- Email: [email protected]
Evaluation of the environmental stress index for physiological variables
Journal of Thermal Biology 28 (2003) 43–49
Evaluation of the environmental stress index for physiological variables D.S. Morana,*, K.B. Pandolfb, Y. Shapiroa, A. Laora, Y. Heleda, R.R. Gonzalezb a
Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer 52621, Israel US Army Research Institute of Environmental Medicine, Natick, MA 01760-5007, USA
b
Received 6 March 2002; accepted 18 June 2002
Abstract Recently, a new environmental stress index (ESI), based on ambient temperature (Ta ), relative humidity and solar radiation, was suggested. ESI was found to be highly correlated to the WBGT index. However, ESI was not correlated with any physiological variable that reflects strain. The purpose of this study was to evaluate ESI for three different physiological variables (rectal temperature (Tre ), heart rate (fc ) and sweat rate (msw )), and with the physiological strain index (PSI). Twelve young men were exposed outdoors for 12 different experimental combinations consisting of three metabolic rates (rest, moderate and hard exercise intensity), two clothing ensembles (cotton clothing and protective overgarments), and two solar radiation levels (shade and sun). Each exposure lasted 120 min with Tre and fc being continuously monitored. High correlations (RX0:838) were found when statistical analysis was done between ESI and Tre ; fc ; msw ; or PSI, which have the potential to be widely accepted and used universally. However, further studies between physiological variables and ESI obtained from other climatic conditions, different exercise intensities, and additional clothing ensembles need to be evaluated. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Heat-stress; Indices; Rectal temperature; Heart rate; Sweat rate
1. Introduction Heat stress evaluation is generally determined through meteorological parameters that enable the estimation of the influence of several environmental factors on thermal comfort and physiological ability. The variables included in heat stress indices and their relative weights have changed over the years. The existing indices can be divided into two main categories (Gonzalez, 1986): effective temperature (ET) scales, which are based on meteorological parameters only (e.g., ambient temperature, wet-bulb temperature, black-globe temperature), and rational heat scales, which include a combination of environmental and physiological parameters
*Corresponding author. Tel.: +972-3-5303564; fax: +972-37377002. E-mail address: [email protected] (D.S. Moran).
(e.g., radiative and convective heat transfer, evaporative capacity of the environment, and metabolic heat production). In 1923, Houghten and Yaglou developed the ET index from which at least five additional indices were derived. Among them, in 1957, Yaglou and Minard introduced the wet bulb globe temperature (WBGT) index, which gained popularity mainly due to its simplicity and convenience of use. This index is obtained primarily from three parameters: black globe temperature (Tg ) which mainly reflects the solar radiation, wet bulb temperature (Tw ), and dry bulb temperature (Ta ). This index is calculated as follows: WBGT ¼ 0:7Tw þ 0:2Tg þ 0:1Ta : The WBGT is in use in the field by the US Army and is the index from which training safety orders are based (Burr, 1991). It has been adapted by the World Health Organization (WHO) and the American College of
0306-4565/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 2 ) 0 0 0 3 5 - 9
44
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
Sports Medicine (ACSM) (1996). In 1972, the National Institute for Occupational Safety and Health (NIOSH) established it as the criterion for occupational exposure to a hot environment. In 1982, it was approved by the ISO organization as an international standard for heat load assessment. Later on, work–rest regime regulations were made in the military based on this index. However, WBGT was found to be limited in evaluating heat stress due to the inconvenience of measuring Tg (Moran and Pandolf, 1999). It is important to note the correlation of this index to physiological responses was only partially tested and was based mainly on the correlation between the number of heatstroke cases during army training and the heat load as calculated by the WBGT (Yaglou and Minard, 1957). In 2001, Moran et al. (2001b) introduced a new environmental stress index (ESI) based on measurements of Ta ; relative humidity (RH), and solar radiaton (SR) as follows: ESI ¼ 0:63Ta 0:03RH þ 0:002SR þ 0:0054ðTa RHÞ 0:073ð0:1 þ SRÞ1 : This newly developed index was validated by using large databases and was found to be highly correlated to the WBGT index (Moran et al., 2001b). However, the ESI was not evaluated for any physiological variables which reflect strain. ESI differs from other indices that have been suggested in the past for two main reasons. First, ESI is based on SR and RH, aside from using Ta : In fact, the WBGT uses Tg for evaluating SR, and Tw is used for estimation of RH. However, the ESI as a stress index, for the first time uses direct measurements of SR and RH. Second, the three meteorological variables used in ESI are characterized by fast reading responses and take only a few seconds to reach equilibrium. Recently, the ESI was validated with measurements from an infrared (IR) light sensor for global radiation. It was concluded in that study that an IR light sensor can be used as a potential substitute for Tg which is incorporated in the WBGT (Moran et al., 2001a). In 1998, Moran et al. (1998b) introduced a new physiological strain index (PSI) based upon the summation in equal weights of individual strains for core temperature (Tc ) and heart rate (fc ) as follows: PSI ¼ 5ðTct Tc0 Þð39:5 Tc0 Þ1 þ 5ðfct fc0 Þð180 fc0 Þ1 ; Where Tct and fct are simultaneous measurements taken at any time during the exposure and Tc0 and fc0 are the initial measurements. This index represents the combined strains of the thermoregulatory and cardiovascular systems. Each strain system was scaled between 0 and 5. The PSI is thus scaled 0–10, and can be used on-line or during data
analysis. The PSI can be applied at any time, including rest or recovery periods, whenever Tc and fc are measured (Moran et al., 1998b). In a recent series of studies, PSI successfully evaluated the strains in different clothing ensembles and climatic conditions during heat stress, during different levels of hydration and exercise intensity, for gender during heat stress, and for different age groups during 10 days of exercise-heat acclimatization (Moran et al., 1998a, b, 1999). The purpose of this study was to evaluate the correlation of ESI for three different physiological variables (sweat rate (msw ), rectal temperature (Tre ), and fc ) and for PSI during different experimental combinations of metabolic rate, clothing and solar radiation.
