Transient response of the human-clothing system

1. 1. A transient clothing model which considers the effects of adsorption and thermal capacitance on the dynamic thermal response of clothing was developed.

2. 2. Moisture adsorption and desorption by the fabric are the major factors that affect the transient response of clothing.

3. 3. This moisture can come from evaporated sweat or from the environment.

4. 4. The clothing model was combined with a modified version of the two-node thermal model of the human body.

5. 5. The combined model shows that, during transients, the mix of latent and sensible heat flow from the skin may differ considerably from the corresponding heat flows from the clothing surface to the environment.

6. 6. The alteration of the heat flows can have a significant impact on the thermal response of the body by changing the sweat rate required to achieve the heat loss necessary to maintain thermal balance.

A computer model of human thermoregulation for a wide range of environmental conditions: the passive system.

A dynamic model predicting human thermal responses in cold, cool, neutral, warm, and hot environments is presented in a two-part study. This, the first paper, is concerned with aspects of the passive system: 1) modeling the human body, 2) modeling heat-transport mechanisms within the body and at its periphery, and 3) the numerical procedure. A paper in preparation will describe the active system and compare the model predictions with experimental data and the predictions by other models. Here, emphasis is given to a detailed modeling of the heat exchange with the environment: local variations of surface convection, directional radiation exchange, evaporation and moisture collection at the skin, and the nonuniformity of clothing ensembles. Other thermal effects are also modeled: the impact of activity level on work efficacy and the change of the effective radiant body area with posture. A stable and accurate hybrid numerical scheme was used to solve the set of differential equations. Predictions of the passive system model are compared with available analytic solutions for cylinders and spheres and show good agreement and stable numerical behavior even for large time steps.     

Climate and physiological heat strain during exercise

A body-atmosphere energy exchange model (BIODEX) using heat transfer theory and empirical relationships is described which predicts the change in body core temperature during exercise as an index of thermal strain. Index values may be interpreted as the length of the period of activity before the heat load on the body causes internal body temperature to rise to critical levels. The performance of the model tested under controlled laboratory conditions using human subjects was found to be reliable. BIODEX is used to show the thermal significance of midsummer climatic conditions in New Zealand for those jogging out-doors.     

Prediction of air temperature for thermal comfort of people in outdoor environments

Current thermal comfort indices do not take into account the effects of wind and body movement on the thermal resistance and vapor resistance of clothing. This may cause public health problem, e.g. cold-related mortality. Based on the energy balance equation and heat exchanges between a clothed body and the outdoor environment, a mathematical model was developed to determine the air temperature at which an average adult, wearing a specific outdoor clothing and engaging in a given activity, attains thermal comfort under outdoor environment condition. The results indicated low clothing insulation, less physical activity and high wind speed lead to high air temperature prediction for thermal comfort. More accurate air temperature prediction is able to prevent wearers from hypothermia under cold conditions.     

A transient thermal model of the human body–clothing–environment system

This paper reports on a new transient thermal model integrating the heat and moisture transfer through clothing as well as the two-node human physiological model to predict the human physiological responses. For the first time, the model considered clothing ventilation and moisture accumulation on the surface of the skin and inner surface of the underwear. The numerical results of the model agreed well with a set of published experimental data and another set of experimental data from our own experiments.     

The free-convective anomaly.

Persons exposed to high temperature, or to equivalent environmental factors, have quantifiable reactions, such as reducing the resistance to both heat and moisture flow in skin tissues and clothing needed to maintain thermal equilibrium. The one-to-one relationship between this resistance in the walking person and temperature, with the other factors neutral, is the basis for the apparent temperature scale and the derived heat index. When this approach is taken to assess the thermal environment for a still person exposed to heat in still air, there is a zone of ambient conditions in which there are three solutions to the heat-balance equation. Extraordinary thermal stress occurs, depending slightly on other conditions, at ambient temperatures near 41 degrees C, especially at high humidity, because of the difficulty in carrying sweat vapor from the person when free convection is minimal. This anomaly is examined for a range of ambient vapor pressures and extra radiation. The rapid rise in heat stress when ambient temperature just exceeds body temperature in still conditions may explain the severity of some observed distress.     

