Habituation of the metabolic and ventilatory responses to cold-water immersion in humans

An experiment was undertaken to answer long-standing questions concerning the nature of metabolic habituation in repeatedly cooled humans. It was hypothesised that repeated skin and deep-body cooling would produce such a habituation that would be specific to the magnitude of the cooling experienced, and that skin cooling alone would dampen the cold-shock but not the metabolic response to cold-water immersion. Twenty-one male participants were divided into three groups, each of which completed two experimental immersions in 12 °C water, lasting until either rectal temperature fell to 35 °C or 90 min had elapsed. Between these two immersions, the control group avoided cold exposures, whilst two experimental groups completed five additional immersions (12 °C). One experimental group repeatedly immersed for 45 min in average, resulting in deep-body (1.18 °C) and skin temperature reductions. The immersions in the second experimental group were designed to result only in skin temperature reductions, and lasted only 5 min. Only the deep-body cooling group displayed a significantly blunted metabolic response during the second experimental immersion until rectal temperature decreased by 1.18 °C, but no habituation was observed when they were cooled further. The skin cooling group showed a significant habituation in the ventilatory response during the initial 5 min of the second experimental immersion, but no alteration in the metabolic response. It is concluded that repeated falls of skin and deep-body temperature can habituate the metabolic response, which shows tissue temperature specificity. However, skin temperature cooling only will lower the cold-shock response, but appears not to elicit an alteration in the metabolic response.     

Evaluation of mean skin temperature formulas by infrared thermography

 To study the reliabiliity of formulas for calculating mean skin temperature (T _sk), values were computed by 18 different techniques and were compared with the mean of 10,841 skin temperatures measured by infrared thermography. One hundred whole-body infrared thermograms were scanned in ten resting males while changing the air temperature from 40° C to 4° C. Local, regional average and mean skin temperatures were obtained using an image processing system. The agreement frequency, defined as the percentage of the calculated T _sk values which agreed with the corresponding infrared thermographic T _sk within ±0.2° C, ranged for with the various formulas from 7% to 80%. In many sites, the local skin temperature did not coincide with the regional average skin temperature. When the local skin temperatures which showed the highest percentage similarity to the regional average skin temperature within ±0.4° C were applied to the formula, the agreement frequency was markedly improved for all formulas. However, the agreement frequency was not affected by changing the weighting factors from specific constants to individually measured values of regional surface area. By applying the physiologically reliable accuracy range of ±0.2° C in the moderate and ±0.4° C in the cool condition, agreement frequencies of at least 95% were observed in formulas involving seven or more skin temperature measurement sites, including the hand and foot. We conclude that calculation of a reliable mean skin temperature must involve more than seven skin temperature measurement sites regardless of ambient temperature. Optimal sites for skin temperature measurement are proposed for various formulas. Received: 2 December 1996 / Accepted: 25 June 1997     

Lessons from the past—human and animal thermal physiology

(1) Primordial lesioning and stimulation experiments established a thermoregulatory centre in the rostral brain stem at the end of the 19th century. (2) A major landmark in understanding how deep-body temperature (T_c) is sensed, came in 1912 when Barbour found that changing rostral brain stem temperature inversely raised or lowered T_c, ultimately leading to a mono-centric concept of hypothalamic thermoregulation, prevailing for about 50 years. (3) The discovery of extrahypothalamic sites of temperature signal generation in the 1960s led to the multiple-input, multiple-controller concept of thermoregulation. (4) During the last 40 years, concepts concerning thermosensory specificity have radically changed from viewing bimodal peripheral thermoreceptors and hypothalamic thermoreceptors as the only relevant signal generators towards a complex picture including monomodality of peripheral warm and cold thermoreceptors and multimodality of deep-body thermosensors.     

