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Islam, Maeen Zakaria
Influence of gender on heart rate and core temperature at critical wbgt for five clothing ensembles at three levels of metabolic rate
h [electronic resource] /
by Maeen Zakaria Islam.
[Tampa, Fla.] :
b University of South Florida,
Thesis (M.S.P.H.)--University of South Florida, 2005.
Includes bibliographical references.
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ABSTRACT: Three main factors that influence heat stress are clothing, work demands and environmental conditions. Gender may also influence the amount of heat stress an individual can tolerate. The purpose of this study was to examine the role of gender in heat stress limits (critical WBGT) and heat strain (heart rate and core temperature). The null hypothesis was that there was no gender difference among critical WBGT, heart rate and core temperature. Fifteen subjects (11 men and 4 women) wore five different clothing ensembles (cotton work clothes, cotton coveralls, particle barrier Tyvek, water-barrier/vapor permeable NexGen LS417, and vapor barrier Tychem QC made by Dupont) at three levels of metabolic rate (115, 175 and 250 W m-2). A treadmill was used to set the metabolic workload. A climatic chamber was used to control the environmental conditions. The participants continued to walk on the treadmill until their core temperature (Tre) reached a steady state.Then the air temperature and humidity were slowly increased. The point at which the core temperature increased steadily was defined as the inflection point. Environmental data as well as core temperature and heart rate were recorded at five minute intervals. The critical conditions were noted at five minutes before the inflection point. Metabolic rate, critical WBGT, core temperature and heart rate were analyzed by 3-way ANOVAs (participants nested by ensemble by metabolic rate) with all two way and three way interactions. Significant differences were observed between genders for metabolic rate and heart rate, but not for core temperature and critical WBGT across metabolic level and ensembles. While there were differences between genders in metabolic rate they did not affect the overall conclusions. The heart rate was significantly higher (12 bpm) for women than for men.
Adviser: Tom E. Bernard.
x Public Health
t USF Electronic Theses and Dissertations.
Influence of Gender on Heart Rate and Core Temperature at Critical WBGT for Five Clothing Ensembles at Th ree Levels of Metabolic Rate by Maeen Zakaria Islam A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Public Health Department of Environmenta l and Occupational Health College of Public Health University of South Florida Major Professor: Thomas E. Bernard, Ph.D. Candi D. Ashley, Ph.D. Steven P. Mlynarek, Ph.D. Date of Approval: April 15, 2005 Keywords: Heat Stress, Gender Difference s, Core Temperature, WBGT, Clothing Copyright 2005, Maeen Z. Islam
Acknowledgements I would like to give a very special tha nk you to Dr. Thomas Bernard. Without his guidance, expertise, patience, and support this paper would not have been possible. The amount of time you spent in guiding me through this paper and my tim e at University of South Florida will always be remembered. I would also like to give a special thank you to Dr. Candi Ashley for your constructive critiq ues of this thesis. To Dr. Steve Mlynarek, I appreciate your help with my thesis as well. Thank you to my wife, Sumaiya without your support in pushing me to get my thesis completed, I am not sure I would have ever completed it. Also, to my parents, thank you for the support you have given me in pursuing my graduate degree. Finally, I would like to thank the Nationa l Institute for Occupational Safety and Health (NIOSH R01-OH03983) for funding this study. The views and opinions presented in this thesis are strictly my own.
Table of Contents List of Tables iii List of Figures iv List of Abbreviations and Acronyms v Abstract vii Introduction 1 Literature Review 3 Heat Stress Factors 3 Metabolic Rate 3 Clothing 4 Environmental Conditions 5 Physiological Responses to Heat Stress 6 Acclimization State 6 Heart Rate and Gender 8 Core Temperature and Gender 9 Sweat Rate and Gender 10 Skin Temperature and Gender 12 Hypothesis 13 Methods 14 Participants 14 Equipment 15 Clothing 16 Experimental Protocol 16 Determination of Critical WBGT 17 Results 19 Metabolic Rate 20 Critical WBGT 22 Mean Heart Rate 22 Mean Core Temperature 24 Discussion 25 Metabolic Rate 25 Critical WBGT 25 i
Heart Rate and Core Temperature 26 Conclusion 28 References 29 ii
List of Tables Table 1. Participant Characteristics 15 Table 2. Summary of p-values for analyses 19 Table 3. Mean Metabolic Rate for Three Levels of Metabolism 20 Table 4. Critical WBGT for Three Levels of Metabolism 22 Table 5. Mean Heart Rate for Three Levels of Metabolism 23 Table 6. Mean Core Temperature for Three Levels of Metabolism 24 iii
List of Figures Figure 1. Time Course of Tre 18 Figure 2. Metabolic Rate between Genders for Three Metabolic Levels 21 Figure 3. Average Heart Rate between Genders for Three Metabolic Levels 23 iv
List of Abbreviations and Acronyms ACGIH American Conference of American Industrial Hygienists ANOVA Analysis of Variance BPM Beats Per Minute HR Heart Rate ISO International Organization for Standardization IRB Institutional Review Board I t Total Clothing Insulation NIOSH National Institute for Occupational Safety and Health R e-t Total evaporative resistance R h Relative Humidity T arm Arm Temperature T calf Calf Temperature T ch Chest Temperature T db Dry Bulb Temperature T g Globe Temperature T th Thigh Temperature T nwb Natural Wet Bulb Temperature T re Core Temperature T pwb Psychometric Wet Bulb Temperature USF University of South Florida v
