The perceived temperature – a versatile index for the assessment of the human thermal environment. Part A: scientific basics

The Perceived Temperature (PT) is an equivalent temperature based on a complete heat budget model of the human body. It has proved its suitability for numerous applications across a wide variety of scales from micro to global and is successfully used both in daily forecasts and climatological studies. PT is designed for staying outdoors and is defined as the air temperature of a reference environment in which the thermal perception would be the same as in the actual environment. The calculation is performed for a reference subject with an internal heat production of 135 W m⁻² (who is walking at 4 km h⁻¹ on flat ground). In the reference environment, the mean radiant temperature equals the air temperature and wind velocity is reduced to a slight draught. The water vapour pressure remains unchanged. Under warm/humid conditions, however, it is implicitly related to a relative humidity of 50%. Clothing is adapted in order to achieve thermal comfort. If this is impossible, cold or heat stress will occur, respectively. The assessment of thermal perception by means of PT is based on Fanger’s Predicted Mean Vote (PMV) together with additional model extensions taking account of stronger deviations from thermal neutrality. This is performed using a parameterisation based on a two-node model. In the cold, it allows the mean skin temperature to drop below the comfort value. In the heat, it assesses additionally the enthalpy of sweat-moistened skin and of wet clothes. PT has the advantages of being self-explanatory due to its deviation from air temperature and being—via PMV—directly linked to a thermo-physiologically-based scale of thermal perception that is widely used and has stood the test of time. This paper explains in detail the basic equations of the human heat budget and the coefficients of the parameterisations.     

Thermal resistance parameters of the air environment at various altitudes

 The thermal properties of atmospheric air surrounding the human body at various altitudes are characterized with a system of parameters. This system comprises resistance of the air to convective heat transfer h _c ^–1, °C (W/m²)⁻¹ and to water vapour transfer h _D ^–1, s/m. The concept of ’evaporative resistance’h _e ^–1, hPa (W/m²)⁻¹) following the similarity of the processes is introduced. In obtaining the altitude dependencies of investigated paramters, a respective heat transfer equation expressing the rate of heat exchange at the boundary body surface – ambient air is applied. The use of the body thermal state of the established altitude dependencies is discussed. The concept of ’thermal stability’ related to the evaporative resistance parameter h _e ^–1 is introduced. This parameter is assumed as: (1) an indicator of the human body thermal stability and (2) distributor and predictor of environmental influence on the body thermal state. Received: 5 January 1996 / Accepted 5 November 1996     

Thermoregulation during swimming and diving in bottlenose dolphins, Tursiops truncatus

Heat transfer from the periphery is an important thermoregulatory response in exercising mammals. However, when marine mammals submerge, peripheral vasoconstriction associated with the dive response may preclude heat dissipation at depth. To determine the effects of exercise and diving on thermoregulation in cetaceans, we measured heat flow and skin temperatures of bottlenose dolphins (Tursiops truncatus) trained to follow a boat and to dive to 15 m. The results demonstrated that skin temperatures usually remained within 1 °C of the water after all exercise levels. Heat flow from peripheral sites (dorsal fin and flukes) increased over resting values immediately after exercise at the water surface and remained elevated for up to 20 min. However, post-exercise values for heat flow from the flukes and dorsal fin decreased by 30–67% when dolphins stationed at 15 m below the surface. The pattern in heat flow was reversed during ascent. For example, mean heat flow from the flukes measured at 5 m depth, 40.10 ± 2.47 W · m⁻², increased by 103.2% upon ascent. There is some flexibility in the balance between thermal and diving responses of dolphins. During high heat loads, heat transfer may momentarily increase during submergence. However, the majority of excess heat in dolphins appears to be dissipated upon resurfacing, thereby preserving the oxygen-conserving benefits of the dive response. Accepted: 4 January 1999     

Temperature and burn injury prediction of human skin exposed to microwaves: a model analysis

A one-dimensional multi-layer model is presented to characterize skin temperature rises and burn processes resulting from skin exposure to microwaves. Temperature variations and damage function analyses in the skin tissue exposed to microwaves were predicted depending on blood perfusion rate, thermal conductivity, power density, and exposure time. Thermal wave model was applied and the bio-heat transfer equation was solved using the finite difference time domain method. The thermal wave model of bio-heat transfer predicts a lower temperature rise than a model that uses Pennes’ equation. When approaching steady state, the solutions overlaps with that obtained using the Pennes’ equation. The results obtained may help to analyze the consequences of short-time high-power MW exposures in biological tissues.     

