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Physiological responses of men during the continuous use of a portable liquid cooling vest

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Title:
Physiological responses of men during the continuous use of a portable liquid cooling vest
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English
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Medina, Theresa J
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University of South Florida
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Subjects / Keywords:
heat stress
metabolic rate
microclimate cooling
heat sink
heat storage
Dissertations, Academic -- Public Health -- Masters -- USF   ( lcsh )
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government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

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Summary:
ABSTRACT: Heat stress is a well documented hazard across industries. The combination of environmental conditions, work demands, and clothing contribute to heat strain. Left unchecked, heat strain causes changes in an individual's physiological state that can lead to serious and fatal conditions with little warning. Although engineering and administrative controls are the first choice to abate this hazard, they frequently are not feasible. In these cases, personal cooling is often employed. There are three main types of personal cooling: liquid, air, and passive. Each has its own advantages and disadvantages. This study focuses on continuous cooling using a portable liquid cooling system (LCS). The LCS used a vest with tubes circulating water from an ice heat sink. The experiment consisted of five males each completing seven tests in random order. The subjects wore work clothes as the control then in conjunction with a firefighter, vapor barrier, and bomb suits. Each suit was tested with and without the benefit of the LCS. All of the tests took place at 35oC dry bulb and 50% relative humidity while attempting to walk 90 minutes on a treadmill at a 300 W metabolic rate. The study found continuous use of the LCS significantly reduced heat storage (S) and the rate of rise of heart rate (rrHR), core temperature (rrTre), and mean skin temperature (rrTsk) for the firefighter and vapor barrier suits as compared to no-cooling. Although the LCS didn't significantly affect the rate of rise for physiological responses with the bomb suit, it did however, significantly increase the endurance time. Interestingly, the study also found when wearing either the vapor barrier or firefighter suits in conjunction with the LCS that the rrHR and rrTre were not significantly different from only wearing work clothes.
Thesis:
Thesis (M.S.P.H.)--University of South Florida, 2004.
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Includes bibliographical references.
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by Theresa J. Medina.
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Title from PDF of title page.
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Document formatted into pages; contains 38 pages.

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notis - AJS2488
usfldc doi - E14-SFE0000444
usfldc handle - e14.444
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Physiological Responses of Men During the Continuous Use of a Portable Liquid Cooling Vest by Theresa J. Medina A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Public Health Department of Environmental and Occupational Health College of Public Health University of South Florida Major Professor: Thomas E. Bernard, Ph.D. Candi D. Ashely, PhD. Steve Mlynarek, PhD. Date of Approval: July 12, 2004 Keywords: heat stress, metabolic rate, micr oclimate cooling, heat sink, heat storage Copyright 2004, Theresa J. Medina

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ACKNOWLEDGMENTS Foremost, I would like to acknowledge th e United States Air Force for providing me the opportunity to pursue my advanced de gree. I want to tha nk the senior officers within the Biomedical Sciences Corp and Bioenvironmental Engineering that had the confidence in selecting me for this assignment. In addition to my senior leadership, I would like to express my gratitude to all t hose in the military I worked with over the years. Each of them ultimately helped me get selected to complete an advance degree. I would like to acknowledge the funding support provided by Med-Eng CardioCOOLTM for without, this research could have not been accomplished. On a more personal level, I am gratef ul to my husband and loving family and friends that provided support thr oughout my program. I would also like to thank the Heat Stress Laboratory workers especially Bumni Oladinni and Christina Luecke along with the study participants for th eir support and time.

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i Table of Contents List of Tables................................................................................................................. .....ii List of Figures................................................................................................................ ....iii Abstract....................................................................................................................... .......iv Introduction................................................................................................................... .......1 Literature Review.............................................................................................................. ...4 Liquid Cooling Systems...........................................................................................4 Air Cooling Systems................................................................................................6 Passive Cooling Systems.........................................................................................7 Intermittent Versus Continuous Cooling.................................................................8 Advantages and Disadvantages of the Different Cooling Systems.........................8 Previous Reports......................................................................................................8 Need for Further Research.......................................................................................9 Methods........................................................................................................................ ......11 Subjects..................................................................................................................11 Experimental Conditions.......................................................................................12 Equipment and Materials.......................................................................................12 Experimental Protocol...........................................................................................13 Results........................................................................................................................ ........16 Discussion..................................................................................................................... .....24 Vapor Barrier Suit..................................................................................................24 Firefighter Turnout Gear........................................................................................25 Bomb Suit..............................................................................................................26 Comparing Work Clothes to PPE with Cooling....................................................27 Conclusions............................................................................................................28 References..................................................................................................................... .....30

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ii List of Tables Table 1 Reports of Personal Cooling Types and Experimental Conditions...........10 Table 2 Types of Tests Performed and Cooling Condition....................................13 Table 3 Results of a 3 Way ANOVA fo r Cooling, Clothing, and Subjects with Respective p-Values..........................................................................19 Table 4 Results of an A Priori Pair ed t-Test for Each Clothing Type Against Cooling Condition with Respective p-Values..............................19 Table 5 Results of an A Priori Pair ed t-Test Compari ng Work Clothes to PPE Plus LCS with Respective p-Values..............................................23

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iii List of Figures Figure 1. Mean Rate of Rise in H eart Rate (rrHR) When Comparing Each Clothing Ensemble Without Cooling to Cooling..............................20 Figure 2. Mean Rate of Rise in Core Temperature (rrTre) When Comparing Each Clothing Ense mble Without Cooling to Cooling.......................................................................................................21 Figure 3. Mean Rate of Rise in Skin Temperature (rrTsk) When Comparing Each Clothing Ense mble Without Cooling to Cooling.......................................................................................................21 Figure 4. Mean Heat Storage (S ) When Comparing Each Clothing Ensemble Without Cooling to Cooling......................................................22 Figure 5. Mean Endurance Time Wh en Comparing Each Clothing Ensemble Without Cooling to Cooling......................................................22