VO 2max Maximum Oxygen Volume WBGT Wet Bulb Globe Temperature vi
Influence of Gender on Heart Rate and Core Temperature at Critical WBGT for Five Clothing Ensembles at Three Levels of Metabolic Rate Maeen Zakaria Islam ABSTRACT Three main factors that influence heat stress are clothing, work demands and environmental conditions. Gender may also influence the amount of heat stress an individual can tolerate. The purpose of this study was to examine the role of gender in heat stress limits (critical WBGT) and heat strain (heart rate and core temperature). The null hypothesis was that there was no gender difference among critical WBGT, heart rate and core temperature. Fifteen subjects (11 men and 4 women) wore five different clothing ensembles (cotton work clothes, cotton coveralls, particle barrier Tyvek, water-barrier/vapor permeable NexGen LS417, and vapor barrier Tychem QC made by Dupont) at three levels of metabolic rate (115, 175 and 250 W m -2 ). A treadmill was used to set the metabolic workload. A climatic chamber was used to control the environmental conditions. The participants continued to walk on the treadmill until their core temperature (T re ) reached a steady state. Then the air temperature and humidity were slowly increased. The point at which the core temperature increased steadily was defined as the inflection point. Environmental data as well as core temperature and heart rate vii
were recorded at five minute intervals. The critical conditions were noted at five minutes before the inflection point. Metabolic rate, critical WBGT, core temperature and heart rate were analyzed by 3-way ANOVAs (participants nested by ensemble by metabolic rate) with all two way and three way interactions. Significant differences were observed between genders for metabolic rate and heart rate, but not for core temperature and critical WBGT across metabolic level and ensembles. While there were differences between genders in metabolic rate they did not affect the overall conclusions. The heart rate was significantly higher (12 bpm) for women than for men. Overall, women had the same upper limit of the prescriptive zone as men, their core temperatures were the same at this limit but women had a greater cardiovascular cost reflected in a higher heart rate. viii
Introduction Among the physical agents that might be present during work, heat stress is well recognized. The job risk factors for heat stress are hot environment, heavy work demands, and protective clothing, alone or in any combination. The evaluation of heat stress is based first on exposure limits that consider the environmental conditions and the metabolic rate. A widely recognized index of environmental conditions is wet bulb globe temperature (WBGT). NIOSH (1986), ACGIH (2004) and ISO (1989) have prescribed exposure limits based on the combination of WBGT and metabolic rate, such that with increasing metabolic rate the ambient WBGT decreases. All of these occupational exposure limits are based on two goals. The first goal is to maintain the physiological responses in the work driven zone, which is a range of environmental conditions for which the body core temperature does not change for a given metabolic rate. The upper end of this range is called the Upper Limit of the Prescriptive Zone (Lind 1963). Beyond that point, the body core temperature increases with the WBGT and this range is called the environmentally driven zone. The second goal is to keep body core temperature below 38 C, and this is accomplished if the exposure occurs in the work driven zone. The occupational exposure limit based on WBGT depends on clothing as well as metabolic rate. To account for clothing, adjustment factors have been put forward that adjust the WBGT limit to represent the change in heat stress due to the clothing. With work clothes as a reference work ensemble, wearing particle-, liquid-, and vapor-barrier 1
clothing ensembles have a progressive increase in the level of heat stress that can be accounted for by the adjustment factors. There is a physiological cost to meeting the demands of heat stress and to maintain thermal equilibrium at the occupational exposure limit. These physiological costs are commonly referred to as heat strain and include heart rate, core temperature and skin temperature. There is good evidence that men and women respond to heat stress differently. This opens a question of whether men or women pay a higher physiological cost at the occupational exposure limit. 2
Literature Review Work in a warm or hot environment in combination with metabolic rate and clothing requirements can bring about heat stress. For a given combination of metabolic rate and clothing, there is a limiting environmental contribution that still allows thermal balance. To achieve the thermal equilibrium, physiological systems respond to the heat stress, and this response is collectively known as heat strain. Heat strain is reflected in heart rate, core temperature and skin temperature. Heat Stress Factors Metabolic Rate Metabolic rate directly affects the amount of heat stress and strain an individual experiences. For heat stress, the metabolic rate represents the internal heat generation that must be dissipated to the environment. On the heat strain side, the metabolic rate drives a need for physiological resources, especially cardiac output, that compete with the need to dissipate heat. In order to study metabolic rate in a laboratory setting, two methods to select metabolic rate are employed; these are absolute and relative. The absolute demand has the advantage of fixing the amount of heat generation for everybody. But the absolute metabolic rate does not account for the aerobic capacity of an individual. For example if 3
