INTRODUCTION

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OVER A WIDE RANGE OF ENVIRONMENTS, body core temperature (T_c) equilibrates at levels proportional to metabolic rate and independent of ambient conditions (21, 26). Thermal environments above this designated "prescriptive zone" (18, 19) force T_c out of equilibrium, resulting in a continuous rise in T_c. Critical conditions that define the upper limit of the prescriptive zone for a given metabolic heat production have been determined for heat-acclimated men (3) and women (13, 14) but not for unacclimated subjects, in part because of perceived methodological concerns.

Belding and Kamon (3) originally used a time-intensive protocol to determine critical ambient vapor pressures at 36°C for a variety of exercise intensities and air movements. A shorter version of their original investigation was proposed (13) and then refined (16) to minimize the number and duration of tests necessary to determine these limits. However, these more time-efficient protocols require the ability to systematically change ambient temperature or water vapor pressure, i.e., with the use of a fairly sophisticated environmental chamber.

In addition to setting environmental limits, physiological responses can be used to determine the effective evaporative coefficient (K_e') at each critical limit based on the assumption that, at those critical limits with high humidity, heat gain equals heat loss and the skin is fully wet. That is, M_net ± (R + C) = E_max, where M_net is the net metabolic heat production, R + C is the combined dry heat gain or loss through radiation and convection, and E_max is the maximal ambient capacity for evaporative cooling. Because E_max = K_e' · v^0.6 · (P_s,sk  P_a), where v is air velocity and (P_s,sk  P_a) is the vapor pressure gradient from fully wetted skin to air, K_e' can be derived and used to model heat exchange with the environment and predict limits for prolonged work-heat exposures. Although some assumptions are required for this sequence of calculations, the resultant values are useful for continuing efforts to model human heat exchange with the environment.

The present study used two innovations of the Kamon and Avellini (13) protocol. In hot dry environments, critical limits are determined by individual sweating capacity as opposed to environmental limits (13), and these limits approach a vertical line on the psychrometric chart, i.e., an isotherm of free evaporation (17). To better predict the dry-bulb temperature (T_db) locus of this line, we held ambient water vapor pressure (P_a) constant and determined a critical T_db at three separate P_a values. Second, in experiments in which T_c did not achieve a true steady state (which occurs more commonly in unacclimated than in fully heat-acclimated subjects), we calculated the heat storage (S) associated with this "pseudo-steady state" and incorporated S into the heat balance calculations, which led to the derivation of K_e', i.e., M_net ± (R + C) ± S = E_max.

The purpose of the present investigation was to determine for the first time the critical environmental limits for unacclimated men and women. In addition, partial calorimetry was used to determine sex-specific values for K_e'. It was hypothesized that 1) critical environments for unacclimated men and women would be shifted downward [toward lower critical water vapor pressure (P_crit)] and leftward [toward lower critical temperature (T_crit)] on a psychrometric chart compared with heat-acclimated subjects because of lower sweating rates, altered sweat distribution, and lower evaporative efficiency; 2) these empirically determined psychrometric limits would fit standard biophysical models of heat exchange; and 3) the derived critical K_e' would be lower for unacclimated subjects than previously published values (3, 13) for fully heat-acclimated subjects.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Subjects. All experimental procedures were approved in advance by the Institutional Review Board at The Pennsylvania State University. After all aspects of the experiment were explained, oral and written informed consent was obtained. Each subject was subsequently approved for the experiment after each completed a medical screening examination. Twenty-one subjects were tested (11 men and 10 women). All subjects were healthy, normotensive, nonsmokers, and not taking any medications that might affect the physiological variables of interest in this study. None of the women was taking oral contraceptives, and no attempt was made to control for menstrual status. Subjects with a maximal aerobic capacity (O_2 max) in the upper or lower 20th percentile for their gender and age (1) were excluded. None of the subjects was physically active outdoors on a regular basis. O_2 max was determined with the use of open-circuit spirometry during a maximal graded exercise test performed on a motor-driven treadmill. Subject characteristics are shown in Table 1. During the experiments, subjects wore thin, short-sleeved cotton tee-shirts, shorts, socks, and walking/running shoes. For consistency with previous studies that have used this minimal clothing ensemble, no clothing corrections were made for this "semi-nude" state (13).

