Prediction of mean skin temperature in warm environments

Summary The data collected by the authors in four experimental series have been analysed together with data from the literature, to study the relationship between mean skin temperature and climatic parameters, subject metabolic rate and clothing insulation. The subjects involved in the various studies were young male subjects, unacclimatized to heat. The range of conditions examined involved mean skin temperatures between 33‡ C and 38‡ C, air temperatures (T_a) between 23‡ C and 50‡ C, ambient water vapour pressures (P_a) between 1 and 4.8 kPa, air velocities (V_a) between 0.2 and 0.9 m · s⁻¹, metabolic rates (M) between 50 and 270 W · m⁻², and Clo values between 0.1 and 0.6. In 95% of the data, mean radiant temperature was within ±3‡ C of air temperature. Based on 190 data averaged over individual values, the following equation was derived by a multiple linear regression technique: ˉT_sk=30.0+0.138T_a+0.254P_a−0.57V_a+1.28 · 10⁻³ M−0.553 Clo. This equation was used to predict mean skin temperature from 629 individual data. The difference between observed and predicted values was within ±0.6‡ C in 70% of the cases and within ±1‡ C in 90% of the cases. It is concluded that the proposed formula may be used to predict mean skin temperature with satisfactory accuracy in nude to lightly clad subjects exposed to warm ambient conditions with no significant radiant heat load.     

Mean skin temperature in warm humid climates

Summary Mean skin temperature was measured in 24 subjects during experiments in a climatic chamber. Three conditions of ambient temperature (T _a=25.6°, 28.9° and 32.2° C), and three of humidity (relative humidity = 50%, 70% and 90%) were studied. A relationship was established by a linear regression technique. It is valid in the 24°–34° C range, for air velocity =0.2 m·s⁻¹, clothing insulation =0.077° C·m²·w⁻¹ (0.5 clo), metabolic rate =64 w·m⁻² (1.1 met) and radiant temperature = air temperature. In these conditions =28.125+0.021P _w+0.210T _a (P _w: ambient water vapour pressure in mb). It shows a small humidity influence. The influences of sex, transition from one condition to the next, and air velocity were also studied. Measurements in Africa confirmed the small influence of humidity. Ethnic life-style differences indicated that a high precision in determination is difficult to achieve.     

The metabolic response to skin temperature

Experiments were done to assess that fraction of the metabolic response to external cold exposure, which is attributable to skin temperature. In 5 conscious and closely clipped goats the metabolic rate was determined at various stable levels of skin temperature in the range from 13 to 41°C, while core temperature was kept constant at 38.8°C. Skin temperature was manipulated by a rapidly circulating shower bath, while core temperature was controlled by means of heat exchangers acting on arterial blood temperature in a chronic arteriovenous shunt. The metabolic response to skin temperature fell into two clearly discernible sections: a first zone with skin temperatures above 25–30°C, within which the metabolic rate rose at a rate of –0.34±0.07 W/kg·°C with decreasing skin temperature, and a second zone with skin temperatures below 25–30°C, within which the metabolic rate either plateaued or even grew smaller with further decreasing skin temperature. It is concluded that the relationship between skin temperature and metabolic rate does not directly reproduce the temperature-response curve of cutaneous cold receptors but also reflects a complex interaction of several factors, including an unspecific temperature effect on muscle metabolism.     

Using skin temperature gradients or skin heat flux measurements to determine thresholds of vasoconstriction and vasodilatation

Forearm–fingertip skin temperature differentials (T _sk-diff) are used to indicate vasomotor tone, vasoconstriction defined as having occurred when T _sk-diff≥4°C (Sessler et al. 1987, 1988a, b). This study was conducted to determine whether T _sk-diff or finger pad heat flux (HF) can be used to predict when vasoconstriction and vasodilatation occur. Seven subjects (one female) sat in water at [mean (SD)] 40.7 (0.8)°C until their core temperature (T _c) increased by 1°C, ensuring vasodilatation. The water was then cooled [at a rate of 0.6 (0.1)°C.min^–1] until T _c fell to 0.5°C below pretesting values, causing vasoconstriction. Subjects were then rewarmed in water [41.2 (1.0)°C]. Skin blood flow (SkBF) was measured using laser Doppler flowmetry (LDF) on the left second finger pad [immersed in water at 10.4 (1.4)°C as part of another experiment], and infrared plethysmography on the third finger pad of both hands. T _sk-diff and HF were measured on the right upper limb, which remained in air. When vasodilated, the subjects had a stable T _sk-diff and HF. During cooling, rapid-onset vasoconstriction occurred coincidental with large gradient changes in HF and T _sk-diff (inflection points). In two subjects the original vasoconstriction definition (T _sk-diff≥4°C) was not attained, in the other five this was achieved 31–51 min after vasoconstriction. During rewarming, the T _sk-diff and HF inflection points less accurately reflected the onset of vasodilatation, although with one exception they were within 5 min of the LDF changes. We conclude that T _sk-diff and HF inflection points predict vasoconstriction accurately, and better than T _sk-diff≥4°C. Electronic Publication     