a study is conducted at 260 W, that is the target for everyone in the study. So, an individual with a low level of fitness will work relatively harder (greater physiological strain) than another individual with a greater aerobic capacity. In contrast a relative metabolic rate, matches the participants VO 2max or a given fraction (%) of VO 2max is used as the target metabolic rate. Matching or assigning a %VO 2max controls for the effects of fitness, but does not control for the heat generation. Clothing Protective clothing can alter the rate of heat dissipation via convection, radiation and evaporation. Clothing characteristics can be described by total clothing insulation (I t ) and total evaporative resistance (R e-t ) In evaluating heat stress, it is important to understand insulation and permeability characteristics of clothing. I t is used to describe the decrease in heat flow due to clothing and air insulation. The insulative properties of clothing depend on three factors. These are the overall thickness of the clothing, the air pockets between the material and the skin, and the air pockets between the layers of clothing. The insulation affects the rate of convection and radiant heat flow. The R e-t describes the permeability characteristics. The more permeable a piece of clothing is, the more it allows sweat to evaporate and promote heat dissipation. Impermeable clothing prevents evaporation thus increasing the level of heat stress. Convective and radiative heat loss accounts for approximately 10% of heat dissipation. The major method of heat dissipation during work is evaporation. 4
Therefore, R e-t is the more important clothing characteristic to consider when evaluating clothing properties. Clothing influences the upper limit of the prescriptive zone (Bernard, et al. 2005). The ACGIH (2004) has suggested clothing adjustments to be added to the ambient WBGT to account for the added heat stress burden and several others have recommended values for different clothing ensembles (Kenney 1988, OConner and Bernard 1999, Bernard et al 2005). Environmental Conditions The wet bulb globe temperature (WBGT) is used to evaluate the contribution of environmental factors to heat stress. The natural wet bulb temperature (T nwb ) assesses the contribution of evaporative cooling. The globe temperature (T g ) is used to evaluate convective and radiant heat. In industrial settings, the equation for WBGT is: WBGT = 0.7 T nwb + 0.3 T g Threshold limit values established by the ACGIH prescribe a relationship between environmental conditions and work demands below which most workers can maintain thermal equilibrium when wearing ordinary work clothes. As metabolic rate increases, the ambient WBGT decreases to maintain thermal balance. The threshold can be shifted depending on the clothing requirements. 5
Physiological Responses to Heat Stress Some individual variations in responses to heat stress may be accounted for by gender. The gender differences in thermoregulation become more apparent with greater thermal loads with females at a disadvantage in very hot environments. In comparison to males, females generally have 1) a greater amount of body fat which acts as insulation and increases heat storage, 2) a higher thermoregulatory set point, and 3) lower aerobic capacity which increases the relative workload of a given task (Reneau et al 1999, NIOSH 1986). Females also have a smaller blood volume and a larger surface area to mass ratio. Peripheral vasodilation results in a relatively larger amount of blood to the periphery. Generally, females rely more on convective heat loss, an advantage in hot wet environments, while males rely more on evaporative heat loss, an advantage in hot dry environments (Kenny et al. 1988, Montain et al. 1994). This is an advantage for females as decreased sweat production may slow dehydration and enhances heat dissipation in humid environments. Acclimatization State The amount of heat strain a worker experiences depends largely on environment, work rate and clothing as well as host factors like acclimatization state. If the worker is not acclimated to the heat, the heat strain is greater. Acclimatization can have a positive impact on heat tolerance by increasing sweat rate, increasing plasma volume, and decreasing heart rate which helps to reduce body temperature and fatigue at a given work rate. 6