2. Materials and methods The material and methods for the original data used in this paper are presented in greater detail in previous articles (Moran et al., 1995; Shapiro et al., 1995). Subjects: Twelve healthy, young (1972 yr), fit men (bw ¼ 68:877:6 kg; VO2max =3.5970.36 l min1) participated in this study. Prior to the experimental exposures, each subject underwent a complete medical examination that included ECG at rest, urine analysis and SMA-12 screening biochemistry laboratory test. A complete medical history was also obtained. Subjects were informed as to the nature of the study and the potential risks of exposure to exercise in a hot climate and all signed a consent form. Protocol: Each subject was tested under 12 experimental combinations (Table 1). The combinations were assigned at random to the subjects. While at rest, the subjects were seated on a bench. The work consisted of walking on a treadmill at a speed of 1.4 m s1 (5 km h1) with no grade at the moderate workload, and with a 5% grade for the heavy workload. Each exposure lasted 120 min consisting of two cycles of 10 min rest and 50 min exercise. A test was terminated before 120 min whenever Tre > 39:01C or fc > 180 bpm: This study was conducted outdoors under a shade (6 6 6 m3) and in an open space where solar radiation was about ten times higher than that in the shaded area. All experiments were conducted in Israel during July and August between 12:00 and 14:00 h. Physiological parameters: During the tests, Tre was recorded every 5 min by using a thermistor probe inserted 10 cm beyond the anal sphincter (YSI 401, Yellow Springs Instrument, USA). Heart rates (fc ) were continuously radio-telemetered to an oscilloscope tachometer (Lifescope 6, Nihon Kohden, Japan) and were recorded every 5 min. To determine metabolic cost, oxygen consumption (VO2 ) was measured after 40 min of each bout of exercise. Expiratory gases were collected
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49 Table 1 The experimental combinations Metabolic rate
Rest (E100 W) Moderate work (E300 W) Heavy work (E450 W)
Solar radiation
Sun (maximal solar radiation E900 W m2) Shade (minimal solar radiation E80 W m2)
Clothing
Cotton clothes (clo=0.99, im/clo=0.75) Protective overgarment (clo=1.64, im/clo=0.43)
and analysed by the automatic metabolic cart (MMCHorizon, SensorMedics). Sweat rate was calculated from weight differences before and after exposure adjusted for water intake and urine output. Environmental variables: Dry bulb (Ta ) and wet bulb temperature (Tw ) were measured at the study location in the shaded area, using a psychometer (Lambrecht). The accuracy of these parameters was cross-tested by measuring ambient temperature and relative humidity with an electronic sensor (Rotronic). All these parameters were measured at approximately 2 m from ground level and recorded every 15 min. Solar radiation (short-wave radiation) was measured every 5 min using the Eppley precision pyranometer model PSP (EPLAB). The black globe temperature (Tg ) was measured using the Vernon black globe thermometer. Tg and SR were measured both in shaded and open areas. Calculations: Heat stress and strain indices were calculated according to the following publications: WBGT was calculated according to Yaglou and Minard (1957); ESI and PSI were calculated as suggested by Moran et al. (1998b, 2001b). Statistical analysis: Data analysis included fitting linear models, which were used to define the correlation between ESI and different physiological variables (Tre ; fc and msw ) and PSI at the end of each experimental exposure after 120 min. These data were analyzed for different combinations of metabolic rate and clothing, whereas for solar radiation the two exposures (sun and shade) were pooled together. All statistical contrasts were accepted at the Po0:05 level of significance. For all correlation and regression analyses, we used SAS 8.0 software procedures CORR and GLM.
3. Results ESI was applied to environmental data collected during the summer in the Israeli coastal regions for 36
45
days between 12:00 and 14:00 h. At this time period, each of the 12 subjects performed different exposures each day as depicted in Table 1. In Figs. 1–6, we compared between the ESI values obtained in the sun and in the shade at different combinations of metabolic rate and clothing. In addition, we showed in these figures the corresponding physiological parameters (Tre ; fc and msw ) and PSI values for ESI. The comparisons of Tre and fc were made from their changes during the sun and the shade exposures, whereas for msw ; we compared between the actual measured values. In general, ESI values in all the tested exposures were significantly higher (Po0:05) in the sun when compared to the shade exposures. However, in all the 12 exposures, values were within a narrow range for the sun (30.1370.101C) and for the shade (28.0670.101C). In contrast, the measured physiological variables (Tre ; fc and msw ) were significantly (Po0:05) affected by exercise intensity and overgarment clothing, which caused metabolic heat production elevation. These physiological variables were also significantly higher (Po0:05) in the sun when compared to the shade exposures. ESI values obtained from the measured climatic parameters were stable during all of the different
Fig. 1. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during resting exposures in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel). (*Po0:05 for the entire exposure (120 min)).