Influence of skin temperature distribution on thermal sensation in a cool environment

Summary Thermal sensation and distribution of skin temperatures in persons exercising at 36.5 W on a bicycle ergometer and resting in a cool environment (10 C) in two different clothings, one with the insulation mainly over the trunk (1.22 clo), and one with well insulated limbs (1.67 clo), were studied. Their general thermal sensations varied from slightly warm to slightly cool. The placing of the insulation had a decisive influence on skin temperature distribution, so that skin temperature was always high in well-insulated areas. When the insulation was placed over the limbs, a greater amount of heat was lost than if a similar insulation was placed on the trunk. Neither nor skin temperature distribution correlated with general thermal sensation. Instead, mean body temperature seemed to be the determinant of general thermal sensation in these conditions. The best prediction of general thermal sensation was obtained by addingT _re with a weighting factor of 0.8–0.9 and with a weighting factor of 0.1–0.2.     

Ratings of perceived exertion and affect in hot and cool environments

Summary The effects of hot and cool environments on perceptual and physiological responses during steady-state exercise were examined in men (n = 14) performing 30 min of constant exercise (cycle ergometry) at a perceived exertion of somewhat hard. Subjects exercised at the same absolute exercise intensity in hot (40°C), neutral (24°C), and cool (8°C) conditions. Data were collected for differential ratings of perceived exertion (RPE), affect, thermal sensation, mean skin ( ) (and rectal temperatures (T _re), and cardiac frequency (f_c). The subjects completed the hot exposure with an average {ei174-2}_sk of 37.5°C (SEM 0.11), while the neutral and cool conditions produced values of 33.8 (SEM 0.09) and 28.2°C (SEM 0.30), respectively. The was significantly higher in the hot than the neutral and cool conditions throughout exercise (P < 0.05). The f_c was significantly lower in the cool than in the other conditions (P < 0.05), and the subjects completed the hot exposure with a mean f_c more than 20 beats · min^–1 greater than observed in the other conditions. The subjects felt worse (lower affect) in the heat throughout exercise (P < 0.05). Overall RPE was significantly lower in the cool than in the heat, while chest RPE scores for the cool and hot conditions were displaced vertically by approximately two points. Subjects perceived work to be harder, felt worse, and experienced greater thermal sensation in the hot condition, compared with the neutral and cool conditions. Changes in cutaneous vasomotor tone and heat-induced influences on the chest may have accounted for the RPE changes observed in the heat.     

Effect of skin temperature on vibrotactile sensitivity

The vibration problems relating to living bodies have so far been studied from the perspectives of engineering physiology and psychology. This study shows the relationship between vibratory sensibility and temperature in the living body. Psychological experiments were carried out by using the vibrometer of an acoustic calibration apparatus in sine, triangular and square waves. The sensibility-threshold measurements were made using 30–700 Hz sine waves, 30–300 Hz triangular and sawtooth waves, or 30–250 Hz square waves. Each of ten subjects was kept seated. The average value of the vibratory levels, varied by ascending and descending steps, was taken as that of the threshold. As the vibrometer in the apparatus used makes a noise at frequencies greater than 250 Hz it was masked from the subject by presenting him with a different noise. The threshold curve for square waves is lower by 12·3 dB than that for sine waves at about 30Hz. The threshold curve of the 26°C sine wave was lower by 10 dB than that of the 58°C sine wave vibration near 200 Hz. For example with a sine wave, at 58°C the amplitude threshold was lowest at about 270 Hz, but at −11°C at about 200 Hz. At frequency stimulation higher than 120 Hz, as the temperature of the contact point was lowered, the amplitude threshold increased and the frequency at which the threshold curve was at a minimum shifted to a lower frequency.     