The process of acclimatization involves exposing workers to work in a hot environment for at least two hours over sequential days. The physiological adaptations of acclimatization will occur with environmental conditions sufficient to raise body core temperature and heart rate. Acclimatization can be accomplished with exposure to a hot environment with low relative humidity (hot-dry) as well as with exposure to a warm environment with high relative humidity (warm-wet). NIOSH (NIOSH 1986) suggests that for workers with previous work experience in the heat, the acclimatization program should begin with 50% exposure on day one, 60% on day two, 80% on day three and 100% on day 4 in recognition of the increased work capacity that comes from a sound acclimatization program. NIOSH (1986) further recommends that the acclimatization program for new workers begin with 20% exposure on day one with a 20% increase in exposure each day, and is more gradual because the worker is also learning the job. Documented physiological responses of acclimatization include improved circulatory efficiency and enhanced thermoregulation. Circulatory changes include an increase in plasma volume and decreased heart rate for a given workload (Montain et al. 1994, Moriomoto et al. 1967, Frye and Kamon 1981, Kamon et al. 1978, McLellan, 1998). In addition, there is a deceased core temperature for a given workload (Morimoto et al. 1986, Frye and Kamon, 1981, Anderson et al. 1995 Kamon et al. 1978, McLellan 1998), and a lower core temperature and skin temperature for the onset of sweating as well as a greater sweat rate (White et al. 1989, Frye and Kamon 1981, Anderson et al. 1995, McLellan, 1998). Increased core temperature is the best marker of limiting conditions for heat stress. As such, a plateau in core temperature has been suggested as the major criteria to designate complete acclimatization. 7
For many heat stress and strain studies, acclimated participants routinely remove acclimatization state as a potential confounder. Heart Rate and Gender Most research has found that men have a lower heart rate for a given level of heat stress than women (Avellini et al. 1980, Shalpiro et al. 1980, Yousef et al. 1984, Kamon et al. 1978, Mcllelan, 1998). The gender difference in heart rate in many studies was attributed to environmental conditions. Shapiro et al (1980) conducted a study under six environmental conditions including a comfortable climate (WBGT = 14.4C, rh = 40%), a mild-wet climate (WBGT=30.3C, rh = 80%), two hot-wet climates (WBGT = 34.0 and 34.5C, rh = 90 and 80%) and two hot-dry conditions (WBGT = 34.0 and 34.2C, rh = 20 and 10%). Men had lower heart rates during hot-dry conditions but not during mild-wet, comfortable or hot-wet conditions. There are a few studies that show a greater heart rate in men than women. Avellini, Kamon and Krajewski (1980) showed that men had higher heart rates than women prior to acclimatization in hot-wet conditions. The mens heart rates were 13-25 bpm higher than the womens. Post-acclimatization, there was no difference in heart rate between men and women. Paolone, Wells and Kelly (1978) also showed higher heart rates in men in neutral, warm and hot conditions. The experimental protocol called for a metabolic rate of 50% VO 2 max As the authors point out, the mean exercise VO 2 of the men was 15% greater in all environments than the females. 8
Typically, aerobic fitness is higher in men than women. Matching participants on aerobic fitness levels the playing field. Studies that matched subjects on aerobic fitness showed no difference in heart rate between men and women (Antunano 1992, McLellan 1998). In addition, although Kamon, Avellini and Krajewski (1978) observed a greater heart rate in men, the authors point out that the difference in heart rate is proportional to the difference in VO 2 max In summary, the literature suggests that there is a difference between males and females with respect to heart rate particularly under hot-dry environmental conditions. Core Temperature and Gender The best indicator of heat stress is core body temperature. During exercise, the increase in core body temperature is proportional to the increase in metabolic rate, heat load and may also be influenced by clothing. There are several methods to measure core body temperature, but the most common laboratory method is rectal temperature (T re ). A number of researchers report a greater T re in semi-nude women than in men (Yousef et al. 1984, Moran et al. 1999, Paolone et al. 1978). In addition, McLellen, (1998) also observed a greater T re in men than women working in NBC clothing in a hot-dry environment. Shapiro et al. (1980) examined heat stress responses under 6 environmental conditions. Under the hot-wet conditions, men had a higher T re Under hot-dry conditions, these researchers reported a significantly greater T re for women. As previously suggested, fitness and acclimatization can have a positive effect on physiological signs of heat strain. In studies where participants were matched on aerobic 9
capacity, the researchers (Frye and Kamon 1981, McLellan 1998) found no significant difference in T re between men and women. Also, when the results of the participants in the McLellans study (1998) were matched on aerobic capacity, gender differences in heat strain (i.e. T re ) disappeared. As previously mentioned, generally men have a higher aerobic capacity than women. Using an equivalent absolute workload results in a greater relative workload for women. A number of researchers have used acclimatized subjects. For example, Avellini, Kamon, and Krajewski (1980), observed a significantly greater T re in men prior to acclimatization. After acclimatization, no significant difference was noted between the genders until 90 minutes of exercise. After that point, T re in males began to increase until test termination at 3 hours. At the end of the exposure, T re of the men was 0.3C higher than the women. Sweat Rate and Gender During work, evaporation is the primary means of heat dissipation. Evaporation is dependent on vapor pressure gradients between the skin, air and clothing. Vapor pressure is a function of relative humidity and ambient temperature. As vapor pressure decreases, less evaporative cooling is possible. The amount of sweat evaporated is dependent on vapor pressure gradients, convection, and the amount of skin wettedness available to the environment. The amount of sweat an individual generates can have an affect on how well the body does to cool itself down. 10