The use of percutaneous thermal sensing microchips for non-invasive measurement of body temperature in foals during summer seasons in a subtropical region

Continuous accurate attainment of the body temperature of foals is important to detect early stages of severe heat stress or fever due to a systemic illness. Among a number of methods to measure body temperature, measuring rectal temperature with a digital thermometer is most frequently used due to being relatively fast and simple method. It is also comparatively accurate and correlates well with the core body temperature. However, this method requires restraining the foal for a few seconds to obtain the temperature, and it can be dangerous for the handling person. Percutaneous thermal sensing microchips (PTSMs) are a means of monitoring the body temperature of horses, which offers a non-invasive, hygienic, quick, and accurate way to measure body temperature and provide an identification number for each individual, once it is implanted. This study tested the hypothesis that PTSM has a strong relationship with a conventional body temperature measurement, i.e., measuring rectal temperature with a digital thermometer of foals during summer seasons. Thirty-two foals in three consecutive foaling seasons (2018, 2019, and 2020 season) were implanted a PTSM into the right pectoral muscle, the right splenius muscle, the right gluteal muscle, and the nuchal ligament as early as two weeks after birth. The four PTSM temperatures, rectal temperature, and climate conditions (air temperature, relative humidity, and wet-bulb globe temperature) were obtained simultaneously during the three summer seasons and paired for comparison analysis. Among the PTSM temperatures, the pectoral muscle had the highest correlation and the least differences with rectal temperature. Using PTSM was safe, easy, and reliable for attaining body temperature in foals.     

Temperatures of skin, subcutaneous tissue, muscle and core in resting men in cold, comfortable and hot conditions

To examine the core-shell model of temperature distribution and the possible role of subcutaneous temperature in heat regulation, comprehensive temperature measurements were made on six nude resting men exposed for 2-3 h to comfort (27 degrees C), cold (15 degrees C) and heat (45 degrees C). Cold produced strong shivering and heat caused heavy sweating. Temperatures were recorded every 10 min from: esophagus, rectum and auditory canal; back muscle and thigh muscle at 20 mm and 40 mm depths; 6 subcutaneous sites; and 16 skin sites. Average temperatures at these 29 sites were tabulated at the ends of comfort, hot and cold and the onsets of sweating and shivering. Body temperature changes were slow to develop, the skin temperatures being fastest, and successively deeper tissues progressively slower. There was occasional after-drop and after-rise. The data were consistent with the core-shell concept. The temperature gradient from subcutaneous tissue to skin, which differed substantially with comfort, the onset of shivering and the onset of sweating, could serve as a regulatory signal. The data are now in computer format and may be of interest to biothermal modelers.     

Thermophysiological consequences of whole body resonant RF exposure (100 MHz) in human volunteers

Thermophysiological responses of heat production and heat loss were measured in seven adult volunteers (six males and one female, aged 31-74 years) during 45 min dorsal exposures of the whole body to 100 MHz continuous wave (CW) radio frequency (RF) energy. Three power densities (PD) (average PD = 4, 6, and 8 mW/cm(2); whole body specific absorption rate [SAR] = 0.068 [W/kg]/[mW/cm(2)]) were tested in each of three ambient temperatures (T(a) = 24, 28, and 31 degrees C), as well as in T(a) controls (no RF). A standardized protocol (30 min baseline, 45 min RF or sham exposure, 10 min baseline) was used. Measured responses included esophageal and seven skin temperatures, metabolic heat production, local sweat rate, and local skin blood flow. No changes in metabolic heat production occurred under any test condition. Unlike published results of similar exposures at 450 and 2450 MHz, local skin temperatures, even those on the back that were irradiated directly, changed little or not at all during 100 MHz exposures. The sole exception was the temperature of the ankle skin, which increased by 3-4 degrees C in some subjects at PD = 8 mW/cm(2). During the 45 min RF exposure, esophageal temperature showed modest changes (range = -0.15 to 0.13 degrees C) and never exceeded 37.2 degrees C. Thermoregulation was principally controlled by appropriate increases in evaporative heat loss (sweating) and, to a lesser extent, by changes in skin blood flow. Because of the deep penetration of RF energy at this frequency, effectively bypassing the skin, these changes must have been stimulated by thermal receptors deep in the body rather than those located in the skin.     

Physiological responses and thermal sensations of the elderly in cold and hot environments

1. 1. 10 elderly and 10 college-aged females served as subjects in cold and heat environments. The subjects changed into the standard clothing (0.63 clo), and stayed in the neutral environment (25°C) for 23 min, thereafter they were exposed to the cold (10°C) or hot (35°C) environment for 49 min.