46
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
Fig. 2. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during resting exposures in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
Fig. 3. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during moderate exercise intensity in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel).
experimental exposures. Therefore, the ESI correlation assessment was done for each exposure after 120 min. High correlations (RX0:838) between ESI120 and the different physiological variables (Tre120 ; fc120; and msw ) and PSI120 were found when analyzed for the different experimental exposures. However, data were analyzed for pooled low and high solar radiation exposures reflecting the radiation intensity, and these affects on the physiological variables (Table 2). During the rest exposures, there were significant differences (Po0:05) in fc changes and msw between the values measured under the sun and in the shade while wearing cotton clothes or protective overgarments (Figs. 1 and 2). However, for DTre significant differences (Po0:05) were found only while wearing protective overgarments (Fig. 2, top panel). Corresponding analysis of ESI and PSI for these rest exposures showed significant differences (Po0:05) between values measured and calculated indices in the entire sun and shade exposures (Figs. 1 and 2, bottom panel). During the moderate exercise intensity, measured changes in msw ; Tre and fc were significantly higher (Po0:05) in the sun exposures when compared to values obtained in the shade. However, significantly higher
(Po0:05) values were found when wearing protective overgarments, with msw of 1.9370.10 l h1, DTre of 1.9370.111C, and Dfc of 9874 bpm, when compared to wearing cotton clothing: msw ¼ 1:5270:08 l h1 ; DTre ¼ 1:0270:111C and Dfc ¼ 7075 bpm (Figs. 3 and 4, top panel). PSI values at the end of the cotton clothing exposures were significantly different (Po0:05) for the shade and sun exposures with values of 3.370.2 and 4.470.4, respectively. Significantly higher (Po0:05) strain was found for these subjects while wearing protective overgarments with PSI values of 5.170.3 and 7.170.7 for the shade and sun exposures, respectively. In accordance, ESI values were significantly (Po0:05) higher in the sun (30.3170.08) exposures when compared to the shade exposures (28.2370.08) (Figs. 3 and 4, bottom panel). Figs. 5 and 6 depict comparisons of the changes between sun and shade exposures for PSI and the physiological variables during hard exercise intensity. In cotton clothing, significant differences (Po0:05) were found in msw and Dfc values between the sun and shade (Fig. 5, top panel). Concomitantly, significant differences were found between sun and shade exposures for ESI and PSI (Fig. 5, bottom panel). Wearing protective
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
47
Fig. 4. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during moderate exercise intensity in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
Fig. 5. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during hard exercise intensity in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel).
overgarments created very high physiological strain especially under the sun, with PSI values at the end of the exposures of 7.570.1 and 9.170.4 for the shade and the sun climate conditions, respectively (Fig. 6, bottom panel). Therefore, eight subjects were not able to tolerate these exposures and they terminated the exercise after less than 1 h. Thus, the data presented in Fig. 6 was obtained from only four subjects who completed the scheduled exposures.
In this study, we evaluated the new suggested ESI with the PSI and for three independent physiological variables (Tre ; fc ; and msw ) under different combinations of metabolic rates, clothing and solar radiation. The high correlations (RX0:838) found after 120 min between these two indices and between ESI and these physiological variables can serve as a validation for ESI that previously was validated only for other stress indices (WBGT and the discomfort index (DI)). In addition, analysis of ESI values for the sun and the shade exposures significantly (Po0:05) corresponded with the values of the tested physiological variables for the respective sun and shade exposures. Heat strain arises from different combinations of work rate, clothing and environmental conditions. Performing exercise while wearing protective garments, which decreases the ability of the body to dissipate heat, under exposure to solar radiation outdoors in hot climates results in a very high physiological strain. Thus, the level of the strain is linear and correlates to exercise intensity. In this study, the strain assessments were in a wide range, however the environmental stress was in a narrow range due to the outdoor design of the study.
4. Discussion During training involving physical activity especially in the military, insufficient work–rest cycles and fluid replacement can lead to heat casualties. Among these is heat stroke, which can be a life threatening situation. The recommended lengths of work–rest cycles and fluid replacement volumes are determined according to exercise intensity and the heat load. The latter is calculated from meteorological parameters consisting of Ta ; Tw or RH and radiation components, either Tg or SR, that reflect the environmental stress.
48
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
The strength of a reliable heat stress index is its ability to perform under various conditions and by its being correlated to a commonly used gold standard index (e.g., WBGT). Although ESI was previously found to be highly correlated to WBGT in a few databases (Moran et al., 2001b), we believe that the strength of ESI stems from its evaluation with PSI and other individual physiological variables (Tre ; fc ; and msw ).
The WBGT and the ESI were derived empirically from observation of meteorological measurements. Both indices are compiled through the use of Ta : However, unlike WBGT, which includes Tg and Tw that are affected by radiation and vapor pressure, respectively, ESI includes solar radiation and relative humidity measurements. Thus, ESI’s meteorological components are commonly used, and are more meaningful and obvious for the layman. Moreover, these three parameters are measured by fast response reading sensors, whereas Tg and Tw are cumbersome to measure and require 20–30 min to reach equilibrium. In conclusion, PSI is a new index already accepted around the world by many military installations and exercise physiologists (Amos, 2000; Baker and Grice, 2000; Baker et al., 2000; Cotter et al., 2000; Moran et al., 1999). We believe that assessment of physiological strain by PSI and environmental stress by ESI complement each other and have the potential to be widely accepted and used universally. However, further studies between physiological variables and ESI obtained from different climate conditions should be evaluated.