Chamber characteristics. The environmental chamber used for this study utilized a three-mode controller (proportional, derivative, and integral) to optimize stability and response. Air is vertically discharged in an even pattern through the ceiling and returned through base molding on three sides. Because air returns behind the walls, wall temperature increases linearly with air temperature. The ceiling was mapped for optimal laminar flow. With no forced air movement in the chamber, velocity in the vicinity of the walking subjects was measured at 0.45 m/s. The same chamber was used in the studies by Kamon and colleagues (9, 13, 14), from which comparative data are presented later in this paper.

Testing procedures. Subjects were asked to refrain from vigorous exercise and alcohol consumption during the 24 h before each experiment and from caffeine on the day of the experiment. On arrival, each subject was weighed and donned a Polar heart rate (HR) monitor. The esophageal probe was inserted as the subject drank 5 ml of room temperature water per kilogram body weight. The subject then moved into the preconditioned environmental chamber for equilibration, and skin thermocouples were attached.

Measurements. T_es was measured with a probe made from a thermistor sealed in a pediatric feeding tube. The probe was inserted nasally and lowered in the esophagus to the level of the left atrium, ~0.25 of the subject's standing height. T_sk was measured at four sites: leg (T_leg), thigh (T_th), arm (T_arm), and chest (T_ch) with copper-constantan thermocouples. A weighted mean T_sk (in °C) was calculated (25) as

(1)

T_es and T_sk were continuously recorded, and HR was recorded at 5-min intervals on a dedicated computer.

1

Determination of T_crit and P_crit. An example of the time course of T_es, T_sk, and environmental conditions for a typical P_crit test is shown in Fig. 1. Typically, T_es began to plateau by about minute 40 and remained at an elevated steady state as P_a was increased 1 Torr per 5 min. The critical T_db or P_a was defined by the subsequent upward inflection of T_es. The inflection point was selected graphically from the raw data (see Fig. 1). A line was drawn between the data points, starting at the 30th min. When the mean slope of the T_es line began to deviate upward from the equilibrium phase slope, a second line was drawn from the point of departure of T_es from the first line. The T_db or P_a 1 min before the point at which the second line departed from the first was defined as the T_crit or P_crit, respectively. To confirm graphical representation of the upward inflection of T_es, the slopes were redrawn with different time axes, including logarithmic scales. The upward rise in T_es was typically preceded by an inflection point in the HR time course (13-15) and thus could be anticipated during the test.

View larger version (17K): [in this window] [in a new window]   Fig. 1.   Data from a representative critical water vapor pressure (Pcrit) test. Dry-bulb temperature (Tdb) is held constant in this case at 34°C while ambient water vapor pressure (Pa) is systematically increased after a 30-min equilibration period. Esophageal temperature (Tes) of the subject rises to a steady state, dependent on exercise intensity and independent of ambient conditions. After a critical environment is reached, Tes again begins to rise. The Pa at which this inflection is seen is designated the Pcrit, which was 27 Torr in this example. The final rise in Tes is preceded by a steady increase in mean skin temperature (Tsk).

Calculation of critical K_e'. All calculations below were performed uniformly across subjects. Metabolic rate (M; W/m²) was derived from the O₂ (l/min) and respiratory exchange ratio (RER; unitless) as

(2)

where A_D is Dubois surface area (m²). External work (W; W/m²) was calculated as

(3)

where m_b is body mass (kg), v_w is walking velocity (m/min), and fg is fractional grade of the treadmill. M_net was then calculated as M  W.

(4)

2

1

(5)

(6)

1

1

(7)

(8)

1

(9)

Statistical analysis. Variables that changed throughout conditions were analyzed with an analysis of covariance with repeated measures. A SAS program was written by using the PROC MIXED statement, with independent variables of sex and the fixed critical condition; the dependent variable was the unknown critical environmental parameter. A one-way analysis of variance was used to compare subject characteristics and variables calculated once per trial, e.g., M_net, (R + C), sweating rate, K_e', and so forth. Where significant F values were found, Tukey's follow-up tests were performed. Differences were considered significant at the 0.05 level.