2. 2. Then they returned to the neutral environment, and stayed there for 47 min. Oral temperature, skin temperatures at 10 sites, blood pressure and thermal sensation were measured during the experiments.

3. 3. In the cold environment, the elderly could not reduce heat loss by vasoconstriction as did young people, and their blood pressures increased more rapidly than in young people. In the hot environment, the elderly could not promote heat loss by vasodilation as did young people. Moreover, there is a delayed sensitivity to cold for the elderly. Therefore, in the houses of the elderly, it is important to have heating and cooling systems which also includes the areas where the people do not stay for a long period of time (e.g. toilet, passageways).

Modelling hand skin temperature in relation to body composition

Skin temperature is a challenging parameter to predict due to the complex interaction of physical and physiological variations. Previous studies concerning the correlation of regional physiological characteristics and body composition showed that obese people have higher hand skin temperature compared to the normal weight people. To predict hand skin temperature in a different environment, a two-node hand thermophysiological model was developed and validated with published experimental data. In addition, a sensitivity analysis was performed which showed that the variations in skin blood flow and blood temperature are most influential on hand skin temperature. The hand model was applied to simulate the hand skin temperature of the obese and normal weight subgroup in different ambient conditions. Higher skin blood flow and blood temperature were used in the simulation of obese people. The results showed a good agreement with experimental data from the literature, with the maximum difference of 0.31 °C. If the difference between blood flow and blood temperature of obese and normal weight people was not taken into account, the hand skin temperature of obese people was predicted with an average deviation of 1.42 °C. In conclusion, when modelling hand skin temperatures, it should be considered that regional skin temperature distribution differs in obese and normal weight people.     

The effect of body fat percentage and body fat distribution on skin surface temperature with infrared thermography

This study aimed to search for relations between body fat percentage and skin temperature and to describe possible effects on skin temperature as a result of fat percentage in each anatomical site. Women (26.11±4.41 years old) (n =123) were tested for: body circumferences; skin temperatures (thermal camera); fat percentage and lean mass from trunk, upper and lower limbs; and body fat percentage (Dual-Energy X-Ray Absorptiometry). Values of minimum (T_Mi), maximum (T_Ma), and mean temperatures (T_Me) were acquired in 30 regions of interest. Pearson's correlation was estimated for body circumferences and skin temperature variables with body fat percentage. Participants were divided into groups of high and low fat percentage of each body segment, of which T_Me values were compared with Student's t-test. Linear regression models for predicting body fat percentage were tested. Body fat percentage was positively correlated with body circumferences and palm temperatures, while it was negatively correlated with most temperatures, such as T_Ma and T_Me of posterior thighs (r =−0.495 and −0.432), T_Me of posterior lower limbs (r =−0.488), T_Ma of anterior thighs (r =−0.406) and T_Mi and T_Me of posterior arms (r =−0.447 and −0.430). Higher fat percentages in the specific anatomical sites tended to decrease T_Me, especially in posterior thighs, shanks and arms. Skin temperatures and body circumferences predicted body fat percentage with 58.3% accuracy (R =0.764 and R² =0.583). This study clarifies that skin temperature distribution is influenced by the fat percentage of each body segment.     

Heat stress and age: Skin blood flow and body temperature

1. 1. The ability to increase skin blood flow is an important mechanism for transferring heat from the body core to the skin for dissipation.

2. 2. During exercise, skin blood flow is typically 20–40% lower in men and women aged 55 and over (compared with 20–30 years old) at a given body core temperature. Yet criterion measures of heat tolerance (changes in core temperature, heat storage) often show minimal or no age-related alterations. From a series of studies conducted in our laboratory over the past 5 years, the following conclusions can be drawn.

3. 3. When fit healthy older subjects are matched with younger subjects of the same gender, size and body composition, V_O2max, acclimation state, and hydration level, age-related differences in skin blood flow are evident. However, these differences often do not translate into “poorer” heat tolerance or higher core temperatures.

4. 4. The larger core-to-skin thermal gradient maintained by the older individuals allows for effective heat transfer at lower skin blood flows.

5. 5. Furthermore, there is an increased coefficient of variation for thermoregulatory response variables with increasing age.