The physiological equivalent temperature – a universal index for the biometeorological assessment of the thermal environment

With considerably increased coverage of weather information in the news media in recent years in many countries, there is also more demand for data that are applicable and useful for everyday life. Both the perception of the thermal component of weather as well as the appropriate clothing for thermal comfort result from the integral effects of all meteorological parameters relevant for heat exchange between the body and its environment. Regulatory physiological processes can affect the relative importance of meteorological parameters, e.g. wind velocity becomes more important when the body is sweating. In order to take into account all these factors, it is necessary to use a heat-balance model of the human body. The physiological equivalent temperature (PET) is based on the Munich Energy-balance Model for Individuals (MEMI), which models the thermal conditions of the human body in a physiologically relevant way. PET is defined as the air temperature at which, in a typical indoor setting (without wind and solar radiation), the heat budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed. This way PET enables a layperson to compare the integral effects of complex thermal conditions outside with his or her own experience indoors. On hot summer days, for example, with direct solar irradiation the PET value may be more than 20 K higher than the air temperature, on a windy day in winter up to 15 K lower. Received: 14 December 1998 / Accepted: 26 May 1999     

UTCI-Fiala multi-node model of human heat transfer and temperature regulation

The UTCI-Fiala mathematical model of human temperature regulation forms the basis of the new Universal Thermal Climate Index (UTC). Following extensive validation tests, adaptations and extensions, such as the inclusion of an adaptive clothing model, the model was used to predict human temperature and regulatory responses for combinations of the prevailing outdoor climate conditions. This paper provides an overview of the underlying algorithms and methods that constitute the multi-node dynamic UTCI-Fiala model of human thermal physiology and comfort. Treated topics include modelling heat and mass transfer within the body, numerical techniques, modelling environmental heat exchanges, thermoregulatory reactions of the central nervous system, and perceptual responses. Other contributions of this special issue describe the validation of the UTCI-Fiala model against measured data and the development of the adaptive clothing model for outdoor climates.     

Effects of a moderate sized city on human thermal bioclimate during clear winter nights

The human thermal bioclimatic effects of urbanization and natural topographic features (the ocean and hills) were investigated during clear winter nights in Christchurch, New Zealand. Results are presented in terms of the amount of clothing insulation required to balance the body heat budget equation of a standing person with no change in body heat storage. The ordering of urban-rural land use zones from lowest to highest clothing requirements was: CBD, light industrial-commercial, residential and rural. Air temperature accounted for most of the variation in clothing requirement with the model used and weather conditions investigated here followed by environmental thermal radiation. The oceans and hill slopes had an effect comparable to that of most of the urban area and required less clothing than did all land use zones except the urban CBD.     

Apparent latent heat of evaporation from clothing: attenuation and "heat pipe" effects.

Investigating claims that a clothed person's mass loss does not always represent their evaporative heat loss (EVAP), a thermal manikin study was performed measuring heat balance components in more detail than human studies would permit. Using clothing with different levels of vapor permeability and measuring heat losses from skin controlled at 34 degrees C in ambient temperatures of 10, 20, and 34 degrees C with constant vapor pressure (1 kPa), additional heat losses from wet skin compared with dry skin were analyzed. EVAP based on mass loss (E(mass)) measurement and direct measurement of the extra heat loss by the manikin due to wet skin (E(app)) were compared. A clear discrepancy was observed. E(mass) overestimated E(app) in warm environments, and both under and overestimations were observed in cool environments, depending on the clothing vapor permeability. At 34 degrees C, apparent latent heat (lambda(app)) of pure evaporative cooling was lower than the physical value (lambda; 2,430 J/g) and reduced with increasing vapor resistance up to 45%. At lower temperatures, lambda(app) increases due to additional skin heat loss via evaporation of moisture that condenses inside the clothing, analogous to a heat pipe. For impermeable clothing, lambda(app) even exceeds lambda by four times that value at 10 degrees C. These findings demonstrate that the traditional way of calculating evaporative heat loss of a clothed person can lead to substantial errors, especially for clothing with low permeability, which can be positive or negative, depending on the climate and clothing type. The model presented explains human subject data on EVAP that previously seemed contradictive.     

The effects of fabric air permeability and moisture absorption on clothing microclimate and subjective sensation in sedentary women at cyclic changes of ambient temperatures from 27 degrees C to 33 degrees C

The present paper aimed at learning the effects of two different levels of air permeability and moisture absorption on clothing microclimate and subjective sensation in sedentary women. Three kinds of clothing ensemble were investigated: 1) polyester clothing with low moisture absorption and low air permeability (A clothing); 2) polyester clothing with low moisture absorption and high air permeability (B clothing); and 3) cotton clothing with high moisture absorption and high air permeability (C clothing). After 20 min of dressing time, the room temperature and humidity began to rise from 27 degrees C and 50% rh to 33 degrees C and 70% rh over 20 min, and it was maintained for 30 min (Section I); it then began to fall to 27 degrees C and 50% rh over 20 min, and it was maintained there for 20 min (Section II). The subject sat quietly on a chair for 110 min. The main findings are summarized as follows: 1) The clothing surface temperature was significantly higher in C clothing than in B clothing during section I, but it was significantly higher in B clothing than in C clothing during section II. 2) Although the positive relationship between the microclimate humidity and forearm sweat rate was significantly confirmed in all three kinds of clothing, the microclimate humidity at the chest for the same sweat rate was lower in C clothing than in A and B clothing. These results were discussed in terms of thermal physiology.     