Acknowledgements
Fig. 6. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during hard exercise intensity in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
The authors wish to acknowledge Mrs. Linda Gans and Ms. Liron Shalit for their technical assistance in preparing this manuscript. Disclaimer The views, opinions and/or findings in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision unless so designated by other official designation. Human subjects participated in these studies after giving their free and informed voluntary consent. Approved for public release; distribution unlimited.
Table 2 The correlation coefficients between ESI120 and Tre120 ; fc120 ; PSI120, and msw obtained from 12 subjects exposed to 12 different combinations of metabolic rate, clothing, and solar radiation Experimental exposure
Tre120
fc120
PSI120
msw
Rest
Cotton Protective overgarment
0.849 0.846
0.844 0.838
0.846 0.893
0.932 0.851
Moderate work
Cotton Protective overgarment
0.864 0.910
0.875 0.859
0.845 0.842
0.955 0.873
Heavy work
Cotton Protective overgarment
0.839 0.899
0.838 0.869
0.875 0.913
0.913 0.947
Analysis data from minimal and maximal solar radiation was pooled together.
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
References American College of Sports Medicine, 1996. Position stand on heat and cold illnesses during distance running. Med. Sci. Sport Exerc. 28(12), i–x. Amos, D., 2000. The measurement of thermal strain in soldiers operating in Northern Australia. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments. Australian Institute of Sport, Canberra, pp. 144–151. Baker, S., Grice, J., 2000. To what extent does thermal sensation reflect physiological heat strain. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments. Australian Institute of Sport, Canberra, pp. 36–38. Baker, S.J., Grice, J., Roby, L., Matthews, C., 2000. Cardiorespiratory and thermoregulatory response of working in fire-fighter protective clothing in a temperate environment. Ergonomics 43, 1350–1358. Burr, R.E., 1991. Heat illness: a handbook for medical officers. USARIEM, Natick, MA. (Rep.no.: T 91-3). Cotter, J.D., Roberts, W.S., Amos, D., Lau, W.M., Prigg, S.K., 2000. Heat strain during combat fitness assessment of soldiers in Northern Australia. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments, Australian Institute of Sport, Canberra, pp. 176–178. Gonzalez, R.R., 1986. Biophysics of heat transfer and clothing considerations. In: Pandolf, K.B., Sawka, M.N., Gonzalez, R.R. (Eds.), Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Cooper Publishing Group, Carmel, IN, pp. 45–95. Houghten, F.C., Yaglou, C.P., 1923. Determining lines of equal comfort. Am. Soc. Heat Ventil. Eng. Trans. 29, 163–176. Moran, D.S., Pandolf, K.B., 1999. Wet bulb globe temperature (WBGT)—to what extent is GT essential? Aviat. Space Environ. Med. 70, 480–484.
49
Moran, D., Shapiro, Y., Epstein, Y., Burstein, R., Stroschein, L., Pandolf, K.B., 1995. Validation and adjustment of the mathematical prediction model for human rectal temperature responses to outdoor environmental conditions. Ergonomics 38, 1011–1018. Moran, D.S., Montain, S.J., Pandolf, K.B., 1998a. Evaluation of different levels of hypohydration using a new physiological strain index. Am J Physiol 275 (Regulatory Integrative Comp. Physiol. 44), R854–R860. Moran, D.S., Shitzer, A., Pandolf, K.B., 1998b. A physiological strain index (PSI) to evaluate heat stress. Am. J. Physiol. 274 (44), R129–R134. Moran, D.S., Shapiro, Y., Laor, A., Israeli, S., Pandolf, K.B., 1999. Can gender differences during exercise-heat stress be assessed by the physiological strain index? Am. J. Physiol. 276 (45), R1798–1804. Moran, D.S., Pandolf, K.B., Shapiro, Y., Frank, A., Heled, Y., Shani, Y., Matthew, W.T., Gonzalez, R.R., 2001a. The role of global radiation measured by a light sensor on heat stress assessment. J. Thermal Biol. 26, 433–436. Moran, D.S., Pandolf, K.B., Shapiro, Y., Heled, Y., Shani, Y., Matthew, W.T., Gonzalez, R.R., 2001b. An environmental stress index (ESI) as a substitute for the wet bulb globe temperature (WBGT). J. Thermal Biol. 26, 427–431. National Institute for Occupational Safety and Health. Occupational Exposure to Hot Environments. Department of Health and Human Services, Washington, DC, 1986 (Rep no. DHHS86–113). Shapiro, Y., Moran, D., Epstein, Y., Stroschein, L., Pandolf, K.B., 1995. Validation and adjustment of the mathematical prediction model for human sweat rate responses to outdoor environmental conditions. Ergonomics 38, 981–986. Yaglou, C.P., Minard, D., 1957. Control of heat casualties at military training centers. Arch. Ind. Hlth. 16, 302–305.