6. 6. Despite differences in the mechanisms underlying thermoregulation, true thermal tolerance is less a function of chronological age than of functional capacity and physiological health status.

7. 7. While this conclusion is based primarily on cross-sectional studies, it is supported by the results of more recent studies using multiple regression analyses.

8. 8. Implicit in this conclusion is the notion that thermal tolerance, at any age, is a modifiable individual characteristic.

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.     

Optimum temperature for egg development of phenotypes in Haemonchus contortus cayugensis as determined by Arrhenius diagrams and Sacher's entropy function

Lejambre L. P. and Whitlock J. H. 1973. Optimum temperature for egg development of phenotypes in Haemonchus contortus cayugensis as determined by Arrhenius diagrams and Sacher's entropy function. International Journal for Parasitology3: 299–310. Oxygen uptake, maturation time and percentage hatch were determined on eggs from “wild-type” Haemonchus contortus cayugensis populations as well as those from the morph types smooth, linguiform B and linguiform A. Sacher's organizational entropy, which is essentially a sum of oxygen utilized by a system corrected for the number of viable units in the system, was used to determine the optimum temperature for development of the eggs. Although the eggs from “wild-type” H. contortus cayugensis hatched well across a broad temperature range; individual phenotypes showed a much narrower range. The optimum temperature for the development of eggs from smooth worms was approximately 30°C while linguiform A and B hatched best at 25°C. Linguiform A eggs had a broader temperature tolerance than linguiform B so that at temperatures of 17°C they hatched with a greater efficiency than did either smooth or linguiform B. It is argued that H. contortus follows a “strategy” of having many phenotypes, each one of which appears to be adapted to a different temperature. This would allow a population to maintain a broad range of optimal temperatures without the expense of maintaining the cybernetic machinery which a single individual would have to have if it were to tolerate the same range of temperature.     

Partial-body exposure of human volunteers to 2450 MHz pulsed or CW fields provokes similar thermoregulatory responses

Many reports describe data showing that continuous wave (CW) and pulsed (PW) radiofrequency (RF) fields, at the same frequency and average power density (PD), yield similar response changes in the exposed organism. During whole-body exposure of squirrel monkeys at 2450 MHz CW and PW fields, heat production and heat loss responses were nearly identical. To explore this question in humans, we exposed two different groups of volunteers to 2450 MHz CW (two females, five males) and PW (65 micros pulse width, 10(4) pps; three females, three males) RF fields. We measured thermophysiological responses of heat production and heat loss (esophageal and six skin temperatures, metabolic heat production, local skin blood flow, and local sweat rate) under a standardized protocol (30 min baseline, 45 min RF or sham exposure, 10 min baseline), conducted in three ambient temperatures (T(a) = 24, 28, and 31 degrees C). At each T(a), average PDs studied were 0, 27, and 35 mW/cm2 (Specific absorption rate (SAR) = 0, 5.94, and 7.7 W/kg). Mean data for each group showed minimal changes in core temperature and metabolic heat production for all test conditions and no reliable differences between CW and PW exposure. Local skin temperatures showed similar trends for CW and PW exposure that were PD-dependent; only the skin temperature of the upper back (facing the antenna) showed a reliably greater increase (P =.005) during PW exposure than during CW exposure. Local sweat rate and skin blood flow were both T(a)- and PD-dependent and showed greater variability than other measures between CW and PW exposures; this variability was attributable primarily to the characteristics of the two subject groups. With one noted exception, no clear evidence for a differential response to CW and PW fields was found.     

Diurnal and menstrual cycles in body temperature are regulated differently: A 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls

