RH), and air velocity (Va) variations yielding 19,200 subjective responses. A thermo-neutral temperature (Tn) of 26.5°C with an acceptability band of ±5.9°C was obtained through the field surveys. The Tn obtained from climate chamber surveys varied from 28.0°C to 29.3°C with an acceptability band of ±3.6°C depending on the ventilation rate. An empirical relationship between thermal sensation vote (TSV) and multiple environmental variables (Calidity) suggests 2°C higher Tn for Caliditychamber, Thermal adaptation correction factor of 2.1°C for warm-dry indoor environment and 1°C for warm-humid indoor environment were deduced through comparative analysis.

Thermal adaptation of southern migrants to the district heating environment in the severe cold area, China: A longitudinally traced investigation

Preference for thermal environments is affected by the thermal histories on their own. A deeper understanding of human thermal adaptation to create comfortable and energy-saving environments is desired. To study the differences in thermal responses of southern migrants and northern locals to district heating environments, a longitudinally traced investigation was performed in the severe cold area, China. 74 freshmen with different thermal histories were invited for six weeks in winter. Physiological parameters and thermal response votes were collected. Results showed that the most significant difference in the thermal neutral temperature between locals and migrants reaches up to 1.1 °C, after acclimation, which decreases to 0.7 °C. Also, southern immigrants are not as sensitive to temperature changes as locals, but the gap is narrowing. Moreover, immigrants tended to wear lighter clothing at the same room temperature compared to natives, who were more flexible in adjusting their clothing. It was also observed that physiological and psychological adaptation processes were not synchronous. The humidity is the leading purpose of physiological discomfort for migrants. Physiological adaptation is a relatively slow process. It concluded that southern immigrants may take approximately 1 week to adapt to the thermal environment with district heating. However, they did not fully adapt to the humid environment until the 6th week. The findings have implications for understanding the mechanisms of thermal adaptation and creating comfortable built environments.

A quantitative evaluation model of outdoor dynamic thermal comfort and adaptation: A year-long longitudinal field study

The understanding of human outdoor thermal comfort demand and thermal adaptation contributes to sustainable urban design as well as city resilience in the context of human health and wellbeing. Humans' past thermal experience influence their outdoor thermal comfort. However, the quantitative relationship between the past thermal experience and outdoor thermal comfort is still not clear. This study aims to reveal quantitative relations of the impact of people's past thermal experience on adaptive thermal comfort and to develop a new outdoor adaptive thermal comfort model. A year-long longitudinal questionnaire survey along with a combination of outdoor thermal environment campaigns was carried out in Chongqing, China. It began on August 15, 2020, and finished on August 19, 2021. Through the analysis of 2240 valid responses to the questionnaire survey, the outdoor thermal adaptation characteristic and dynamic thermal comfort evaluation of the respondents were revealed. The results show that the quantified temperature of past outdoor thermal experience is the quadratic correlation with the thermal sensitivity coefficient and deviation constant, and the linear correlated with outdoor thermal demands. Based on the quantitative analysis, a new outdoor adaptive thermal comfort model has been developed as a function of the exponentially weighted sum of historical mean air temperature series (MeanTrm). The outdoor adaptive thermal comfort zones by 80% and 90% satisfactions thereby have been first drawn based on the Universal Thermal Climate Index (UTCI). The study developed a methodology for the evaluation of dynamic outdoor thermal comfort which can be used for different climate regions.

Climate chamber investigation of the effect of indoor thermal histories on thermal adaptation in different seasons

The indoor thermal history of residents in the hot summer and cold winter climate zone in China have undergone a significant change in recent years, which also changes their seasonal thermal adaptations and this has formed feed-back effects to the increasing usage of air conditioning units in this area. To study the seasonal variations of thermal adaptation, the thermal comfort experiments were conducted on two groups of participants. Each groups included 20 participants who had indoor history mainly with natural ventilation (NV group) and air-conditioning (AC group), respectively. The results demonstrated that the thermal sensation vote (TSV) in warm environments did not differ between AC and NV subjects in summer. However, the TSV of AC subjects were much lower than that of NV subjects in the same standard effective temperature and skin temperature in a cold environment in winter. Overall, the participants who spent most of their time in AC space in winter showed a low level of thermal adaptation with a narrower acceptable skin temperature range of 32.6 – 33.5 °C. Thus, this study presents the basic information regarding the seasonal effects on human thermal adaptation due to different long-term indoor thermal histories.