Evaluation of the environmental stress index for physiological variables D.S. Morana,*, K.B. Pandolfb, Y. Shapiroa, A. Laora, Y. Heleda, R.R. Gonzalezb a
Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer 52621, Israel US Army Research Institute of Environmental Medicine, Natick, MA 01760-5007, USA
b
Received 6 March 2002; accepted 18 June 2002
Abstract Recently, a new environmental stress index (ESI), based on ambient temperature (Ta ), relative humidity and solar radiation, was suggested. ESI was found to be highly correlated to the WBGT index. However, ESI was not correlated with any physiological variable that reflects strain. The purpose of this study was to evaluate ESI for three different physiological variables (rectal temperature (Tre ), heart rate (fc ) and sweat rate (msw )), and with the physiological strain index (PSI). Twelve young men were exposed outdoors for 12 different experimental combinations consisting of three metabolic rates (rest, moderate and hard exercise intensity), two clothing ensembles (cotton clothing and protective overgarments), and two solar radiation levels (shade and sun). Each exposure lasted 120 min with Tre and fc being continuously monitored. High correlations (RX0:838) were found when statistical analysis was done between ESI and Tre ; fc ; msw ; or PSI, which have the potential to be widely accepted and used universally. However, further studies between physiological variables and ESI obtained from other climatic conditions, different exercise intensities, and additional clothing ensembles need to be evaluated. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Heat-stress; Indices; Rectal temperature; Heart rate; Sweat rate
1. Introduction Heat stress evaluation is generally determined through meteorological parameters that enable the estimation of the influence of several environmental factors on thermal comfort and physiological ability. The variables included in heat stress indices and their relative weights have changed over the years. The existing indices can be divided into two main categories (Gonzalez, 1986): effective temperature (ET) scales, which are based on meteorological parameters only (e.g., ambient temperature, wet-bulb temperature, black-globe temperature), and rational heat scales, which include a combination of environmental and physiological parameters
*Corresponding author. Tel.: +972-3-5303564; fax: +972-37377002. E-mail address: [email protected] (D.S. Moran).
(e.g., radiative and convective heat transfer, evaporative capacity of the environment, and metabolic heat production). In 1923, Houghten and Yaglou developed the ET index from which at least five additional indices were derived. Among them, in 1957, Yaglou and Minard introduced the wet bulb globe temperature (WBGT) index, which gained popularity mainly due to its simplicity and convenience of use. This index is obtained primarily from three parameters: black globe temperature (Tg ) which mainly reflects the solar radiation, wet bulb temperature (Tw ), and dry bulb temperature (Ta ). This index is calculated as follows: WBGT ¼ 0:7Tw þ 0:2Tg þ 0:1Ta : The WBGT is in use in the field by the US Army and is the index from which training safety orders are based (Burr, 1991). It has been adapted by the World Health Organization (WHO) and the American College of
0306-4565/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 2 ) 0 0 0 3 5 - 9
44
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
Sports Medicine (ACSM) (1996). In 1972, the National Institute for Occupational Safety and Health (NIOSH) established it as the criterion for occupational exposure to a hot environment. In 1982, it was approved by the ISO organization as an international standard for heat load assessment. Later on, work–rest regime regulations were made in the military based on this index. However, WBGT was found to be limited in evaluating heat stress due to the inconvenience of measuring Tg (Moran and Pandolf, 1999). It is important to note the correlation of this index to physiological responses was only partially tested and was based mainly on the correlation between the number of heatstroke cases during army training and the heat load as calculated by the WBGT (Yaglou and Minard, 1957). In 2001, Moran et al. (2001b) introduced a new environmental stress index (ESI) based on measurements of Ta ; relative humidity (RH), and solar radiaton (SR) as follows: ESI ¼ 0:63Ta 0:03RH þ 0:002SR þ 0:0054ðTa RHÞ 0:073ð0:1 þ SRÞ1 : This newly developed index was validated by using large databases and was found to be highly correlated to the WBGT index (Moran et al., 2001b). However, the ESI was not evaluated for any physiological variables which reflect strain. ESI differs from other indices that have been suggested in the past for two main reasons. First, ESI is based on SR and RH, aside from using Ta : In fact, the WBGT uses Tg for evaluating SR, and Tw is used for estimation of RH. However, the ESI as a stress index, for the first time uses direct measurements of SR and RH. Second, the three meteorological variables used in ESI are characterized by fast reading responses and take only a few seconds to reach equilibrium. Recently, the ESI was validated with measurements from an infrared (IR) light sensor for global radiation. It was concluded in that study that an IR light sensor can be used as a potential substitute for Tg which is incorporated in the WBGT (Moran et al., 2001a). In 1998, Moran et al. (1998b) introduced a new physiological strain index (PSI) based upon the summation in equal weights of individual strains for core temperature (Tc ) and heart rate (fc ) as follows: PSI ¼ 5ðTct Tc0 Þð39:5 Tc0 Þ1 þ 5ðfct fc0 Þð180 fc0 Þ1 ; Where Tct and fct are simultaneous measurements taken at any time during the exposure and Tc0 and fc0 are the initial measurements. This index represents the combined strains of the thermoregulatory and cardiovascular systems. Each strain system was scaled between 0 and 5. The PSI is thus scaled 0–10, and can be used on-line or during data
analysis. The PSI can be applied at any time, including rest or recovery periods, whenever Tc and fc are measured (Moran et al., 1998b). In a recent series of studies, PSI successfully evaluated the strains in different clothing ensembles and climatic conditions during heat stress, during different levels of hydration and exercise intensity, for gender during heat stress, and for different age groups during 10 days of exercise-heat acclimatization (Moran et al., 1998a, b, 1999). The purpose of this study was to evaluate the correlation of ESI for three different physiological variables (sweat rate (msw ), rectal temperature (Tre ), and fc ) and for PSI during different experimental combinations of metabolic rate, clothing and solar radiation.