Quantification of thermal comfort parameters using a behavioural indicator

This study is a pilot attempt to propose an alternative approach to the question of work in hot or cold ambient atmospheres. Thermal alliesthesial responses were used to calibrate the thermal characteristics of man undergoing exogenous and/or endogenous thermal loads. Ambient thermal conditions, activity levels, clothing (CLO-values), exposure durations, core or mean skin temperatures associated with thermal comfort were estimated for five subjects at six different room temperatures. One practical field application in the steel industry is discussed. The results indicate that the present method is most suited for the optimization of work/rest regimens during industrial excessive heat exposures when the improvement of thermal environment is not possible or economically not feasible.     

The interrealtionship between locally applied heat,ageing and skin blood flow on heat transfer into and from the skin

In response to a thermal stress, skin blood flow (BF) increases to protect the skin from damage. When a very warm, noxious, heat source (44°C) is applied to the skin, the BF increases disproportionately faster than the heat stress that was applied, creating a safety mechanism for protecting the skin. In the present investigation, the rate of rise of BF in response to applied heat at temperatures between 32°C and 40°C was examined as well as the thermal transfer to and from the skin with and without BF in younger and older subjects to see how the skin responds to a non-noxious heat source. Twenty male and female subjects (10 – 20–35 years, 10 – 40–70 years) were examined. The arms of the subjects were passively heated for 6?min with and without vascular occlusion by a thermode at temperatures of 32, 36, 38 or 40°C. When occlusion was not used during the 6?min exposure to heat, there was an exponential rise in skin temperature and BF in both groups of subjects over the 6-min period. However, the older subjects achieved similar skin temperatures but with the expenditure of fewer calories from the thermode than was seen for the younger subjects (p?<?0.05). BF was significantly less in the older group than the younger group at rest and after exposure to each of the three warmest thermode temperatures (p?<?0.05). As was seen for noxious temperatures, after a delay, the rate of rise of BF at the three warmest thermode temperatures was faster than the rise in skin temperature in the younger group but less in the older group of subjects. Thus, a consequence of ageing is reduced excess BF in response to thermal stress increasing susceptibility to thermal damage. This must be considered in modelling of BF.     

Effect of non-uniform skin temperature on thermoregulatory response during water immersion

The present study investigated the effect of non-uniform skin temperature distribution on thermoregulatory responses and subjective thermal sensation during water immersion. Ten healthy male subjects carried out 60 min water immersion twice, once with uniform (UST) and once with non-uniform (NUST) skin temperature. In UST condition, subjects immersed at 29 degrees C in naked condition, while in NUST condition, subjects immersed at 26 degrees C with partial coverage wetsuit (PCWS). The PCWS covers trunk region, upper arms, and thighs. The non-uniform skin temperature distribution, higher at trunk and lower at distal extremities, was observed in NUST condition. Shivering thermogenesis was not influenced by the skin temperature distribution at the experimental condition of this study. On the other hand, the tissue insulation (I (tissue)) was significantly higher in NUST condition compared to the UST condition. The increment of I (tissue) might have been caused by the peripheral vasoconstriction induced by the cold input from the distal extremities in NUST condition. The higher I (tissue) in NUST condition might lead to the significantly higher esophageal temperature compared to UST condition. No difference was observed in thermal sensation between the two conditions. Subjects felt slightly more comfortable in NUST condition than in UST condition. In conclusion, the non-uniform skin temperature distribution, higher at trunk and lower at distal extremities, might affect the peripheral vasoconstriction to increase the I (tissue). On the other hand, shivering thermogenesis and subjective thermal sensation were not affected by the non-uniform skin temperature distribution at the present experimental condition.     

Thermographic studies on patterns of skin temperature after exercise

Summary In six subjects thermograms of the thighs and the forearms were taken before, during and after 10 min ergometer exercise at 100 W at an ambient temperature of 23°C. During exercise, an intraindividually constant and reproducible skin temperature pattern with local temperature differences exceeding 3°C evolved. Reactions after external local cooling or after occlusion of blood flow and measurements with a laser Dopplerflow-meter showed dispersed convective heat transport to be the source of this irregular pattern. Temperature differences of 3.6°C and deviations of blood flow in the skin microcirculation of 300% within a distance of a few centimetres reduce the value of single-spot measurements of skin temperature with reference to the whole extremity.     