Effects of room temperature on physiological and subjective responses during whole-body bathing,half-body bathing and showering

The effects of bathroom thermal conditions on physiological and subjective responses were evaluated before, during, and after whole-body bath (W-bath), half-body bath (H-bath) and showering. The air temperature of the dressing room and bathroom was controlled at 10 degrees C, 17.5 degrees C, and 25 degrees C. Eight healthy males bathed for 10 min under nine conditions on separate days. The water temperature of the bathtub and shower was controlled at 40 degrees C and 41 degrees C, respectively. Rectal temperature (Tre), mean skin temperature (Tsk), blood pressure (BP), heart rate (HR), body weight loss and blood characteristics (hematocrit: Hct, hemoglobin: Hb) were evaluated. Also, thermal sensation (TS), thermal comfort (TC) and thermal acceptability (TA) were recorded. BP decreased rapidly during W-bath and H-bath compared to showering. HR during W-bath was significantly higher than for H-bath and showering (p < 0.01). The double products due to W-bath during bathing were also greater than for H-bath and showering (p < 0.05). There were no distinct differences in Hct and Hb among the nine conditions. However, significant differences in body weight loss were observed among the bathing methods: W-bath > H-bath > showering (p < 0.001). W-bath showed the largest increase in Tre and Tsk, followed by H-bath, and showering. Significant differences in Tre after bathing among the room temperatures were found only at H-bath. The changes in Tre after bathing for H-bath at 25 degrees C were similar to those for W-bath at 17.5 degrees C and 10 degrees C. TS and TC after bathing significantly differed for the three bathing methods at 17.5 degrees C and 10 degrees C (TS: p < 0.01 TC: p < 0.001). Especially, for showering, the largest number of subjects felt "cold" and "uncomfortable". Even though all of the subjects could accept the 10 degrees C condition after W-bath, such conditions were intolerable to half of them after showering. These results suggested that the physiological strains during H-bath and showering were smaller than during W-bath. However, colder room temperatures made it more difficult to retain body warmth after H-bath and created thermal discomfort after showering. It is particularly important for H-bath and showering to maintain an acceptable temperature in the dressing room and bathroom, in order to bathe comfortably and ensure warmth.     

Development and validity of a universal empirical equation to predict skin surface temperature on thermal manikins

Clothing evaporative resistance is an important input in thermal comfort models. Thermal manikin tests give the most accurate and reliable evaporative resistance values for clothing. The calculation methods of clothing evaporative resistance require the sweating skin surface temperature (i.e., options 1 and 2). However, prevailing calculation methods of clothing evaporative resistance (i.e., options 3 and 4) are based on the controlled nude manikin surface temperature due to the sensory measurement difficulty. In order to overcome the difficulty of attaching temperature sensors to the wet skin surface and to enhance the calculation accuracy on evaporative resistance, we conducted an intensive skin study on a thermal manikin ‘Tore’. The relationship among the nude manikin surface temperature, the total heat loss and the wet skin surface temperature in three ambient conditions was investigated. A universal empirical equation to predict the wet skin surface temperature of a sweating thermal manikin was developed and validated on the manikin dressed in six different clothing ensembles. The skin surface temperature prediction equation in an ambient temperature range between 25.0 and 34.0 °C is T_sk=34.0–0.0132HL. It is demonstrated that the universal empirical equation is a good alternative to predicting the wet skin surface temperature and facilitates calculating the evaporative resistance of permeable clothing ensembles. Further studies on the validation of the empirical equation on different thermal manikins are needed however.     

Effect of temperature difference between manikin and wet fabric skin surfaces on clothing evaporative resistance: how much error is there?