The ability of the body to cool itself by sweating is influenced by environmental conditions. In a humid environment, the perspiration does not readily evaporate due to a low water vapor pressure gradient between the skin and air. In contrast if an individual is in a hot dry climate, the sweat easily evaporates off the skin. Morimoto (1967) observed a significant increase in sweat rates of men compared to women in both humid and dry conditions. They did observe though, that sweat rates decreased in both genders during humid conditions. Although women have a greater number of sweat glands than men, men tend to have a higher sweat rate than women. In the literature, there is a significant difference in sweat rate between genders. Generally, the vast majority showed that men sweat much more than women (Morimoto et al. 1967, Avellini et al. 1980, Frye and Kamon 1981, Shapiro et al. 1980, Fox et al. 1969, Yousef et al. 1984, Anderson et al. 1995, Kamon et al. 1978, McLellan 1998, Paolone et al. 1978). Avellini, Kamon, and Krajewski (1980) observed that prior to acclimatization, men sweated significantly more than women. After hot humid acclimatization, the difference in sweat rate between men and women was even more significant. Sweat rate in males after acclimatization increased 35% while that of females increased 18%. In a similar study Frye and Kamon (1981) observed a greater sweat rate in men than women prior to acclimatization. After acclimatization, the sweat rate between men and women was not significantly different. Both males and females showed increased sweating after acclimatization, but in their study a significant difference between the genders did not exist. Shapiro et al (1980) observed that acclimatized men sweated more than acclimatized women in humid environments. Under hot-wet environmental conditions 11
men sweated 40% more than women. Under mild-wet conditions, men sweated an average of 23% more than women. In hot-dry dry conditions men sweated more than women but the difference between the sexes was not significantly different. Fox et al (1969) and Yousef et al. (1984) studied sweat rates in hot-dry environmental conditions. They observed that females had a higher onset of sweating threshold for sweating. Females also had a lower sweat rate than males when they were exercising at the same rate. Kenny and Zeman (2002) measured sweat rate for unacclimatized males and females. They observed that males sweat significantly more than females and males had a higher evaporative rate of sweat than females. In a study examining heat strain of subjects with equivalent aerobic capacities, Moran et al. (1999) observed equivalent sweat rates between men and women. However, there was also a third group consisting of men with a higher aerobic capacity. These subjects had a higher sweat rate than the men and women with equivalent aerobic capacities. Skin Temperature and Gender Skin temperature has also been used as an indication of heat stress. The majority of studies concluded that there is no significant difference in skin temperature between males and females (Avellini et al. 1980, Frye and Kamon 1981, Kamon et al 1978, Paolone et al 1978). A few studies observed a higher skin temperature in women than men. Yousef et al (1984) and McLellan (1998) observed that women have a higher skin temperature than 12
males. Yousef et al. (1984) attributed the higher skin temperature in the women to the lower sweat rate observed. McLellan (1998) concluded that the higher skin temperature in women was due to the NBC clothing worn during the experiment. Due to the impermeable characteristics of the clothing, there was an increase in the vapor pressure inside the suit. As suggested earlier, as women rely more on convective heat transfer, heat dissipation in humid environments is compromised. In addition, the researchers used an equivalent workload for all subjects. This may have equated to a higher relative workload for the women. In general, the majority of studies concluded that there was no significant difference observed for skin temperature between the two sexes. The two studies that did observe a difference attributed the difference to other protocol factors. Hypothesis The purpose of this study was to examine the effect of gender on heart rate and core temperature at critical WBGT in five ensembles at three levels of metabolic rate. The clothing ensembles varied from cotton work clothes to those with high evaporative resistance based on past laboratory experience to represent the range of clothing used in industry. The metabolic levels were chosen to represent light, moderate and heavy work of industrial settings. The null hypothesis was that there was no gender difference among critical WBGT, heart rate and core temperature at three levels of metabolic rate and five levels of clothing ensembles. 13