Research Progress and Hotspot Evolution Analysis of Landscape Microclimate: Visual Analysis Based on CNKI and WOS.

With the increasing requirements of healthy habitat environments, landscape microclimates have been widely concerned. To comprehensively grasp the development history and research status of the landscape microclimates in China and other countries, CiteSpace software was used to comparatively analyze and visually present the literature related to landscape microclimates in CNKI and WOS databases for the past 20 years. The results show that: (1) The number of publications on landscape microclimate research shows an increasing trend in China and other countries, and the number of publications increased significantly after 2016. Although the number of publications by Chinese scholars is less than that of foreign scholars, they have developed rapidly in recent years, and their international influence has increased significantly. (2) A positive exchange has been formed among international scholars, and the number of collaborative studies has been increasing. China's microclimate research has formed relatively stable teams that have conducted numerous studies in the fields of urban communities, street greening, and plant communities, respectively. Although the links between research teams and institutions in China and other countries are relatively close, they still need to be further strengthened. (3) In the past decade, the theoretical system of landscape microclimates has been improved, and the research themes have become more concentrated, but it still has remained close to the early basic research. (4) Future research will remain centered on "mitigating the urban heat island effect" and "optimizing human heat perception". New topics such as "biodiversity", "carbon cycle", and "climate change" have been added. In conclusion, the research needs to continue to explore the evaluation system of microclimates and verify the evaluation index of outdoor thermal comfort for human thermal adaptation in different regions. The standards and systems of human habitat adapted to different regional characteristics should be formed.

Thermal adaptation of different set point temperature modes and energy saving potential in split air-conditioned office buildings during summer

The Chinese government issued a regulation that the cooling set point temperature (SPT) in office buildings cannot be lower than 26 °C. Therefore, SPTs were divided into different modes: SPT≥26 °C and SPT<26 °C. To study the human thermal adaptation of different SPT modes and determine the comfort and energy saving of the policy, a field test and subjective questionnaires were conducted in 21 split air-conditioned (SAC) office buildings during the warm season in the cold zone of China. We measured the environmental parameters and subjects' skin temperatures, and the subjective thermal responses were also investigated. The results showed that 45% of the air conditioner SPTs were less than 26 °C. In the high SPT mode, people's physiological adaptability to the warm environment was stronger, and subjects were more active in reducing clothes and increasing air velocity to improve thermal discomfort. Because the two modes had similar thermal histories and the same perceptual control, in the same SET ranges, no significant differences were found in thermal sensation, acceptability and comfort between the two SPT modes. The effects of long-term indoor thermal history on human thermal adaptation varied with the thermal exposure type, mode and intensity. From the analysis of thermal adaptation, the rationality of the policy was fully proved, and it was determined that SAC office buildings could save 26.4–35.2% of power energy consumption by reasonably increasing SPT in the cold zone. The results can enrich the thermal adaptation theory and provide a theoretical basis for the government's energy-saving policies in SAC office buildings.

An investigation of human thermal comfort and adaptation in naturally ventilated residential buildings and its implication for energy use in tropical climates of Ethiopia

A human thermal comfort study was conducted in naturally ventilated residential buildings of Jimma town, southwest Ethiopia. A field survey was undertaken in dry seasons (February 1 to May 31, 2020) of a warm temperate climate. In this study, 104 residential buildings were purposely selected and 430 residents, 213 (49.5%) male and 217 (50.5%) female participated. The subjective responses on the prevailing thermal conditions and adaptation methods were collected using the questionnaire. Measurements of indoor thermal conditions were undertaken using handheld digital instruments following ASHRAE class II protocols. In the study periods, the outdoor thermal data were obtained from the town's meteorological station. The neutral temperature was 20.4 °C and the comfort range of indoor air temperature was 14.6 °C to 26.3 °C. The study participants used simple environmental controls and personal adjustments to set indoor thermal comfort. Our study indicates that people in developing countries like Ethiopia can achieve thermal comfort with natural ventilation and hence less demand for energy-intensive mechanical ventilation. Further study on cooling/heating energy consumptions of households of developing countries is recommended.