	RESULTS

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Subjects for this study were recruited to be representative of the population with respect to body size, adiposity, and aerobic fitness. Thus the women were shorter, weighed less, and had a higher percent body fat and a lower body surface area than the men (all at P < 0.05; Table 1). Body fatness ranged from 11 to 26% for the men and from 16 to 29% for the women. With respect to body mass index, the men ranged from the 18th to the 71st percentile for their age group, whereas the women ranged from the 24th to the 87th percentile (1). The women also had a 13-14% lower O_2 max, which translated to a significantly lower M, M_net (139 ± 3 vs. 191 ± 6 W/m², P < 0.001) and E_req during exercise at 30% O_2 max (Table 2) in each critical environment. The measured exercise intensity ranged from 29.9 ± 1.1 to 32.3 ± 1.0% O_2 max for the men and from 28.3 ± 0.9 to 30.0 ± 1.0% O_2 max for the women across trials (P > 0.05). Mean sweating rate was consistently and significantly (P < 0.05) lower in the women, as was the E_sk of the sweat produced (Table 2).

Table 3 presents the T_db and P_a psychrometric locus values for each critical environment as well as the mean P_s,sk  P_a. In the tests conducted at T_db = 34, 36, and 38°C (coinciding with the more humid environments), each respective P_crit was significantly higher and each P_s,sk  P_a was significantly lower for the women (P < 0.05). No significant sex differences were seen in the critical P_s,sk  P_a for the tests at fixed low ambient vapor pressures. These critical environmental loci are plotted on a standard psychrometric chart in Fig. 2.

View larger version (25K): [in this window] [in a new window]   Fig. 2.   A standard psychrometric chart showing the mean Pcrit and values determined empirically for 11 unacclimated men () and 10 unacclimated women (). Pcrit values were significantly higher for the women in the humid conditions because of their lower absolute heat production at 30% maximal aerobic capacity (O2 max), whereas no differences were seen in drier environments (see text for full explanation). The dotted arrowheads point to separate biophysical anchor points with coordinates [Tdb = Tsk  (Esk/Mnet), Pa = Ps,sk] and have a calculated slope () = hr+c/Ke' · v0.6, where Esk is skin evaporative capacity, Mnet is net metabolic heat production, Ps,sk is vapor pressure gradient, hr+c is the combined radiative and convective heat transfer coefficient, Ke' is the critical evaporative coefficient, and v is air velocity. Data fit the calculated relation for both the men and the women. On the other hand, the dotted isotherms curving toward a limit for free evaporation at Tdb values above 40°C were significantly lower than the experimental data. These curves transition from full wettedness (w  1) to free evaporation (w = 0) and are a function of Tsk and Esk. rh, Relative humidity.

Also shown in Fig. 2 are calculated isothermal lines from previously published biophysical models of heat exchange as described in METHODS. The arrowheads in Fig. 2 extrapolate to two separately calculated biophysical anchor points [T_db = T_sk  (E_sk/M_net), P_a = P_s,sk], and the lines have slopes of  = h_r+c/K_e' · v^0.6. The curved portion of the isotherms at higher T_db and lower P_a values reflect progressive changes in skin wettedness approaching the T_db for free evaporation, a vertical line at T_db + (E_sk  M_net)/h_r+c. These lines of free evaporation were calculated from the highest mean sweating rate for each group, as was done by Kamon and Avellini (13), and are presented for illustrative purposes. Each subject would have his or her own line based on E_sk. It is obvious that the isotherms fit the data well at high humidities but underpredict the empirical data in the hotter, drier environments.

Derived K_e' values are listed for men and women in Table 2. There were no sex differences for K_e' (men = 17.4, 15.5, and 14.2 W · m² · Torr¹ and women = 16.8, 15.5, and 14.2 W · m² · Torr¹ at 34, 36, and 38°C, respectively). These K_e' values were lower than those previously published for fully heat-acclimated men (18.4 W · m² · Torr¹ at 36°C) (3) and women (17.7 W · m² · Torr¹ at 36°C and 15.5 W · m² · Torr¹ at 38°C) (13). No K_e' values are presented for the T_crit tests, since the requirement that the skin be fully wet was not met (see DISCUSSION). Further proof is given by the estimated skin wettedness values (w = E_sk/E_max) provided in Table 2. In the humid environments, the estimated skin wettedness value exceeds 1.0 by 30-50%, suggesting dripped sweat, whereas this value is <1.0 for the three less humid environments.