Reflex changes in discharge activities of gamma efferents to varying skin temperatures in cats

Summary Changes in spontaneous discharges of single -efferents dissected from the lumbrosacral ventral root of spinal cats were observed during cooling of warming of a small area of the foot pad skin of the ipsilateral hind limb. In a certain range of skin temperature, varying from 34°C to 38°C from preparation to preparation, temperature changes resulted in either an increase or decrease in discharge frequency, whereas above this temperature range, heating produced an increase in discharge frequency. By contrast, all -efferents examined responded with a negative temperature coefficient to a change in skin temperature below the respective temperature. The thermal activation of these efferents elicited by a small temperature change of the skin was affected considerably by the velocity rather than by the degree of temperature changes but the effect of both variables differed according to the skin temperature at which the temperature change occurred. Ratios of increment or decrement in discharge frequency (± imp./s) to degree (± °C) and velocity of temperature change (± °C/s) respectively were plotted as a function of skin temperature, and indicated a directional relationship between stimulus and response by attaching to each ratio either a positive or negative temperature coefficient. The maximum activity response in the range of 20–30°C was –19 imp./s per °C at 22°C and the average was –8.33±4.77 imp./s per °C. In the warmer range, the coefficients were positive and the average was 23.79±12.21 imp./s per °C in the range of 40–44°C. The steady state activity at a constant temperature showed no sign of dependency on skin temperature. On the basis of these results, the possible mechanisms for thermal inflow contributing to a thermally-induced reflex of the -motoneurons and the functional implications of these reflexes for the thermoregulatory system are discussed.     

Assessment of male anthropometric trends and the effects on simulated heat stress responses

Assessing temporal changes in anthropometrics and body composition of US Army soldiers is important because these changes may affect fitness, performance, and safety. This study investigated differences in body dimensions (height, weight, percent body fat (%BF)) of US Army male soldiers by comparing 2004 and 1988 databases. Anthropometric somatotypes were identified and physiological responses of the different somatotypes to simulated heat stress (35 degrees C/50%rh, approximately 550 W work rate, carrying 12 kg load including battle dress uniform and body armor, rest for 30 min and walk for 70 min) using a thermal regulatory model were evaluated. A significant increase in body weight (2.4 kg) was observed between the 2004 and 1988 data (P < 0.05, after Bonferroni correction). However, changes in height and circumference measurements for %BF were insignificant, with the magnitude of the changes not exceeding inter-observer errors. Multivariate analyses demonstrated that anthropometric distributions did not differ between the two databases and identified five primary somatotypes: "tall-fat", "tall-lean", "average", "short-lean", and "short-fat." Within each database, anthropometric values differed among the somatotypes. However, simulated physiological responses to heat stress in each somatotype were similar in the 2004 and 1988 populations. In conclusion, an increase in body weight was the primary change observed in this sample of US Army male soldiers. Temporal changes in somatotypes of soldiers over a 16-year period had minimal impact on simulated physiological response to heat stress using a thermal regulatory model.     

Brief skin temperature changes towards thermoneutrality trigger REM sleep in rats

When rats were in slow-wave sleep (SWS) at an environmental temperature (23°C) below their thermoneutral zone (27–31°C), brief skin warming by either radiant heating, or forced air convection resulted in REM sleep on 79–80% of the trials. During control nonwarmed SWS bouts, the animals went into REM sleep on only 22–24% of the trials. When the environmental temperature was above thermoneutrality, 34°C, lowering skin temperature by convective cooling resulted in REM sleep entry 68% of the time, compared to 21% for noncooled, control trials. Skin warming and cooling at 29°C decreased the percent occurrence of REM sleep to 22% and 9% respectively, for at this thermoneutral temperature 46% of the control SWS bouts ended in REM sleep. Thus, peripheral temperature changes towards thermoneutrality trigger REM sleep in mildly thermally stressed rats.     

Monitoring the pathophysiological correlates of postmenopausal hot flushes

Clinical assessment of the severity and frequency of post-menopausal hot flushes can be made objectively by measuring the associated changes in skin conductance and skin and core temperature. Such measurements provide a more reliable index of the response to therapy than does subjective reporting which has been employed in the past. The design and use of a working analyzer is presented that is sufficiently simple, rugged, safe and portable to be used under normal clinical conditions to provide a permanent record of the attacks.     