Predictive control for indoor environment based on thermal adaptation.

Previous studies show that the indoor environment quality (IEQ) of buildings directly affects human health and comfort. This study aims to predict the change of indoor parameters at the next moment under the influences of the current indoor climate and outdoor climate and control the IEQ parameters based on the human thermal adaption in advance. We combine the simulation and the mathematical method to establish the office building model with air-conditioning and lighting systems and construct the bilinear model of the IEQ parameters and control variables. Unknown parameters are identified using the experimental method. Model predictive control (MPC) based on human thermal comfort is discussed by considering human thermal adaptation, and the neutral temperature is calculated through the dynamic relationship between outdoor and indoor temperatures. Results show that the temperature setpoint is adjusted in accordance with human adaptability, and the air-conditioning, fan, and lighting systems are controlled via MPC. The usage time of air-conditioning and light is reduced, and thus, energy is saved.

Energetics as a driver of human morphological thermal adaptation; evidence from female ultra-endurance athletes.

Functional benefits of the morphologies described by Bergmann's and Allen's rules in human males have recently been reported. However, the functional implications of ecogeographical patterning in females remain poorly understood. Here, we report the findings of preliminary work analysing the association between body shape and performance in female ultramarathon runners (n = 36) competing in hot and cold environments. The body shapes differed between finishers of hot and cold races, and also between hot race finishers and non-finishers. Variability in race performance across different settings supports the notion that human phenotype is adapted to different thermal environments as ecogeographical patterns have reported previously. This report provides support for the recent hypothesis that the heightened thermal strain associated with prolonged physical activity in hot/cold environments may have driven the emergence of thermally adaptive phenotypes in our evolutionary past. These results also tentatively suggest that the relationship between morphology and performance may be stronger in female vs. male athletes. This potential sex difference is discussed with reference to the evolved unique energetic context of human female reproduction. Further work, with a larger sample size, is required to investigate the observed potential sex differences in the strength of the relationship between phenotype and performance.

A Passive Building Energy-Efficiency Evaluation Method Considering Human Thermal Adaptation with Seasonal Climate Change

Under the premise of satisfying the human thermal comfort in the whole year, accurately evaluating the building energy-efficiency (BEE) through passive design can provide direct feedback and guidance for the passive design process. The scientific relationship between dynamic indoor thermal comfort benchmarks, outdoor climate change and passive building energy-efficiency design (PBEED) is a prerequisite for calculating BEE with passive design method. However, the current energy-efficiency effect evaluation method cannot reflect the dynamic change characteristics of the thermal comfort benchmark linked with the continuous dynamic change of the climate throughout the year. Firstly, BEE evaluation method in China was introduced, and energy conservation ratio calculating method was illustrated. Then, analyse and conclude 4 disadvantages of BEE evaluation method. Secondly, a new calculating method of PBEED evaluation considering human thermal adaptation with seasonal climate change was invented, including calculating model, some new concepts appeared in the model and calculating process of PBEED evaluation index. Finally, obtaining method of the PBEED evaluation index was proposed, including temperatures outdoors and indoors, dynamic thermal comfort zone. The results of this study will expand the application field of adaptive thermal comfort in building energy conservation, and will provide scientific support for accurately evaluating the passive building energy-efficiency design.

Measurement of Personal Experienced Temperature Variations in Rural Households Using Wearable Monitors: A Pilot Study.