Figure 3 shows the critical environments for the unacclimated young women in the present study along with data from a group of heat-acclimated women tested by Kamon and Avellini (13). We believe that this comparison is justified because 1) these data were collected in the same environmental chamber used in our laboratory and 2) the heat-acclimated women were similar in terms of anthropometry, exercised at a similar metabolic rate (150 ± 5 W/m), and were semi-nude, as in our study. Although the psychrometric limits are similar at higher P_a values, the prescriptive zone for the unacclimated women is contracted as T_db increases and P_a decreases, i.e., as environments become less humid.

View larger version (23K): [in this window] [in a new window]   Fig. 3.   Data from the young women tested in the present study ( and solid line) are contrasted with data from a group of fully acclimated women previously tested in our laboratory ( and dashed line). Although the psychrometric limits are similar at higher Pa values, the prescriptive zone for the unacclimated women is contracted as Tdb increases and Pa decreases, i.e., as environments become less humid. This illustrates that the advantages conveyed by improved sweat evaporation after rigorous heat acclimation are most prominent in environments that allow free evaporation of sweat.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The objective of the present study was to determine, for unacclimated subjects, the range of hot ambient environments beyond which T_c would not equilibrate at a level set by exercise intensity. The protocol used was a modification of that developed by Belding and Kamon (3) and refined by Kamon and Avellini (13) and then again refined by Kenney (15, 16). The underlying basis was to empirically determine a family of ambient conditions under which M_net ± (R + C) = E_max. At full skin wettedness, E_max = K_e' · v^0.6 · (P_s,sk  P_a) and K_e' can be derived by solving the heat balance equations involved. Before this investigation, this approach had only been used for fully heat-acclimatized men and women (3, 13, 14).

Potential limitations. The approach utilized in the present paper is built on heat balance calculations that, although theoretically sound, are based on numerous previous investigations, each of which made unique assumptions. Coefficients and calculations chosen for this investigation in some cases followed the previous studies of Kamon and colleagues (3, 13) so that comparisons could be made more directly between the studies. Some coefficients were directly determined, i.e., by naphthalene sublimation (22), whereas others were based on indirect calculations from empirical data. With the exception of Kamon and colleagues (13, 14), subjects in those previous studies were predominantly men.

Psychrometric limits. When T_db is close to T_sk, dry heat transfer is minimal and heat exposure at a given metabolic rate is limited by the vapor pressure gradient, P_s,sk  P_a. In such environments, one would predict that higher sweating rates typically exhibited by men (2, 4, 6-8, 10, 12, 20, 24) would not be beneficial and that critical limits would be similar between unmatched men and women. Under hotter, drier ambient environments where (R + C) is positive and E_max is high, the limit to prolonged exposure is defined by sweating capacity; men, who typically have higher maximal sweating rates, should have an advantage in these environments. However, such was not the case in this experiment.

Evaporative coefficients. As shown in Table 2, K_e' values at T_db = 34-38°C were not significantly different between men and women. Calculation of K_e' requires conditions of fully wet skin, and thus only the K_e' values calculated in the more humid environments are meaningful. In the dry environments, the use of P_s,sk when the skin was not fully saturated would result in an overestimate of E_max and an artificially low K_e'. As in previous studies, K_e' decreases slightly with increasing T_db and decreasing P_a within this temperature range. The values of K_e' for unacclimated men (17.4, 15.5, and 14.2 W · m² · Torr¹) and women (16.8, 15.5, and 14.2 W · m² · Torr¹ ) at 34, 36, or 38°C, respectively, were lower than those previously published for fully heat-acclimated men (18.4 W · m² · Torr¹ at 36°C) (3) and women (17.7 W · m² · Torr¹ at 36°C and 15.5 W · m² · Torr¹ at 38°C) (13). These values are more appropriate for future models of heat transfer between humans and the environment when heat acclimation is absent.