The Response of Human Thermal Sensation and Its Prediction to Temperature Step-Change (Cool-Neutral-Cool)

This paper reports on studies of the effect of temperature step-change (between a cool and a neutral environment) on human thermal sensation and skin temperature. Experiments with three temperature conditions were carried out in a climate chamber during the period in winter. Twelve subjects participated in the experiments simulating moving inside and outside of rooms or cabins with air conditioning. Skin temperatures and thermal sensation were recorded. Results showed overshoot and asymmetry of TSV due to the step-change. Skin temperature changed immediately when subjects entered a new environment. When moving into a neutral environment from cool, dynamic thermal sensation was in the thermal comfort zone and overshoot was not obvious. Air-conditioning in a transitional area should be considered to limit temperature difference to not more than 5°C to decrease the unacceptability of temperature step-change. The linear relationship between thermal sensation and skin temperature or gradient of skin temperature does not apply in a step-change environment. There is a significant linear correlation between TSV and Q_loss in the transient environment. Heat loss from the human skin surface can be used to predict dynamic thermal sensation instead of the heat transfer of the whole human body.     

Effects of thermal environment on sleep and circadian rhythm

ABSTRACT: The thermal environment is one of the most important factors that can affect human sleep. The stereotypical effects of heat or cold exposure are increased wakefulness and decreased rapid eye movement sleep and slow wave sleep. These effects of the thermal environment on sleep stages are strongly linked to thermoregulation, which affects the mechanism regulating sleep. The effects on sleep stages also differ depending on the use of bedding and/or clothing. In semi-nude subjects, sleep stages are more affected by cold exposure than heat exposure. In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation. On the other hand, cold exposure does not affect sleep stages, though the use of beddings and clothing during sleep is critical in supporting thermoregulation and sleep in cold exposure. However, cold exposure affects cardiac autonomic response during sleep without affecting sleep stages and subjective sensations. These results indicate that the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.     

The UTCI-clothing model

The Universal Thermal Climate Index (UTCI) was conceived as a thermal index covering the whole climate range from heat to cold. This would be impossible without considering clothing as the interface between the person (here, the physiological model of thermoregulation) and the environment. It was decided to develop a clothing model for this application in which the following three factors were considered: (1) typical dressing behaviour in different temperatures, as observed in the field, resulting in a model of the distribution of clothing over the different body segments in relation to the ambient temperature, (2) the changes in clothing insulation and vapour resistance caused by wind and body movement, and (3) the change in wind speed in relation to the height above ground. The outcome was a clothing model that defines in detail the effective clothing insulation and vapour resistance for each of the thermo-physiological model’s body segments over a wide range of climatic conditions. This paper details this model’s conception and documents its definitions.     

Assessment of human bioclimate based on thermal response

To assess the thermal component of human climate, the adequacy of two body-environment energy balance models is tested. Requirements are that these models should include the full range of atmospheric variables using a system which embraces both the attributes of those exposed and the functional attributes of the environment. They should also account for thermal stress on the body as well as the feedback thermophysiological responses as a result of that stress, and produce a unitary thermal index. Based on detailed field assessment, the study provides an interpretation of index values in terms of thermal sensation and pleasantness using standardised verbal scales. Detailed statistical analysis shows that the associations between index values and thermal sensation votes are strong giving correlation coefficients in the order of 0.86 which is considerably larger than those from comparable results reported in the literature. Optimal thermal conditions for sedentary activity appear to be located in the zone of vasomotor regulation against heat, subjectly interpreted as slightly warm, rather than precisely at the point of minimum heat stress. Sensitivity of model output and thermal sensation are greater in the zone of heat stress than in the zone of cold stress.     