The time-varying data of air temperatures experienced by people in their daily lives is an important basis for studying human thermal sensation, adaptation, comfort, and health. It is also very important for designing targeted strategies to help people reduce uncomfortable experience. In this study, a small (98 mm × 49 mm × 25 mm), lightweight (~100 g), and portable temperature logger with a wide measurement range (−40 to 100 °C) and appropriate accuracy (±0.3 °C precision) was combined with a phone holder that was adapted as an armband sleeve to constitute a wearable monitor. Fourteen monitors were worn by 14 residents in 6 different households in rural Beijing, China, to monitor their personal thermal environment. In the context of having very similar living habits in winter and coping strategies for thermal discomfort, the temperatures that 14 residents experienced exhibited wide ranges and large variations during the two-day test period. The highest and lowest temperatures experienced by residents reached 30.6 and −16.6 °C, respectively. This paper provided new data and evidences about various temperatures experienced by residents, even though they were from the same family and lived together for decades. In terms of methodology, as an exploration, the present study indicated that using personal wearable monitors is a viable method to capture the real experienced thermal environment, which extended the method for collecting data regarding complex experiences in different environments to aid the study of human responses to the real-world thermal environment.

Effects of indoor thermal exposure on human dynamic thermal adaptation process

The effects of indoor thermal exposure on human thermal adaptation is a time-dependent process, and deconstructing the human thermal response in a dynamic process helps to reveal the thermal adaptation mechanism. Although many researchers have studied human thermal comfort under different indoor thermal conditions in cross-sectional field investigations, the temporal variation of the human thermal response in the adaptation process remains to be further studied. To study the combined effect of indoor thermal stress and exposure duration on human thermal adaptation, a follow-up study was performed on 30 subjects for nine weeks: they were exposed to three different heating intensities in winter (unheated room, low-level heated room, and high-level heated room). Subjects’ physiological responses to the thermal environmental changes were monitored, and psychological assessment was conducted with questionnaires at indoor temperatures of 14, 18, and 22 °C in a climate chamber in each week. The results showed that indoor thermal exposure exhibited a temporal cumulative effect on human thermal adaptation, and the physiological and psychological adaptation processes were not necessarily synchronous. Prolonged indoor cold exposure not only increased the acceptability of human body to cold exposure, but also elevated the vigilance to warm exposure; skin temperature was increased and sympathetic activity was decreased characterized by lower LF/HF ratio in cold exposure. The research results provide implications for the control of indoor thermal environments based on human adaptability.

Energy flexibility for heating and cooling in traditional Chinese dwellings based on adaptive thermal comfort: A case study in Nanjing

Human thermal adaptation is widely recognized to achieve building energy saving. However, the energy-saving potential of traditional dwellings based on adaptive thermal comfort of residents in cooling and heating periods has not been fully explored. This paper takes the traditional dwellings in the Small West Lake area in Nanjing, China as an example, and explores the thermal comfort of local residents. Then, based on the annual thermal neutral temperature of local residents, a dynamic control strategy of temperature setting points throughout the cooling and heating periods was proposed, and compared with the traditional constant indoor condition from the perspective of heat transfer. The results show that the proposed dynamic controlling method can reduce the energy consumption for maintaining heating or cooling by 26.87%–36.51% in the whole year under the climatic environment of Nanjing, China. The average thermal neutral temperatures are 27.63 °C in summer and 14.44 °C in winter. The thermal sensitivity of residents in traditional Chinese dwellings in winter is lower than that in summer. This study shows the applicability of the adaptive thermal comfort model to traditional Chinese dwellings and a relaxation in the presence of strict set-point temperatures. It is recommended to consider the adaptive thermal comfort of residents into the design of air conditioning or heating systems, which can achieve great energy-saving effect.

Seasonal Rhythm of Human Thermal Adaptation for Chinese Youth in Cold Climate Zone

Seasonal Rhythm of Human Thermal Adaptation for Chinese Youth in Cold Climate Zone

The effect of indoor thermal history on human thermal responses in cold environments of early winter.