Clothing evaporative resistance is one of the inherent factors that impede heat exchange by sweating evaporation. It is widely used as a basic input in physiological heat strain models. Previous studies showed a large variability in clothing evaporative resistance both at intra-laboratory and inter-laboratory testing. The errors in evaporative resistance may cause severe problems in the determination of heat stress level of the wearers. In this paper, the effect of temperature difference between the manikin nude surface and wet textile skin surface on clothing evaporative resistance was investigated by both theoretical analysis and thermal manikin measurements. It was found that the temperature difference between the skin surface and the manikin nude surface could lead to an error of up to 35.9% in evaporative resistance of the boundary air layer. Similarly, this temperature difference could also introduce an error of up to 23.7% in the real clothing total evaporative resistance (R _{et_real}  < 0.1287 kPa m²/W). Finally, it is evident that one major error in the calculation of evaporative resistance comes from the use of the manikin surface temperature instead of the wet textile fabric skin temperature.     

Suitability of frequency modulated thermal wave imaging for skin cancer detection—A theoretical prediction

A theoretical study on the quantification of surface thermal response of cancerous human skin using the frequency modulated thermal wave imaging (FMTWI) technique has been presented in this article. For the first time, the use of the FMTWI technique for the detection and the differentiation of skin cancer has been demonstrated in this article. A three dimensional multilayered skin has been considered with the counter-current blood vessels in individual skin layers along with different stages of cancerous lesions based on geometrical, thermal and physical parameters available in the literature. Transient surface thermal responses of melanoma during FMTWI of skin cancer have been obtained by integrating the heat transfer model for biological tissue along with the flow model for blood vessels. It has been observed from the numerical results that, flow of blood in the subsurface region leads to a substantial alteration on the surface thermal response of the human skin. The alteration due to blood flow further causes a reduction in the performance of the thermal imaging technique during the thermal evaluation of earliest melanoma stages (small volume) compared to relatively large volume. Based on theoretical study, it has been predicted that the method is suitable for detection and differentiation of melanoma with comparatively large volume than the earliest development stages (small volume). The study has also performed phase based image analysis of the raw thermograms to resolve the different stages of melanoma volume. The phase images have been found to be clearly individuate the different development stages of melanoma compared to raw thermograms.     

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.     

A model of evaporative cooling of wet skin surface and fur layer

(1) A theoretical model that simultaneously solves heat and mass transfer in a wet skin surface and fur layer that occurs when an animal is cooled by blowing air over its wetted skin surface and hair coat is presented. (2) The model predicts evaporative and convective heat losses for different levels of wetness, air velocity, ambient temperature, relative humidity and fur properties. (3) Model predictions provide insight about evaporative and convective cooling processes of cows in stressful hot environments.     

Human morphology and temperature regulation

For nearly a century individuals have believed that there is a link between human morphology and one’s thermoregulatory response in adverse environments. Most early research was focussed on the rate of core cooling in a male adult population and the role of subcutaneous adipose tissue, surface area and the surface-area-to-mass ratio in one’s ability to withstand varying degrees of cold stress. More recently research has addressed heat tolerance in various populations, exploring the role of subcutaneous adipose tissue, surface area and the surface-area-to-mass ratio in one’s ability to maintain thermal equilibrium in warm and hot, dry and humid environments. Since the late 1970s an emphasis has been placed on the role of muscle and muscle perfusion in total-body thermal insulation. Yet, despite the history of research pertaining to human morphology and temperature regulation there is little consensus as to the impact of variations in human morphology on thermoregulatory responses. Individuals differing in body size, shape and composition appear to respond quantitatively differently to variations in both ambient and core temperatures but the interrelations between morphological components and temperature regulation are complex. It is the purpose of this paper to examine the literature pertaining to the impact of variations in muscularity, adipose tissue thickness and patterning, surface area and the surface-area-to-mass ratio on thermoregulation and thermal stability in response to both heat and cold stress. Received: 6 May 1999 / Accepted: 14 July 1999     