An integrated model for simulating interactive thermal processes in human–clothing system

Stolwijk's multi-node model is modified by considering the sweat accumulation on the skin surface and is applied to simulate the human physiological regulatory response. This human model is interfaced with a coupled heat and moisture model of clothing materials that takes into consideration the adsorption of water vapor in the fibers. Furthermore, a multi-layer clothing system is developed and integrated with the human model. The result agrees well with the published experimental data.     

Thermal bioclimatic conditions and patterns of behaviour in an urban park in Göteborg,Sweden

People in urban areas frequently use parks for recreation and outdoor activities. Owing to the complexity of the outdoor environment, there have only been a few attempts to understand the effect of the thermal environment on people's use of outdoor spaces. This paper therefore seeks to determine the relationship between the thermal environment, park use and behavioural patterns in an urban area of Sweden. The methods used include structured interviews, unobtrusive observations of the naturally occurring behaviour and simultaneous measurements of thermal comfort variables, i.e., air temperature, air humidity, wind speed and global radiation. The thermal environment is investigated through the mean radiant temperature (T_mrt) and the predicted mean vote (PMV) index. The outcome is compared to the subjective behaviour and thermal sensation of the interviewees. It is found that the thermal environment, access and design are important factors in the use of the park. In order to continue to use the park when the thermal conditions become too cold or too hot for comfort, people improve their comfort conditions by modifying their clothing and by choosing the most supportive thermal opportunities available within the place. The study also shows that psychological aspects such as time of exposure, expectations, experience and perceived control may influence the subjective assessment. Comparison between the thermal sensation of the interviewees and the thermal sensation assessed by the PMV index indicates that steady-state models such as the PMV index may not be appropriate for the assessment of short-term outdoor thermal comfort, mainly because they are unable to analyse transient exposure.     

A thermal model of the human body exposed to an electromagnetic field

The human body was modeled by numerical procedures to determine the thermal response under varied electromagnetic (EM) exposures. The basic approach taken was to modify the heat transfer equations for man in air to account for thermal loading due to the energy absorbed from the EM field. The human body was represented in an EM model by a large number of small cubical cells of tissue, and the energy density was determined for each cell. This information was then analyzed by a thermal response model consisting of a series of two-dimensional transient conduction equations with internal heat generation due to metabolism, internal convective heat transfer due to blood flow, external interaction by convection and radiation, and cooling of the skin by sweating and evaporation. This model simulated the human body by a series of cylindrical segments. The local temperature at 61 discrete locations as well as the thermoregulatory responses of vasodilatation and sweating were computed for a number of EM field intensities and two frequencies, one near whole-body resonance.     

The effects of clothing thermal resistance and operative temperature on human skin temperature

Skin temperature is an essential physiological parameter of thermal comfort. The purpose of this research was to reveal the effects of clothing thermal resistance and operative temperature on local skin temperature (LST) and mean skin temperature (MST). The LSTs (at 32 sites) in stable condition were measured for different clothing thermal resistances 1.39, 0.5 and 0.1 clo. To study the effect of environmental temperature on LST and MST, the LSTs were also measured for operative temperatures 23, 26 and 33 °C. The experimental data showed that the effect of clothing thermal resistance on the foot was greater compared to the other human parts, and the effect of operative temperature on many parts of the human body was great, such as foot, hand, trunk, and arm. The MSTs measured on the conditions that air speed was under 0.1 m/s, RH was about 30–70%, and metabolic rate was about 1 met, were collected from previous studies. On the basis of these experimental data, a MST prediction equation with the operative temperature and clothing thermal resistance as independent variables, was obtained by multiple linear regression. This equation was a good alternative and provided convenience to predict the MST in different operative temperatures and clothing thermal resistances.     

Prediction of human thermophysiological responses during shower bathing

This study develops a model to predict the thermophysiological response of the human body during shower bathing. Despite the needs for the quantitative evaluation of human body response during bathing for thermal comfort and safety, the complicated mechanisms of heat transfer at the skin surface, especially during shower bathing, have disturbed the development of adequate models. In this study, an initial modeling approach is proposed by developing a simple heat transfer model at the skin surface during shower bathing applied to Stolwijk’s human thermal model. The main feature of the model is the division of the skin surface into three parts: a dry part, a wet part without water flow, and a wet part with water flow. The area ratio of each part is decided by a simple formula developed from a geometrical approach based on the shape of the Stolwijk’s human thermal model. At the same time, the convective heat transfer coefficient between the skin and the flowing water is determined experimentally. The proposed model is validated by a comparison with the results of human subject experiments under controlled and free shower conditions. The model predicts the mean skin temperature during shower fairly well both for controlled and free shower bathing styles.