Human thermal adaptation is an important factor of indoor thermal comfort and energy conservation. To study the effect of indoor thermal history on cold adaptation in the early winter, climate chamber tests were conducted in cold environments at 16 °C with two different thermal experience groups. The groups are divided as follows: the natural ventilation (NV) group consisted of subjects living in naturally ventilated buildings (approximately 11.8 ± 3.4 °C in winter (Liu, H., Wu, Y., Li, B., Cheng, Y., Yao, R., 2017. Seasonal variation of thermal sensations in residential buildings in the Hot Summer and Cold Winter zone of China. Energy and Buildings 140, 9-18)) and the air conditioning (AC) group consisted of subjects living in air-conditioned buildings for at least one year before the climate chamber experiments. The experiments on the NV and AC groups were conducted between December 1-13 and December 15-25, respectively. Each group consisted of 20 subjects wearing winter clothes (1.15 ± 0.05 clo). The thermal sensation votes (TSVs) and thermal comfort votes (TCVs) in both groups were investigated and the subjects' skin temperatures were monitored during the experiments. The results showed that the mean TCV and TSV of both groups were not significantly different in the early winter. However, differences were observed in the subjects' localized body parts. The skin temperatures of the chest and arms of subjects in the NV group were higher than those in the AC group after exposure for 60 min at 16 °C, while calves skin temperatures of subjects in the NV group were lower. In addition, subjects in the AC group were found to feel colder compared to those in the NV group in cold environments at the same skin temperature. Thus, this study provides information about thermal comfort based on thermal experience in early winter.

Energy flexibility for heating and cooling based on seasonal occupant thermal adaptation in mixed-mode residential buildings

The energy flexibility for heating and cooling has not been fully explored though human thermal adaptation has been acknowledged in achieving energy savings in buildings. The aim of this study is to explore the capacity for heating/cooling flexibility in residential buildings in the hot summer and cold winter zone in China, by investigating the year-round dynamic changes in the thermal adaptation of occupants. A 13,005-set data set was extracted from a nation-wide field survey database. The results showed that the measured indoor temperatures were linearly related to the outdoor temperature in transient seasons but were discrete in the summer/winter seasons due to the mixed-mode operations of heating/cooling devices. The occupants’ neutral temperatures varied with outdoor temperatures in step with seasonal changes. Flexibility of temperature settings during the whole heating and cooling periods have been demonstrated, incorporating the dynamic thermal adaptation changes of occupants; such implementation has been estimated with great energy saving potential (e.g. 34.4% in Nanjing). This work contributes to the quantitative understanding of the role of human thermal adaptation in the smart control of residential energy management. It provides evidence for policy-making for flexible thermal design codes in building, to discourage excessive cooling/heating demands.

Daylight affects human thermal perception

Understanding the factors that affect human thermal responses is necessary to properly design and operate low-energy buildings. It has been suggested that factors not related to the thermal environment can affect thermal responses of occupants, but these factors have not been integrated in thermal comfort models due to a lack of knowledge of indoor factor interactions. While some studies have investigated the effect of electric light on thermal responses, no study exists on the effect of daylight. This study presents the first controlled experimental investigation on the effect of daylight quantity on thermal responses, combining three levels of daylight illuminance (low ~130 lx, medium ~600 lx, and high ~1400 lx) with three temperature levels (19, 23, 27 °C). Subjective and objective thermal responses of 84 participants were collected through subjective ratings on thermal perception and physiological measurements, respectively. Results indicate that the quantity of daylight influences the thermal perception of people specifically resulting in a cross-modal effect, with a low daylight illuminance leading to a less comfortable and less acceptable thermal environment in cold conditions and to a more comfortable one in warm conditions. No effect on their physiological responses was observed. Moreover, it is hypothesised that a warm thermal environment could be tolerated more whenever daylight is present in the room, as compared to the same thermal condition in a room lit with electric lights. Findings further the understanding of factors affecting human thermal responses and thermal adaptation processes in indoor environments and are relevant for both research and practice. The findings suggest that daylight should be considered as a factor in thermal comfort models and in all thermal comfort investigations, as well as that thermal and daylight illuminance conditions should be tuned and changed through the operation and design strategy of the building to guarantee its occupants’ thermal comfort in existing and future structures.

Ultra-endurance athletic performance suggests that energetics drive human morphological thermal adaptation.