Host identification by Schistosoma japonicum cercariae

Attachment, the first phase of host identification by Schistosoma japonicum cercariae, can occur in 2 different ways. Cercariae clinging to the water surface simply swing around and transfer to the host skin. Free-swimming cercariae behave like S. mansoni: upon touching a substrate, they switch from tailward to forward movement, swim in an arc, and attach to it with the penetration organ. Neither type of attachment is influenced by chemical, thermal, or specific mechanical stimuli from the host. The second phase, remaining on the host, requires a solid hydrophobic surface and seems to depend only on the cercaria's ability to cling to it. This phase is not influenced by chemical or thermal stimuli. The third phase, creeping across the host surface, is independent of chemical and mechanical stimuli. Cercariae migrate in thermal gradients to a preferred temperature of 37 +/- 3 C and then attempt to penetrate. Penetration, the fourth phase, was evoked by human skin surface lipids. The free fatty acid (FA) fraction was identified as exclusively stimulating components. Saturated FA's were effective at chain lengths between 10 and 14 carbon atoms (pH 7.0), and unsaturated FA's were effective at longer chains and their activity increased with increasing number of double bonds. Dog skin surface components did not stimulate cercarial penetration, which can be attributed to the lack of free FA's. A temperature of 32-40 C also stimulated penetration responses, which might be the main stimulus in animal hosts, whose skin surfaces contain no or only a few free FA's. FA's and heat evoked a transformation of cercarial tegument simultaneous with penetration behavior, making the organisms osmotically sensitive. The host identification of S. japonicum cercariae is very nonspecific compared with the differentiated host recognition of S. mansoni.     

Comparison of heat dissipation response between Malaysian and Japanese males during exercise in humid heat stress

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T _re) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T _re in MY may be explained by the presence of a significantly greater core–skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core–skin temperature gradient observed in MY is responsible for the smaller increase in T _re.     

Physiological functions of the effects of the different bathing method on recovery from local muscle fatigue

Background

Recently, mist saunas have been used in the home as a new bathing style in Japan. However, there are still few reports on the effects of bathing methods on recovery from muscle fatigue. Furthermore, the effect of mist sauna bathing on human physiological function has not yet been revealed. Therefore, we measured the physiological effects of bathing methods including the mist sauna on recovery from muscle fatigue.

Methods

The bathing methods studied included four conditions: full immersion bath, shower, mist sauna, and no bathing as a control. Ten men participated in this study. The participants completed four consecutive sessions: a 30-min rest period, a 10-min all out elbow flexion task period, a 10-min bathing period, and a 10-min recovery period. We evaluated the mean power frequency (MNF) of the electromyogram (EMG), rectal temperature (Tre), skin temperature (Tsk), skin blood flow (SBF), concentration of oxygenated hemoglobin (O2Hb), and subjective evaluation.

Results

We found that the MNF under the full immersion bath condition was significantly higher than those under the other conditions. Furthermore, Tre, SBF, and O2Hb under the full immersion bath condition were significantly higher than under the other conditions.

Conclusions

Following the results for the full immersion bath condition, the SBF and O2Hb of the mist sauna condition were significantly higher than those for the shower and no bathing conditions. These results suggest that full immersion bath and mist sauna are effective in facilitating recovery from muscle fatigue.     

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.     

Morphometry and trophic state modify the thermal response of lakes to meteorological forcing

We hypothesised that (i) a summer heat wave would increase the thermal stability of lakes and (ii) the size and trophic state differences would modify the lakes’ responses to heating. Within 2 years, 2008 and 2009, we studied the thermal and optical regimes of two adjacent stratified lakes in northern Italy, the oligo-mesotrophic Lake Monate (2.5 km², max. depth 34 m) and the eutrophic Lake Varese (14.8 km², 26 m). After the cold winter 2008–2009, a heat wave starting in May turned the whole year 2009 the second hottest after 2003. The particular sequence of meteorological events resulted in extreme vertical temperature gradients and unusually high thermal stability of both lakes. All calculated thermal parameters showed the highest values in 2009 while also the values for 2008 exceeded considerably those published for these lakes in the past. Due to the large wind exposed surface, wind mixing was supposedly the dominating mechanism of heat transfer in the shallower eutrophic Lake Varese where, due to low water transparency, large amount of solar energy trapped in the upper layers markedly increased the thermal stability. In the deeper and more transparent Lake Monate, the deeper penetrating solar irradiance contributed to better energy dissipation within the water column and smaller interannual differences in thermal stability.     