2. Materials and methods The material and methods for the original data used in this paper are presented in greater detail in previous articles (Moran et al., 1995; Shapiro et al., 1995). Subjects: Twelve healthy, young (1972 yr), fit men (bw ¼ 68:877:6 kg; VO2max =3.5970.36 l min1) participated in this study. Prior to the experimental exposures, each subject underwent a complete medical examination that included ECG at rest, urine analysis and SMA-12 screening biochemistry laboratory test. A complete medical history was also obtained. Subjects were informed as to the nature of the study and the potential risks of exposure to exercise in a hot climate and all signed a consent form. Protocol: Each subject was tested under 12 experimental combinations (Table 1). The combinations were assigned at random to the subjects. While at rest, the subjects were seated on a bench. The work consisted of walking on a treadmill at a speed of 1.4 m s1 (5 km h1) with no grade at the moderate workload, and with a 5% grade for the heavy workload. Each exposure lasted 120 min consisting of two cycles of 10 min rest and 50 min exercise. A test was terminated before 120 min whenever Tre > 39:01C or fc > 180 bpm: This study was conducted outdoors under a shade (6 6 6 m3) and in an open space where solar radiation was about ten times higher than that in the shaded area. All experiments were conducted in Israel during July and August between 12:00 and 14:00 h. Physiological parameters: During the tests, Tre was recorded every 5 min by using a thermistor probe inserted 10 cm beyond the anal sphincter (YSI 401, Yellow Springs Instrument, USA). Heart rates (fc ) were continuously radio-telemetered to an oscilloscope tachometer (Lifescope 6, Nihon Kohden, Japan) and were recorded every 5 min. To determine metabolic cost, oxygen consumption (VO2 ) was measured after 40 min of each bout of exercise. Expiratory gases were collected
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49 Table 1 The experimental combinations Metabolic rate
Rest (E100 W) Moderate work (E300 W) Heavy work (E450 W)
Solar radiation
Sun (maximal solar radiation E900 W m2) Shade (minimal solar radiation E80 W m2)
Clothing
Cotton clothes (clo=0.99, im/clo=0.75) Protective overgarment (clo=1.64, im/clo=0.43)
and analysed by the automatic metabolic cart (MMCHorizon, SensorMedics). Sweat rate was calculated from weight differences before and after exposure adjusted for water intake and urine output. Environmental variables: Dry bulb (Ta ) and wet bulb temperature (Tw ) were measured at the study location in the shaded area, using a psychometer (Lambrecht). The accuracy of these parameters was cross-tested by measuring ambient temperature and relative humidity with an electronic sensor (Rotronic). All these parameters were measured at approximately 2 m from ground level and recorded every 15 min. Solar radiation (short-wave radiation) was measured every 5 min using the Eppley precision pyranometer model PSP (EPLAB). The black globe temperature (Tg ) was measured using the Vernon black globe thermometer. Tg and SR were measured both in shaded and open areas. Calculations: Heat stress and strain indices were calculated according to the following publications: WBGT was calculated according to Yaglou and Minard (1957); ESI and PSI were calculated as suggested by Moran et al. (1998b, 2001b). Statistical analysis: Data analysis included fitting linear models, which were used to define the correlation between ESI and different physiological variables (Tre ; fc and msw ) and PSI at the end of each experimental exposure after 120 min. These data were analyzed for different combinations of metabolic rate and clothing, whereas for solar radiation the two exposures (sun and shade) were pooled together. All statistical contrasts were accepted at the Po0:05 level of significance. For all correlation and regression analyses, we used SAS 8.0 software procedures CORR and GLM.
3. Results ESI was applied to environmental data collected during the summer in the Israeli coastal regions for 36
45
days between 12:00 and 14:00 h. At this time period, each of the 12 subjects performed different exposures each day as depicted in Table 1. In Figs. 1–6, we compared between the ESI values obtained in the sun and in the shade at different combinations of metabolic rate and clothing. In addition, we showed in these figures the corresponding physiological parameters (Tre ; fc and msw ) and PSI values for ESI. The comparisons of Tre and fc were made from their changes during the sun and the shade exposures, whereas for msw ; we compared between the actual measured values. In general, ESI values in all the tested exposures were significantly higher (Po0:05) in the sun when compared to the shade exposures. However, in all the 12 exposures, values were within a narrow range for the sun (30.1370.101C) and for the shade (28.0670.101C). In contrast, the measured physiological variables (Tre ; fc and msw ) were significantly (Po0:05) affected by exercise intensity and overgarment clothing, which caused metabolic heat production elevation. These physiological variables were also significantly higher (Po0:05) in the sun when compared to the shade exposures. ESI values obtained from the measured climatic parameters were stable during all of the different
Fig. 1. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during resting exposures in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel). (*Po0:05 for the entire exposure (120 min)).