Both extinct and extant hominin populations display morphological features consistent with Bergmann's and Allen's Rules. However, the functional implications of the morphologies described by these ecological laws are poorly understood. We examined this through the lens of endurance running. Previous research concerning endurance running has focused on locomotor energetic economy. We considered a less-studied dimension of functionality, thermoregulation. The performance of male ultra-marathon runners (n = 88) competing in hot and cold environments was analysed with reference to expected thermoregulatory energy costs and the optimal morphologies predicted by Bergmann's and Allen's Rules. Ecogeographical patterning supporting both principles was observed in thermally challenging environments. Finishers of hot-condition events had significantly longer legs than finishers of cold-condition events. Furthermore, hot-condition finishers had significantly longer legs than those failing to complete hot-condition events. A degree of niche-picking was evident; athletes may have tailored their event entry choices in accordance with their previous race experiences. We propose that the interaction between prolonged physical exertion and hot or cold climates may induce powerful selective pressures driving morphological adaptation. The resulting phenotypes reduce thermoregulatory energetic expenditure, allowing diversion of energy to other functional outcomes such as faster running.

Global pattern of human thermal adaptation and limit of thermal neutrality: Systematic analysis of outdoor neutral temperature

Neutral temperature is the temperature at which people feel neither cool nor warm. Previous regional studies have found that neutral temperature can change with season, which can be attributed to the psychological adaptation effect of people. However, few studies have examined whether the adaptation pattern can be observed across wide geographical and climatic regions. In some cases, outdoor comfort surveys cannot determine neutral temperatures because thermal sensation votes obtained do not reach neutrality. It reflects that thermal adaptation has a temperature limit, and thermal neutrality can hardly be achieved outside the limit. This study aims to establish linear models between neutral temperature and background temperature (survey‐period and climate normal) using neutral temperature data reported in international studies. The limit of thermal neutrality will also be determined in the best‐fit model. The neutral temperature data in 19 empirical outdoor thermal comfort studies published between 2009 and 2017 were used in the linear regression models. Survey‐period mean air temperature was found to be the best predictor of neutral temperature (R2 = 0.539, p = 0.007). The effect of long‐term acclimatization may be less important than it was previously thought because survey‐period temperature can explain over a half of the variability in neutral temperature. The temperature limit of thermal neutrality was defined by the lowest and the highest neutral temperatures in the best‐fit model; the limit was between 12.9 and 32.3 °C in air temperature. The findings indicate that the adaptation effect has a worldwide pattern as neutral temperature always lies closely to background air temperature.

Influence of human thermal adaptation and its development on human thermal responses to warm environments

The human thermal adaptation has been documented while the underlying mechanisms of the adaptive processes remain incompletely understood. This study was to evaluate human thermal adaptation and take a step further to reveal the development via a repeated heat exposure design. Two series of experiments were conducted in a well-controlled climate chamber, one in summer and the other, in autumn. Sixteen subjects were selected in Series I (temperature: 26 °C/29 °C/32 °C, relative humidity (RH): 15%/50%/85%) and twenty-four were engaged in a 7-day repeated experiment in Series II (32 °C/20% RH vs 32 °C/80% RH). Results show that subjects' mean skin temperatures (MST) and thermal sensation votes (TSV) were higher at 32 °C in Series I. In Series II, subjects who were de-acclimated showed quicker and stronger responses with regards to MST during the first 30 min and higher change rates of MST (32 °C/80% RH: peak value of 0.49 °C/min, 32 °C/20% RH: 0.16 °C/min) during the first exposure. This was accompanied by a higher skin blood flow (SBF) and total water loss (32 °C/80% RH: 292 g ± 63 g, 32 °C/20% RH: 307 g ± 47 g) on Day 1. The physiological regulations for MST, SBF, and total water loss attenuated continuously over the seven days, especially at 32 °C/20% RH. The repeated exposure consequently ameliorated subjects' thermal and sweating sensations. The findings indicate the mechanism of human physiological and psychological adaptation, which provides deeper understandings of people's dynamic thermal adaptation and application in thermal environment designs.