Diurnal cycle variations in body-heat loss and heat production, and their resulting core body temperature (CBT), are relatively well investigated; however, little is known about their variations across the menstrual cycle under ambulatory conditions. The main purpose of this study was to determine whether menstrual cycle variations in distal and proximal skin temperatures exhibit similar patterns to those of diurnal variations, with lower internal heat conductance when CBT is high, i.e. during the luteal phase. Furthermore, we tested these relationships in two groups of women, with and without thermal discomfort of cold extremities (TDCE). In total, 19 healthy eumenorrheic women with regular menstrual cycles (28–32 days), 9 with habitual TDCE (ages 29?±?1.5 year; BMI 20.1?±?0.4) and 10 controls without these symptoms (CON: aged 27?±?0.8 year; BMI 22.7?±?0.6; p?<?0.004 different to TDCE) took part in the study. Twenty-eight days continuous ambulatory skin temperature measurements of distal (mean of hands and feet) and proximal (mean of sternum and infraclavicular regions) skin regions, thighs, and calves were carried out under real-life, ambulatory conditions (i-Buttons® skin probes, sampling rate: 2.5?min). The distal minus proximal skin temperature gradient (DPG) provided a valuable measure for heat redistribution from the core to the shell, and, hence, for internal heat conduction. Additionally, basal body temperature was measured sublingually directly after waking up in bed. Mean diurnal amplitudes in skin temperatures increased from proximal to distal skin regions and the 24-h mean values were inversely related. TDCE compared to CON showed significantly lower hand skin temperatures and DPG during daytime. However, menstrual cycle phase did not modify these diurnal patterns, indicating that menstrual and diurnal cycle variations in skin temperatures reveal additive effects. Most striking was the finding that all measured skin temperatures, together with basal body temperature, revealed a similar menstrual cycle variation (independent of BMI), with highest and lowest values during the luteal and follicular phases, respectively. These findings lead to the conclusion that in contrast to diurnal cycle, variations in CBT variation across the menstrual cycle cannot be explained by changes in internal heat conduction under ambulatory conditions. Although no measurements of metabolic heat production were carried out increased metabolic heat generation during the luteal phase seems to be the most plausible explanation for similar body temperature increases.     

Thermal variability in body temperature in an ectotherm: Are cloacal temperatures good indicators of tortoise body temperature?

Historically, studies of reptilian thermal biology have compared ambient temperatures (T_a) to body temperatures (T_b) from the animal under study, with T_b usually taken from the cloaca and various instruments being used to measure T_b. The advent of surgically implanted miniature temperature loggers has offered the opportunity to test the efficacy of cloacal T_b as a measurement in thermoregulatory studies. We expected that there was a difference between skin, cloacal, and core T_b's. Temperatures were measured from various positions on leopard tortoises (Stigmochelys pardalis) using thermocouples and miniature temperature loggers, including surgically implanted temperature loggers. Measurements of temperature from various positions on and in the tortoise were significantly different from T_a. Cloacal T_b's were significantly lower than all other body temperatures measured, and core T_b's were significantly different from cloacal T_b, skin and carapace temperatures. In addition, significant differences were found between measures of temperature from other parts of the body. The variations between core T_b, cloacal T_b and other measures of T_b indicated that there are large thermal gradients within the body of a relatively large tortoise at any given time with cloacal T_b not an accurate measure of core T_b.     

Changes in body core temperatures and heat balance after an abrupt release of lower body negative pressure in humans

Changes in body core temperature (T _cor) and heat balance after an abrupt release of lower body negative pressure (LBNP) were investigated in 5 volunteers under the following conditions: (1) an ambient temperature (T _a) of 20 °C or (2) 35 °C, and (3)T _a of 25 °C with a leg skin temperature of 30°C or (4) 35°C. The leg skin temperature was controlled with water perfusion devices wound around the legs. Rectal (T _re), tympanic (T _ty) and esophageal (T _es) temperatures, skin temperatures (7 sites) and oxygen consumption were measured. The intensity of LBNP was adjusted so that the amount of blood pooled in the legs was the same under all conditions. When a thermal balance was attained during LBNP, application of LBNP was suddenly halted. The skin temperatures increased significantly after the release of LBNP under all conditions, while oxygen consumption hardly changed. The release of LBNP caused significant falls inT _cor s under conditions (1) and (3), but loweredT _cor s very slightly under conditions (2) and (4). The changes inT _es were always more rapid and greater than those ofT _ty andT _re. The falls inT _ty andT _re appeared to be explained by changes in heat balance, whereas the sharp drop ofT _es could not be explained especially during the first 8 min after the release of LBNP. The results suggest that a fall inT _cor after a release of LBNP is attributed to an increase in heat loss due to reflexive skin vasodilation and is dependent on the temperature of venous blood returning from the lower body. It is presumed thatT _es may not be an appropriate indicator forT _cor when venous return changes rapidly.     

