Dusan Fiala

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.

Deriving the operational procedure for the Universal Thermal Climate Index (UTCI)

The Universal Thermal Climate Index (UTCI) aimed for a one-dimensional quantity adequately reflecting the human physiological reaction to the multi-dimensionally defined actual outdoor thermal environment. The human reaction was simulated by the UTCI-Fiala multi-node model of human thermoregulation, which was integrated with an adaptive clothing model. Following the concept of an equivalent temperature, UTCI for a given combination of wind speed, radiation, humidity and air temperature was defined as the air temperature of the reference environment, which according to the model produces an equivalent dynamic physiological response. Operationalising this concept involved (1) the definition of a reference environment with 50% relative humidity (but vapour pressure capped at 20xa0hPa), with calm air and radiant temperature equalling air temperature and (2) the development of a one-dimensional representation of the multivariate model output at different exposure times. The latter was achieved by principal component analyses showing that the linear combination of 7 parameters of thermophysiological strain (core, mean and facial skin temperatures, sweat production, skin wettedness, skin blood flow, shivering) after 30 and 120xa0min exposure time accounted for two-thirds of the total variation in the multi-dimensional dynamic physiological response. The operational procedure was completed by a scale categorising UTCI equivalent temperature values in terms of thermal stress, and by providing simplified routines for fast but sufficiently accurate calculation, which included look-up tables of pre-calculated UTCI values for a grid of all relevant combinations of climate parameters and polynomial regression equations predicting UTCI over the same grid. The analyses of the sensitivity of UTCI to humidity, radiation and wind speed showed plausible reactions in the heat as well as in the cold, and indicate that UTCI may in this regard be universally useable in the major areas of research and application in human biometeorology.

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.

Predicting urban outdoor thermal comfort by the Universal Thermal Climate Index UTCI—a case study in Southern Brazil

Recognising that modifications to the physical attributes of urban space are able to promote improved thermal outdoor conditions and thus positively influence the use of open spaces, a survey to define optimal thermal comfort ranges for passers-by in pedestrian streets was conducted in Curitiba, Brazil. We applied general additive models to study the impact of temperature, humidity, and wind, as well as long-wave and short-wave radiant heat fluxes as summarised by the recently developed Universal Thermal Climate Index (UTCI) on the choice of clothing insulation by fitting LOESS smoothers to observations from 944 males and 710 females aged from 13 to 91xa0years. We further analysed votes of thermal sensation compared to predictions of UTCI. The results showed that females chose less insulating clothing in warm conditions compared to males and that observed values of clothing insulation depended on temperature, but also on season and potentially on solar radiation. The overall pattern of clothing choice was well reflected by UTCI, which also provided for good predictions of thermal sensation votes depending on the meteorological conditions. Analysing subgroups indicated that the goodness-of-fit of the UTCI was independent of gender and age, and with only limited influence of season and body composition as assessed by body mass index. This suggests that UTCI can serve as a suitable planning tool for urban thermal comfort in sub-tropical regions.

Validation of the Fiala multi-node thermophysiological model for UTCI application.

The important requirement that COST Action 730 demanded of the physiological model to be used for the Universal Thermal Climate Index (UTCI) was its capability of accurate simulation of human thermophysiological responses across a wide range of relevant environmental conditions, such as conditions corresponding to the selection of all habitable climates and their seasonal changes, and transient conditions representing the temporal variation of outdoor conditions. In the first part of this study, available heat budget/two-node models and multi-node thermophysiological models were evaluated by direct comparison over a wide spectrum of climatic conditions. The UTCI-Fiala model predicted most reliably the average human thermal response, as shown by least deviations from physiologically plausible responses when compared to other models. In the second part of the study, this model was subjected to extensive validation using the results of human subject experiments for a range of relevant (steady-state and transient) environmental conditions. The UTCI-Fiala multi-node model proved its ability to predict adequately the human physiological response for a variety of moderate and extreme conditions represented in the COST 730 database. The mean skin and core temperatures were predicted with average root-mean-square deviations of 1.35u2009±u20091.00°C and 0.32u2009±u20090.20°C, respectively.

Single-sector thermophysiological human simulator.