46
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
Fig. 2. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during resting exposures in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
Fig. 3. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during moderate exercise intensity in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel).
experimental exposures. Therefore, the ESI correlation assessment was done for each exposure after 120 min. High correlations (RX0:838) between ESI120 and the different physiological variables (Tre120 ; fc120; and msw ) and PSI120 were found when analyzed for the different experimental exposures. However, data were analyzed for pooled low and high solar radiation exposures reflecting the radiation intensity, and these affects on the physiological variables (Table 2). During the rest exposures, there were significant differences (Po0:05) in fc changes and msw between the values measured under the sun and in the shade while wearing cotton clothes or protective overgarments (Figs. 1 and 2). However, for DTre significant differences (Po0:05) were found only while wearing protective overgarments (Fig. 2, top panel). Corresponding analysis of ESI and PSI for these rest exposures showed significant differences (Po0:05) between values measured and calculated indices in the entire sun and shade exposures (Figs. 1 and 2, bottom panel). During the moderate exercise intensity, measured changes in msw ; Tre and fc were significantly higher (Po0:05) in the sun exposures when compared to values obtained in the shade. However, significantly higher
(Po0:05) values were found when wearing protective overgarments, with msw of 1.9370.10 l h1, DTre of 1.9370.111C, and Dfc of 9874 bpm, when compared to wearing cotton clothing: msw ¼ 1:5270:08 l h1 ; DTre ¼ 1:0270:111C and Dfc ¼ 7075 bpm (Figs. 3 and 4, top panel). PSI values at the end of the cotton clothing exposures were significantly different (Po0:05) for the shade and sun exposures with values of 3.370.2 and 4.470.4, respectively. Significantly higher (Po0:05) strain was found for these subjects while wearing protective overgarments with PSI values of 5.170.3 and 7.170.7 for the shade and sun exposures, respectively. In accordance, ESI values were significantly (Po0:05) higher in the sun (30.3170.08) exposures when compared to the shade exposures (28.2370.08) (Figs. 3 and 4, bottom panel). Figs. 5 and 6 depict comparisons of the changes between sun and shade exposures for PSI and the physiological variables during hard exercise intensity. In cotton clothing, significant differences (Po0:05) were found in msw and Dfc values between the sun and shade (Fig. 5, top panel). Concomitantly, significant differences were found between sun and shade exposures for ESI and PSI (Fig. 5, bottom panel). Wearing protective
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
47
Fig. 4. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during moderate exercise intensity in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
Fig. 5. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during hard exercise intensity in the shade and sun while wearing cotton clothing (top panel) and the obtained PSI and ESI indices (bottom panel).
overgarments created very high physiological strain especially under the sun, with PSI values at the end of the exposures of 7.570.1 and 9.170.4 for the shade and the sun climate conditions, respectively (Fig. 6, bottom panel). Therefore, eight subjects were not able to tolerate these exposures and they terminated the exercise after less than 1 h. Thus, the data presented in Fig. 6 was obtained from only four subjects who completed the scheduled exposures.
In this study, we evaluated the new suggested ESI with the PSI and for three independent physiological variables (Tre ; fc ; and msw ) under different combinations of metabolic rates, clothing and solar radiation. The high correlations (RX0:838) found after 120 min between these two indices and between ESI and these physiological variables can serve as a validation for ESI that previously was validated only for other stress indices (WBGT and the discomfort index (DI)). In addition, analysis of ESI values for the sun and the shade exposures significantly (Po0:05) corresponded with the values of the tested physiological variables for the respective sun and shade exposures. Heat strain arises from different combinations of work rate, clothing and environmental conditions. Performing exercise while wearing protective garments, which decreases the ability of the body to dissipate heat, under exposure to solar radiation outdoors in hot climates results in a very high physiological strain. Thus, the level of the strain is linear and correlates to exercise intensity. In this study, the strain assessments were in a wide range, however the environmental stress was in a narrow range due to the outdoor design of the study.
4. Discussion During training involving physical activity especially in the military, insufficient work–rest cycles and fluid replacement can lead to heat casualties. Among these is heat stroke, which can be a life threatening situation. The recommended lengths of work–rest cycles and fluid replacement volumes are determined according to exercise intensity and the heat load. The latter is calculated from meteorological parameters consisting of Ta ; Tw or RH and radiation components, either Tg or SR, that reflect the environmental stress.
48
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
The strength of a reliable heat stress index is its ability to perform under various conditions and by its being correlated to a commonly used gold standard index (e.g., WBGT). Although ESI was previously found to be highly correlated to WBGT in a few databases (Moran et al., 2001b), we believe that the strength of ESI stems from its evaluation with PSI and other individual physiological variables (Tre ; fc ; and msw ).
The WBGT and the ESI were derived empirically from observation of meteorological measurements. Both indices are compiled through the use of Ta : However, unlike WBGT, which includes Tg and Tw that are affected by radiation and vapor pressure, respectively, ESI includes solar radiation and relative humidity measurements. Thus, ESI’s meteorological components are commonly used, and are more meaningful and obvious for the layman. Moreover, these three parameters are measured by fast response reading sensors, whereas Tg and Tw are cumbersome to measure and require 20–30 min to reach equilibrium. In conclusion, PSI is a new index already accepted around the world by many military installations and exercise physiologists (Amos, 2000; Baker and Grice, 2000; Baker et al., 2000; Cotter et al., 2000; Moran et al., 1999). We believe that assessment of physiological strain by PSI and environmental stress by ESI complement each other and have the potential to be widely accepted and used universally. However, further studies between physiological variables and ESI obtained from different climate conditions should be evaluated.