Effects of skin temperature on cold defense after cutaneous denervation of the trunk

In intact goats the core temperature threshold below which heat production increases with falling core temperature, is inversely related to the temperature of the water bath in which they stand and is therefore assumed to be indicative of the central integration of signals from skin and core temperature receptors. The present study shows that a difference in core temperature thresholds for bath temperatures of 35°C and 40°C persisted after denervation of about two-thirds of the skin of the trunk and limbs. Also, for a given combination of skin and core temperatures, heat production was as great or greater after cutaneous denervation as before. It is concluded that, following denervation of the trunk and upper limbs, intact temperature receptors in the non-denervated skin of the legs and tail, and/or also in tissues between the skin and core, provide important and significant inputs to the temperature regulating system. But these inputs cannot explain fully the thermoregulatory responses observed unless it is assumed that the thermosensitivity of these tissues increased.     

Thermal control of blood flow through capillaries and arteriovenous anastomoses in skin of sheep

Using radioactive microsphere and electromagnetic techniques, hindleg vascular responses were studied in 38 conscious, chronically prepared sheep subjected to either exposure to a warm environment, and/or local warming of the hypothalamus, spinal cord, forelegs or hindlegs. The total proportion of cardiac output passing through AVA's was increased by all treatments. AVA flow in hindleg skin was increased but capillary flow was unchanged by warming the hypothalamus, spinal cord or forelegs. AVA flow was unchanged but capillary flow was increased by warming the ambient air or the hindlegs alone. Equivalent cooling treatments resulted in AVA and capillary flow changes converse to warming.It is concluded that, in sheep, blood flow through cutaneous AVA's is controlled by specific thermoregulatory reflexes, whereas capillary flow is the target of local temperature effects. A significant role for the direction of the thermal gradient across the skin is implicated.     

Differential thermal sensitivity in the human skin

Summary Thermal irradiation was applied to selected skin areas to determine whether particular areas demonstrate a greater thermal sensitivity than others in determination of a physiological thermoregulatory response. Modifications in thigh sweating rate were related to the change in temperature of the irradiated skin and the area of skin irradiated by computing a sensitivity coefficient for each skin area. Thermal sensitivity of the face, as measured by its effect on sweating rate change from the thigh, was found to be approximately three times that of the chest, abdomen men and thigh. Lower legs were found to have about one-half the thermal sensitivity of the thigh. A table of weighting factors for calculation of physiological mean skin temperature, based upon thermal sensitivity and area, is presented.Supported in part by NIH Fellowship 1 F03 ES47944 BENG and NIH Grant ES-00354.     

Simulation of hand cooling due to touching cold materials

Summary A simple analytical model has been developed to simulate the cooling of the hands due to touching various types of cold material. The model consisted of a slab of tissue, covered on both sides with skin. The only active mechanism was the skin blood flow. The blood flow was controlled by body core temperature, mean skin temperature, and local hand temperature. The blood flowed along the palm before returning via the back of the hand. The control function was adapted from an earlier study, dealing with feet, but enhanced with a cold induced vasodilatation term. The palm of the hand was touching materials that were specified by conductivity and heat capacity. The hand was initially at a steady-state in a neutral environment and then suddenly grabbed the material. The resulting cooling curves have been compared to data from an experiment including six materials (foam, wood, nylon, steel, aluminium and metal at a constant temperature), three temperatures (–10, 0, and 10° C), two thermal states of the body (neutral and 0.4°C raised), and with and without gloves. There was a fair general agreement between the model and the experiment but the model failed to predict three specific effects: the unequal effect of equal 10° C steps in cold surface temperature on the temperature of the palm of the hand, the cooling effect of nylon, and the rapid drop in back of the hand temperature. Nevertheless the overall regression was 0.88 with a standard deviation between model and experiment of about 2.5°C.     

Skin and core temperatures as determinants of heat production and heat loss in the goat

In 82 experiments on 10 goats body core temperature (T _core) was altered between 35° and 42°C by external heat exchangers acting on blood temperature while skin temperature (T_skin) was maintained constant, by a circulating shower bath, at different levels between 32° and 44°C. At all skin temperatures at least fourfold increases of heat production (M) and respiratory evaporative heat loss (REHL) occurred whenT _core was lowered or raised, respectively. The lower T_skin was, the higher were the thresholds ofT _core, at which M or REHL exceeded resting levels. The lower T_skin was, the higher were the slopes, at which M or REHL changed per unit ofT _core. At a given T_skin, the slopes decreased with increasing M or REHL, and were dependent on the range ofT _core. The higher the range ofT _core, the steeper changed M and REHL with changingT _core, if all other variables were held constant. The results support the concept that an exponential relationship betweenT _core and the rate of core temperature signals is the primary cause of the effects exerted by T_skin on the slopes, at which M or REHL change per unit ofT _core.