Methods To determine the critical conditions of men and women wearing protective clothing in heat stress at different work demands, an experimental protocol was formulated using humans subjected to three different metabolic rates wearing five clothing ensembles. Participants Participants (11 men and 4 women) were recruited using local printed media. A licensed physician gave each participant a physical and approved him or her for participation. Each person participating in this study signed an informed consent, which followed University of South Florida (USF) Institutional Review Board (IRB) guidelines. Table 1 provides a summary of the physical characteristics of the participants. 14
Table 1. Means and standard deviations of participant age, height, weight and estimated body surface area by gender and overall. Number of Participants Age (years) Height (cm) Weight (kg) Body Surface Area (m 2 ) Males 11 28.0 9.5 176 11 81.9 .7 1.98 0.18 Females 4 23 4.7 165 6 64.2 18 1.70 .22 All 15 26.7 8.6 173 11 77.2 15.3 1.91 0.22 Each participant was acclimatized to dry heat for five days wearing athletic shorts and a T-shirt. Chamber conditions were set to a dry bulb temperature (T db ) of 50C and a relative humidity of 20%. After five days of acclimatization, each participant followed a random schedule of trials for metabolic rates and clothing ensembles. Equipment The experimental environment was controlled in a Forma Scientific climatic chamber where the humidity and dry bulb temperature were controlled. Environmental conditions were measured as dry bulb (T db ), psychrometric wet bulb (T pwb ), and globe temperatures (T g ) using mercury thermometers. The work consisted of walking on a motorized treadmill. Metabolic rate was assessed every 30 minutes by measuring the volume and composition of expired air using a Douglas Bag. The percentage of oxygen in the expired air was collected in the bag and analyzed with a Beckman E-2 Oxygen Analyzer, which was calibrated before each 15
experimental trial. The volume of expired air was measured using a Rayfield Dry Gas meter. The core temperature (T re ) was measured using a flexible YSI rectal thermistor (YSI 401AC) inserted 10 cm past the anal sphincter muscle. Thermistors were calibrated before each experiment. Skin temperatures at the chest (T ch ), arm (T arm ), thigh (T th ) and calf (T calf ) were measured using a YSI skin probe taped to the body at each location. The heart rate was measured using a Polar heart monitor. Clothing The five different clothing ensembles worn by participants during the study were cotton work clothes (6 oz/yd 2 cotton shirt and 8 oz/yd 2 cotton pants), cotton coveralls (9 oz/yd 2 ), particle barrier Tyvek 1427 made by Dupont, water-barrier/vapor-permeable NexGen LS417 manufactured by Kappler, and vapor-barrier Tychem QC made by Dupont. Experimental Procedure Prior to beginning the experimental protocol, participants underwent an acclimatization period for 5 days. Acclimatization consisted of walking on a treadmill at a metabolic rate of approximately 160 W m -2 in the climatic chamber (50 C, 20% rh) for 2 hours. Participants wore shorts, t-shirts and/or a sports bra. The target metabolic rates for the experimental trials were 115, 175, and 250 W m -2 These rates were chosen to provide a reasonable range of work demands centered on a moderate rate of work used in previous research. Prior to the first experiment, treadmill 16
speed and grade were adjusted for each participant to yield target metabolic rates and were used throughout the experimental trials. The 15 combinations of metabolic rate and clothing ensemble were randomized for each participant. The conditions for the progressive heat stress protocol began with a T db of 34 C, and a psychrometric wet bulb (T pwb ) at 25.5 C (relative humidity of 50%). When a physiological steady state was achieved, as evidenced by a steady T re T db was increased by 0.8 C every five minutes and the relative humidity was held at 50%. This increased heat stress by limiting evaporative cooling and increasing the dry heat gain. The participants were allowed to drink water or a commercial fluid replacement beverage at will throughout the experimental session. Environmental data and participant physiological data were collected and recorded every five minutes. The experiment continued until there was loss of thermal equilibrium evidenced by at least a 0.3 C increase in core temperature (T re ) within any continuous 15 minute interval, a T re of 39 C was reached, 90% of the age-estimated maximum heart rate was sustained, or the participant wished to stop the experiment. Determination of Critical WBGT The inflection point marks the transition from the work-driven zone to the environmentally-driven zone, which is the basis for WBGT occupational exposure limits. After the inflection point, core temperature continued to rise. Figure 1 illustrates core temperature versus time for one trial. The chamber conditions five minutes before the noted increase in core temperature was taken as the critical condition. Usually one 17
investigator noted the critical condition, and the decisions were randomly reviewed by a second investigator. The critical WBGT in C was computed as 0.7 (T pwb + 1.0) + 0.3 T g following the method described in OConnor and Bernard (1999). 36.537.037.538.038.50306090120Time (min)Tre (C) CriticalCondition Figure 1. The time course of T re for an example trial with an arrow to indicate the critical condition. 18