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.     

Heat transfer in elephants: thermal partitioning based on skin temperature profiles

The elephant with its low surface-to-volume ratio presents an interesting problem concerning heat dissipation. To understand how such large mammals remain in thermal balance, we determined the major avenues of heat loss for an adult African elephant and an immature Indian elephant. Because conventional physiological measurements are difficult for these animals, the present study used a non-invasive technique, infrared thermography, to measure skin temperatures of each elephant. Detailed surface temperature profiles and surface area measurements of each elephant were used in standard equations for convective, conductive and radiant heat transfer. Results demonstrated that heat transfer by free convection and radiation accounted for 86% of the total heat loss for the elephants at T _a= 12·6 °C. Heat transfer across the ears, an important thermal window at high ambient temperatures, represented less than 8% of the total heat loss. Surface area of the animals, and metabolic heat production calculated from total heat loss of the African elephant, scaled predictably with body mass. In contrast, the thermal conductance of the elephants (71·6 W /°C, African; 84·5 W /°C, Indian) was three to five times higher than predicted from an allometric relationship for smaller mammals. The high thermal conductance of elephants is attributed to the absence of fur and appears to counteract reduced heat transfer associated with a low surface-to-volume ratio.     

Heat and water transfer in a rotating drum containing solid substrate particles

In previous work we reported on the simulation of mixing behavior of a slowly rotating drum for solid-state fermentation (SSF) using a discrete particle model. In this investigation the discrete particle model is extended with heat and moisture transfer. Heat transfer is implemented in the model via interparticle contacts and the interparticle heat transfer coefficient is determined experimentally. The model is shown to accurately predict heat transfer and resulting temperature gradients in a mixed wheat grain bed. In addition to heat transfer, the addition and subsequent distribution of water in the substrate bed is also studied. The water is added to the bed via spray nozzles to overcome desiccation of the bed during evaporative cooling. The development of moisture profiles in the bed during spraying and mixing are studied experimentally with a water-soluble fluorescent tracer. Two processes that affect the water distribution are considered in the model: the intraparticle absorption process, and the interparticle transfer of free water. It is found that optimum distribution can be achieved when the free water present at the surface of the grains is quickly distributed in the bed, for example, by fast mixing. Alternatively, a short spraying period, followed by a period of mixing without water addition, can be applied. The discrete particle model developed is used successfully to examine the influence of process operation on the moisture distribution (e.g., fill level and rotation rate). It is concluded that the extended discrete particle model can be used as a powerful predictive tool to derive operating strategies and criteria for design and scale-up for mixed SSF and other processes with granular media.     

Thermal insulation and body temperature wearing a thermal swimsuit during water immersion

This study evaluated the effects of a thermal swimsuit on body temperatures, thermoregulatory responses and thermal insulation during 60 min water immersion at rest. Ten healthy male subjects wearing either thermal swimsuits or normal swimsuits were immersed in water (26 degrees C or 29 degrees C). Esophageal temperature, skin temperatures and oxygen consumption were measured during the experiments. Metabolic heat production was calculated from oxygen consumption. Heat loss from skin to the water was calculated from the metabolic heat production and the change in mean body temperature during water immersion. Total insulation and tissue insulation were estimated by dividing the temperature difference between the esophagus and the water or the esophagus and the skin with heat loss from the skin. Esophageal temperature with a thermal swimsuit was higher than that with a normal swimsuit at the end of immersion in both water temperature conditions (p<0.05). Oxygen consumption, metabolic heat production and heat loss from the skin were less with the thermal swimsuit than with a normal swimsuit in both water temperatures (p<0.05). Total insulation with the thermal swimsuit was higher than that with a normal swimsuit due to insulation of the suit at both water temperatures (p<0.05). Tissue insulation was similar in all four conditions, but significantly higher with the thermal swimsuit in both water temperature conditions (p<0.05), perhaps due to of the attenuation of shivering during immersion with a thermal swimsuit. A thermal swimsuit can increase total insulation and reduce heat loss from the skin. Therefore, subjects with thermal swimsuits can maintain higher body temperatures than with a normal swimsuit and reduce shivering thermo-genesis.