Active cutaneous vasodilation in resting humans during mild heat stress.

The role of skin temperature in reflex control of the active cutaneous vasodilator system was examined in six subjects during mild graded heat stress imposed by perfusing water at 34, 36, 38, and 40 degrees C through a tube-lined garment. Skin sympathetic nerve activity (SSNA) was recorded from the peroneal nerve with microneurography. While monitoring esophageal, mean skin, and local skin temperatures, we recorded skin blood flow at bretylium-treated and untreated skin sites by using laser-Doppler velocimetry and local sweat rate by using capacitance hygrometry on the dorsal foot. Cutaneous vascular conductance (CVC) was calculated by dividing skin blood flow by mean arterial pressure. Mild heat stress increased mean skin temperature by 0.2 or 0.3 degrees C every stage, but esophageal and local skin temperature did not change during the first three stages. CVC at the bretylium tosylate-treated site (CVC(BT)) and sweat expulsion number increased at 38 and 40 degrees C compared with 34 degrees C (P < 0.05); however, CVC at the untreated site did not change. SSNA increased at 40 degrees C (P < 0.05, different from 34 degrees C). However, SSNA burst amplitude increased (P < 0.05), whereas SSNA burst duration decreased (P < 0.05), at the same time as we observed the increase in CVC(BT) and sweat expulsion number. These data support the hypothesis that the active vasodilator system is activated by changes in mean skin temperature, even at normal core temperature, and illustrate the intricate competition between active vasodilator and the vasoconstrictor system for control of skin blood flow during mild heat stress.     

Monitoring changes in skin temperature associated with exercise in horses on a water treadmill by use of infrared thermography

Infrared thermography (IRT) was used to assess surface temperature change as an indirect measure of muscle activity and exercise associated changes in blood flow in the working hind limb muscles of horses (n=7) undergoing water treadmill exercise. Three treatments were investigated including the treadmill ran dry (TD), water at the height of the proximal interphalangeal joint (PIP) and water at the height of the carpus (CP). Maximum skin surface temperature was recorded from the region of the semitendinosus muscle during exercise at each water height. There was a significant difference in surface hind limb temperature between exercise on the water treadmill ran dry and with water at the height of the PIP and CP (P<0.0001) with hotter temperatures recorded during the TD treatment. There was a greater increase in surface temperature of the hind limbs from pre exercise to maximum temperature during the PIP and CP treatments when compared to the TD treatment, however, this was not significant (P=0.58). There was no significant difference in surface hind limb temperature found between exercise in water at the height of the PIP and water at the height of the CP. The findings from this study suggest that IRT is able to non-invasively detect muscle activity and associated changes in blood flow whilst horses are exercised on a water treadmill. IRT could potentially be used as an alternative method to assess muscle activity and temperature change in an aquatic environment where existing methods present methodological challenges.     

Prediction of human core body temperature using non-invasive measurement methods

The measurement of core body temperature is an efficient method for monitoring heat stress amongst workers in hot conditions. However, invasive measurement of core body temperature (e.g. rectal, intestinal, oesophageal temperature) is impractical for such applications. Therefore, the aim of this study was to define relevant non-invasive measures to predict core body temperature under various conditions. We conducted two human subject studies with different experimental protocols, different environmental temperatures (10 °C, 30 °C) and different subjects. In both studies the same non-invasive measurement methods (skin temperature, skin heat flux, heart rate) were applied. A principle component analysis was conducted to extract independent factors, which were then used in a linear regression model. We identified six parameters (three skin temperatures, two skin heat fluxes and heart rate), which were included for the calculation of two factors. The predictive value of these factors for core body temperature was evaluated by a multiple regression analysis. The calculated root mean square deviation (rmsd) was in the range from 0.28 °C to 0.34 °C for all environmental conditions. These errors are similar to previous models using non-invasive measures to predict core body temperature. The results from this study illustrate that multiple physiological parameters (e.g. skin temperature and skin heat fluxes) are needed to predict core body temperature. In addition, the physiological measurements chosen in this study and the algorithm defined in this work are potentially applicable as real-time core body temperature monitoring to assess health risk in broad range of working conditions.