Results The study protocol called for assessing the critical WBGT and noting the concurrent heart rate, body core temperature and average skin temperature at the critical WBGT. Data was collected across 15 combinations of clothing and metabolic rate for 11 men and 4 women, the summary of p-values is shown in Table 2. Table 2. Summary of p-values for analyses p-Value Metabolic Rate Critical WBGT Heart Rate Core Temperature Ensemble 0.78 <0.0001 0.059 0.18 Metabolic Level <0.0001 <0.0001 <0.0001 <0.0001 Gender 0.0004 0.28 <0.0001 0.98 Subject <0.0001 <0.0001 <0.0001 <0.0001 Gender/ Metabolic Level 0.84 0.10 0.12 0.39 Gender/Ensemble 0.53 0.97 0.092 0.19 Metabolic Level /Ensemble 0.37 0.63 0.43 0.94 Gender/ Metabolic Level /Ensemble 0.73 0.43 0.17 0.37 19
Metabolic Rate Table 3 provides the mean metabolic rate with standard deviation for the combinations of Gender, Ensemble and Metabolic Level. Metabolic rates were compared by a 3-way ANOVA (participants nested in gender by Ensemble by Metabolic Level) with all two-way and the three-way interactions. There were significant differences, as shown in Table 2, between Genders (170 versus 184 W m -2 for females versus males) and among Metabolic Levels (114, 176, and 250 for Low, Moderate and High) as well as participants within Gender. Table 3. Means and standard deviations of metabolic rate (W/m -2 ) by level, clothing ensemble and gender. Metabolic Level Low Moderate High Ensemble Female Male Female Male Female Male Work Clothes 1109 12530 16224 17835 2474.8 25237 Coveralls 11749 11813 1582.3 18112 22621 24752 Particle Barrier 9811 11216 16915 18119 25834 24837 Liquid Barrier 9526 11615 18210 17614 24817 26232 Vapor Barrier 10016 12018 16230 18122 23216 25544 Gender by Metabolic Level 10427 11819 16719 17922 24230 25340 20
The relationships for metabolic rate between Genders for the three Metabolic Levels are illustrated in Figure 2 050100150200250300LowModerateHighMetabolic LevelMetabolic Rate Male Female Figure 2. Average metabolic rates for males and females at each metabolic level. 21
Critical WBGT Table 4 provides the critical WBGT for the combinations of Gender, Ensemble and Metabolic Level. Critical WBGTs were compared by a 3-way ANOVA (participants nested within Gender by Ensemble by Metabolic Level) with all two-way and the three-way interactions. From Table 2, there were significant differences for Ensemble and Metabolic Level as well as participants within Gender. There was no significant difference for Gender (p = 0.28). Table 4. Means and standard deviations of critical WBGT (C) by metabolic rate level, clothing ensemble and gender. Metabolic Level Low Moderate High Ensemble Female Male Female Male Female Male Work Clothes 36.91.0 36.51.2 33.40.9 33.71.6 30.50.6 31.31.8 Coveralls 35.81.3 35.71.5 34.01.3 33.51.2 30.31.4 31.21.9 Particle Barrier 36.20.9 35.51.8 33.10.5 33.71.5 29.52.2 31.11.4 Liquid Barrier 34.71.2 34.30.8 31.80.6 31.31.3 28.30.7 29.31.6 Vapor Barrier 30.92.0 30.71.6 26.42.5 27.51.6 25.02.0 24.42.2 Gender by Metabolic Level 34.92.6 34.52.5 31.73.2 31.92.8 28.72.6 29.53.2 Heart Rate Table 5 provides the mean heart rate with standard deviation for the combinations of Gender, Ensemble and Metabolic Level. Heart rates were compared by a 3-way ANOVA (participants nested in Gender by Ensemble by Metabolic Level) with all two22
way and the three-way interactions. There were significant differences for Gender and Metabolic Level as well as participants within Gender (Table 2). Table 5. Means and standard deviations of heart rate by level, clothing ensemble and gender. Metabolic Level Low Moderate High Ensemble Female Male Female Male Female Male Work Clothes 10820 10712 1225 11419 12161 12421 Coveralls 11726 10315 12212 11213 15015 12117 Particle Barrier 11819 10819 12720 11415 14417 12316 Liquid Barrier 11522 10916 12825 11111 12814 12920 Vapor Barrier 13030 10815 13218 11817 14924 12813 Gender by Metabolic Level 11823 10715 12716 11415 13930 12517 The relationships for heart rate by Gender for the three Metabolic Levels are illustrated in Figure 3. 100110120130140LowModerateHighMetabolic LevelHeart Rate (bpm) Male Female Figure 3. Average heart rates of males and females at the three metabolic levels. 23
Core Temperature Table 6 provides the mean core temperature with standard deviation for the combinations of Gender, Ensemble and Metabolic Level. Core temperatures were compared by a 3-way ANOVA (participants nested in Gender by Ensemble by Metabolic Level) with all two-way and the three-way interactions. There were significant differences for Metabolic Level as well as participants within Gender (Table 2). The average core temperature for males and females was 37.75C. Table 6 Means and standard deviations of core temperature by level, clothing ensemble and gender. Metabolic Level Low Moderate High Ensemble Female Male Female Male Female Male Work Clothes 37.40.3 37.50.2 37.70.3 37.80.1 37.80.3 38.00.3 Coveralls 37.60.2 37.30.2 37.70.1 37.70.4 37.90.2 37.90.2 Particle Barrier 37.50.3 37.40.4 37.70.4 37.80.3 38.00.3 37.90.3 Liquid Barrier 37.40.4 37.60.2 37.90.4 37.70.2 37.70.2 38.10.4 Vapor Barrier 37.70.4 37.70.3 37.90.3 37.80.4 38.10.5 37.90.4 Gender by Metabolic Level 37.50.3 37.50.3 37.80.3 37.80.3 37.90.3 38.00.3 24
Discussion To examine the possible effects of gender on the critical WBGT and physiological strain, trials were completed for 15 participants over 15 combinations of ensemble (5) and metabolic rate (3). Metabolic Rate There were significant differences in the three metabolic levels, which were 114, 176, and 250 W m -2 and this was part of the design. The differences among participants were expected, and the effects were minimized with the factorial design of the experiment. The difference between genders was important. Men were about 11 to 14 W m -2 greater than women at each level of metabolic rate. This might bias men to a lower critical WBGT and higher core temperatures and heart rates. Consequently, the interpretation of results must consider the differences in metabolic rate. Critical WBGT The critical WBGT was significantly different for ensemble and metabolic level as well as participants. These differences were expected. The critical WBGT values between genders was not significant, the females had an average critical WBGT of 31.9C-WBGT while the males had an average critical WBGT of 31.7C-WBGT. A regression of mean values for men yielded a slope of -0.037 C-WBGT/W m -2 In this 25