Acknowledgements
Fig. 6. Mean7SE of sweat rate (msw ), the change in rectal temperature (DTre ), and in heart rate (Dfc ) during hard exercise intensity in the shade and sun while wearing protective overgarments (top panel) and the obtained PSI and ESI indices (bottom panel).
The authors wish to acknowledge Mrs. Linda Gans and Ms. Liron Shalit for their technical assistance in preparing this manuscript. Disclaimer The views, opinions and/or findings in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision unless so designated by other official designation. Human subjects participated in these studies after giving their free and informed voluntary consent. Approved for public release; distribution unlimited.
Table 2 The correlation coefficients between ESI120 and Tre120 ; fc120 ; PSI120, and msw obtained from 12 subjects exposed to 12 different combinations of metabolic rate, clothing, and solar radiation Experimental exposure
Tre120
fc120
PSI120
msw
Rest
Cotton Protective overgarment
0.849 0.846
0.844 0.838
0.846 0.893
0.932 0.851
Moderate work
Cotton Protective overgarment
0.864 0.910
0.875 0.859
0.845 0.842
0.955 0.873
Heavy work
Cotton Protective overgarment
0.839 0.899
0.838 0.869
0.875 0.913
0.913 0.947
Analysis data from minimal and maximal solar radiation was pooled together.
D.S. Moran et al. / Journal of Thermal Biology 28 (2003) 43–49
References American College of Sports Medicine, 1996. Position stand on heat and cold illnesses during distance running. Med. Sci. Sport Exerc. 28(12), i–x. Amos, D., 2000. The measurement of thermal strain in soldiers operating in Northern Australia. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments. Australian Institute of Sport, Canberra, pp. 144–151. Baker, S., Grice, J., 2000. To what extent does thermal sensation reflect physiological heat strain. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments. Australian Institute of Sport, Canberra, pp. 36–38. Baker, S.J., Grice, J., Roby, L., Matthews, C., 2000. Cardiorespiratory and thermoregulatory response of working in fire-fighter protective clothing in a temperate environment. Ergonomics 43, 1350–1358. Burr, R.E., 1991. Heat illness: a handbook for medical officers. USARIEM, Natick, MA. (Rep.no.: T 91-3). Cotter, J.D., Roberts, W.S., Amos, D., Lau, W.M., Prigg, S.K., 2000. Heat strain during combat fitness assessment of soldiers in Northern Australia. Proceedings of the International Conference on Physiological and Cognitive Performance in Extreme Environments, Australian Institute of Sport, Canberra, pp. 176–178. Gonzalez, R.R., 1986. Biophysics of heat transfer and clothing considerations. In: Pandolf, K.B., Sawka, M.N., Gonzalez, R.R. (Eds.), Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Cooper Publishing Group, Carmel, IN, pp. 45–95. Houghten, F.C., Yaglou, C.P., 1923. Determining lines of equal comfort. Am. Soc. Heat Ventil. Eng. Trans. 29, 163–176. Moran, D.S., Pandolf, K.B., 1999. Wet bulb globe temperature (WBGT)—to what extent is GT essential? Aviat. Space Environ. Med. 70, 480–484.
49
Moran, D., Shapiro, Y., Epstein, Y., Burstein, R., Stroschein, L., Pandolf, K.B., 1995. Validation and adjustment of the mathematical prediction model for human rectal temperature responses to outdoor environmental conditions. Ergonomics 38, 1011–1018. Moran, D.S., Montain, S.J., Pandolf, K.B., 1998a. Evaluation of different levels of hypohydration using a new physiological strain index. Am J Physiol 275 (Regulatory Integrative Comp. Physiol. 44), R854–R860. Moran, D.S., Shitzer, A., Pandolf, K.B., 1998b. A physiological strain index (PSI) to evaluate heat stress. Am. J. Physiol. 274 (44), R129–R134. Moran, D.S., Shapiro, Y., Laor, A., Israeli, S., Pandolf, K.B., 1999. Can gender differences during exercise-heat stress be assessed by the physiological strain index? Am. J. Physiol. 276 (45), R1798–1804. Moran, D.S., Pandolf, K.B., Shapiro, Y., Frank, A., Heled, Y., Shani, Y., Matthew, W.T., Gonzalez, R.R., 2001a. The role of global radiation measured by a light sensor on heat stress assessment. J. Thermal Biol. 26, 433–436. Moran, D.S., Pandolf, K.B., Shapiro, Y., Heled, Y., Shani, Y., Matthew, W.T., Gonzalez, R.R., 2001b. An environmental stress index (ESI) as a substitute for the wet bulb globe temperature (WBGT). J. Thermal Biol. 26, 427–431. National Institute for Occupational Safety and Health. Occupational Exposure to Hot Environments. Department of Health and Human Services, Washington, DC, 1986 (Rep no. DHHS86–113). Shapiro, Y., Moran, D., Epstein, Y., Stroschein, L., Pandolf, K.B., 1995. Validation and adjustment of the mathematical prediction model for human sweat rate responses to outdoor environmental conditions. Ergonomics 38, 981–986. Yaglou, C.P., Minard, D., 1957. Control of heat casualties at military training centers. Arch. Ind. Hlth. 16, 302–305.