Thermal sweating manikins are used to analyse the heat and mass transfer phenomena in the skin-clothing-environment system. However, the limiting factor of present thermal manikins is their inability to simulate adequately the human thermal behaviour, which has a significant effect on the clothing microenvironment. A mathematical model of the human physiology was, therefore, incorporated into the system control to simulate human thermoregulatory responses and the perception of thermal comfort over a wide range of environmental and personal conditions. Thereby, the computer model provides the physiological intelligence, while the hardware is used to measure the required calorimetric states relevant to the human heat exchange with the environment. This paper describes the development of a single-sector thermophysiological human simulator, which consists of a sweating heated cylinder Torso coupled with the iesd-Fiala multi-node model of human physiology and thermal comfort. Validation tests conducted for steady-state and, to some extent, transient conditions ranging from cold to hot revealed good agreement with the corresponding experimental results obtained for semi-nude subjects. The new coupled system enables overall physiological and comfort responses, health risk and survival conditions to be predicted for adult humans for various scenarios.

Physiological responses to temperature and humidity compared to the assessment by UTCI, WGBT and PHS

In COST Action 730, a multi-segmental thermophysiological model was used to describe physiological strain reactions for different climatic conditions in order to develop a Universal Thermal Climate Index (UTCI). UTCI predictions for warm climates were compared with empirical data from the laboratory tests. The comparison was performed by means of equivalence lines within a psychrometric chart so that the combined influence of air temperature and humidity on physiological strain may be assessed. Within a reasonable regime of air temperatures and relative humidities (RH), the differences between simulated and measured values were as follows: for rectal temperatures below 0.3°C, for skin temperatures below 1.5°C, for sweat rates below 200xa0g/h and for heart rates (estimated from relative cardiac output) below 30 min−1. This characterises the validity of the model with respect to the description of the influence of heat and humidity on physiological strain. The same comparison to physiological data was also conducted for the equivalent temperature calculated for UTCI. In order to compare UTCI with other thermal indices used in occupational health, the physiological data have also been compared to equivalence lines of WBGT (Wet Bulb Globe Temperature) and PHS (Predicted Heat Strain) indices.

Thermal Indices and Thermophysiological Modeling for Heat Stress

The assessment of the risk of human exposure to heat is a topic as relevant today as a century ago. The introduction and use of heat stress indices and models to predict and quantify heat stress and heat strain has helped to reduce morbidity and mortality in industrial, military, sports, and leisure activities dramatically. Models used range from simple instruments that attempt to mimic the human-environment heat exchange to complex thermophysiological models that simulate both internal and external heat and mass transfer, including related processes through (protective) clothing. This article discusses the most commonly used indices and models and looks at how these are deployed in the different contexts of industrial, military, and biometeorological applications, with focus on use to predict related thermal sensations, acute risk of heat illness, and epidemiological analysis of morbidity and mortality. A critical assessment is made of tendencies to use simple indices such as WBGT in more complex conditions (e.g., while wearing protective clothing), or when employed in conjunction with inappropriate sensors. Regarding the more complex thermophysiological models, the article discusses more recent developments including model individualization approaches and advanced systems that combine simulation models with (body worn) sensors to provide real-time risk assessment. The models discussed in the article range from historical indices to recent developments in using thermophysiological models in (bio) meteorological applications as an indicator of the combined effect of outdoor weather settings on humans.

Modelling Human Heat Transfer and Temperature Regulation

In recent years there has been a growing demand from research, military and the industry for robust, reliable models predicting human thermophysiological responses. This chapter discusses the various aspects of- and approaches to- modelling human heat transfer and thermoregulation including the passive and the active system, numerical tissue heat transfer, environmental heat exchange, and clothing. Attention is also paid to advanced modelling topics such as model personalisation to predict responses of individuals, and methods for coupling with other simulation models and measurement systems. Several application examples of coupled systems are illustrated including numerical and physical simulation systems and a system for non-invasive assessment of internal temperature using signals from wearable sensors. The predictive performance of the model is discussed based on validation examples covering different exposure scenarios, personal characteristics, physical activities and in conjunction with non-invasive determination of rectal temperature with measured skin temperatures as model input. It is concluded that the model is a robust predictor of human thermophysiological responses, and, the proposed numerical simulation approach to non-invasive assessment of body core temperature, a reliable method applicable to a broad range of exposure conditions, personal characteristics, exercise intensities and types of clothing.

Modified wind chill temperatures determined by a whole body thermoregulation model and human-based facial convective coefficients

Wind chill equivalent temperatures (WCETs) were estimated by a modified Fiala’s whole body thermoregulation model of a clothed person. Facial convective heat exchange coefficients applied in the computations concurrently with environmental radiation effects were taken from a recently derived human-based correlation. Apart from these, the analysis followed the methodology used in the derivation of the currently used wind chill charts. WCET values are summarized by the following equation:WCET=12.87+0.5334∗To−12.66−0.4414∗To∗Ureported0.1228