study the average metabolic rate was 12 W m -2 greater for men which would then lower the critical WBGT by 0.4C for men. By adding 0.4 C, it is reasonable to adjust for this bias, which would make the critical WBGT for men 32.1C. The adjusted difference is still the same absolute difference and therefore not significantly different. Heart Rate and Core Temperature With equivalent WBGTs and similar metabolic rates normalized to body surface area, a comparison of physiological strain between genders can be made. As shown in Figure 3, the heart rates for women are about 12 bpm higher than for men at each of the metabolic rate levels. In this study the men had greater normalized (and absolute) metabolic rates. A regression analysis on mean metabolic rates to mean heart rates yielded slope of 0.134 bpm/ W m -2 If the normalized rates had been the same, the mean male heart rate would be 2 bpm lower. The effect is small but the differences between men and women would have been greater. Using a similar study design as the current study, Kamon, Avellini and Krajewski (1978) reported a greater HR at the critical condition in females wearing cotton work clothes. On the other hand, Frye and Kamon (1981) reported equivalent HRs for acclimatized males and females at the critical condition. Their subjects, however, were matched on aerobic capacity which would reduce differences in thermoregulation and equalize heat strain between males and females. Studies that examined the gender differences in HR in semi-nude acclimatized (Moran 2000, Shapiro et al 1980) and unacclimatized subjects (McLellan 1998, Paolone et al 1978, Yousef et al. 1984) under controlled conditions of uncompensable heat stress reported higher HRs for females than males at a specific time into the trial. In looking at 26
gender differences in HR in response to compensable or uncompensable heat stress, the findings in the current study are in line with others in finding a higher HR response for women when there is no matching of subjects based on aerobic capacity. Core temperature presented a different profile. There was not a significant main effect for Gender. Inspection of Table 6 shows virtually no difference in T re between males and females at each Metabolic Rate Level. A regression analysis on mean metabolic rates to mean core temperature yielded slope of 0.0037C / W m -2 If the normalized rates had been the same, the mean male heart rate would be 0.04C lower. There was no absolute difference between genders and the standard deviation was 0.3C, a difference of 0.04C would not likely change the statistical determination of no difference. Others report a greater T re for females in hot-dry environments (Shapiro et al. 1980, Yousef et al. 1984) and a greater T re for males in warm-humid environments (Avellini et al. 1980, Shapiro et al. 1980). The results of our study suggest that T re was equivalent for both genders at the critical condition averaged over the Metabolic Rate Levels. Studies that evaluated T re under compensable heat stress conditions similar to those in the present study reported equivalent T re for males and females (Frye and Kamon 1981, Kamon et al. 1978). Frye and Kamon (1981) matched their acclimated subjects on aerobic fitness (VO 2 max = 54 and 56 ml kg -1 min -1 for females and males, respectively) and they were acclimatized. The matching removed an important difference due to gender and explained the absence of a difference in T re The lack of significance in the Kamon, Avellini and Krajewski (1978) study may have been due to the small sample size (4M and 4W). Our results differ from that of McLellan (1998) who studied unacclimatized female and male subjects wearing NBC clothing working in a hot-dry 27
environment (40C, 30% rh) for intermittent exercise up to 5 hours. Initial T re for males and females was not significantly different, yet T re was greater for females than males after 30 minutes of heat exposure. In summary, a difference was not observed in core temperature between genders; this equivalence was observed in two other studies that used a similar protocol to the current study. Studies of uncompensable heat stress did show differences. Conclusion The main objective of this study was to investigate if there are gender differences for critical WBGT, heart rate and core temperature at the upper limit of compensable heat stress across three levels of metabolic rate and five levels of clothing. There was no statistical difference observed between genders in critical WBGT and core temperature. The only outcome that was statistically different was heart rate. The heart rate was significantly higher (12 bpm) for women than for men. Overall, women had the same upper limit of the prescriptive zone as men, their core temperatures were the same at this limit but women had a greater cardiovascular cost reflected in a higher heart rate. 28
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