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iv Physiological Responses of Men Duri ng the Continuous Use of a Portable Liquid Cooling Vest Theresa J. Medina ABSTRACT Heat stress is a well documented hazard across industries. The combination of environmental conditions, work demands, and cl othing contribute to heat strain. Left unchecked, heat strain causes changes in an individual’s physiological state that can lead to serious and fatal conditions with l ittle warning. Alt hough engineering and administrative controls are the first choice to abate this hazard, they frequently are not feasible. In these cases, personal cooling is often employed. There are three main types of personal cooling: liquid, air, and pa ssive. Each has its own advantages and disadvantages. This study focuses on continuous cooling using a portable liquid cooling system (LCS). The LCS used a vest with tubes ci rculating water from an ice heat sink. The experiment consisted of five males each co mpleting seven tests in random order. The subjects wore work clothes as the control then in conjunction with a firefighter, vapor barrier, and bomb suits. Each suit was tested with and without the benefit of the LCS. All of the tests took place at 35oC dry bulb and 50% relative humidity while attempting to walk 90 minutes on a treadmill at a 300 W metabolic rate. The study found continuous use of the LCS significantly reduced heat storage (S) and the rate of rise of heart ra te (rrHR), core temperature (rrTre), and mean skin

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v temperature (rrTsk) for the firefighter and vapor barrier suits as compared to no-cooling. Although the LCS didn’t significantly affect the rate of rise for physiological responses with the bomb suit, it did however, signi ficantly increase the endurance time. Interestingly, the study also found when weari ng either the vapor barrier or firefighter suits in conjunction with th e LCS that the rrHR and rrTre were not significantly different from only wearing work clothes.

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1 INTRODUCTION A wide range of occupations incl uding firefighters, HAZMAT workers, and explosive ordinance technician s have potentially dangerous heat stress exposures. Heat stress is the net load on the worker from th e metabolic demands, environmental factors, and clothing. Increasing the work load w ill increase the metabolic rate and in turn generate heat in the body. Air temperature, movement, and humidity along with radiant heat exchange are all environmental factors th at contribute to heat stress. Clothing can drastically alter the heat stress an individual experi ences. Unfortunately, many occupations require additional layers of personal protective equipment (PPE) as a barrier against hazards that cannot otherwise be controlled. PPE is often multilayered, impervious to water vapor and air, encapsulating, and thermally insulated. This drastically affects heat stress by significantly reducing the ability of the body to cool itself through the evaporation of sweat. The internal temperature of the human body remains fairly constant even when exposed to widely varying environmental condi tions. Safe limits for the fluctuation of core temperatures are small, and therefore, the body must get rid of excess heat to keep the internal temperature within safe limits. The primary mechanisms the body uses to maintain heat balance are to vary the rate and amount of blood circulating to the skin by increasing the heart rate and to release water onto the skin through sweat glands. As the sweat evaporates, the skin cools thereby elim inating large quantities of heat from the

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2 body. In order to achieve the cooling effects from sweating, sweat must be removed by evaporation. High humidity environments or protective clothing with high evaporative resistance may significantly diminish evapor ation and the body’s ability to dissipate excess heat. These defensive mechanisms of the body can also cause adverse effects. With large amounts of blood going to the skin and less to active muscles and the brain, muscle strength and alertness may decline. Left unchecked, heat strain can lead to serious and even fatal conditions sometimes with little warning. The exchange of heat between the body and the environment is governed by the fundamental laws of thermodynamics. A co mmon equation employed to express heat stress is the heat balance equation(1): S = M + C + R – E (1) The change in body heat storag e (S) is a function of the me tabolic rate (M), convective heat exchange (C), radiant heat exchange (R ), and evaporative heat loss (E). Whenever the change in body heat storage is positive, the individual is gaining heat. Engineering controls are often employed to control heat gains. In the case of metabolic heat gains, work stations can be designed to limit the physical effort the employee must use to perform the job. Conv ective heat gain can be reduced by lowering the air temperature so that the environm ental temperature is less then the skin temperature and increasing air ve locities. Radiant heat gain is typically controlled with shielding to block heat flow. In addition, administrative controls are used. These include frequent breaks and monito ring both environmental and phys iological conditions. The American Conference of Governmental I ndustrial Hygienists (ACGIH) has published guidelines to determine work-rest cycles wh en evaluating work load and environmental

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3 conditions. Unfortunately, as in all disciplines of industrial hygiene, it is not always technically or economically possible to limit excessive heat stress by the use of engineering and administrative controls. Wh en engineering and administrative controls do not adequately reduce heat stress, personal protective equipment is necessary. Cooling garments are typically used to meet this need. It is often impossible to implement ad equate engineering and administrative controls for firefighters who must wear in sulating turnout gear, enter extremely hot environments, and perform heavy labor. The same can be said of explosive ordinance personnel who are required to wear heavy bomb suits to protect from flying debris and the impact of an explosion. Many jobs require the use of a chemical resistance suit to protect the skin. This vapor barrier causes evaporative re sistance which reduces cooling by evaporation. When wearing turnout gear, a bomb suit, or vapor ba rrier suit, a cooling garment is one approach to decrease heat st rain. This would a llow the individual to perform work longer with reduced risk of excessive heat strain.

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4 LITERATURE REVIEW Since effective engineering and administrati ve controls are ofte n not feasible for chemical, physical and biological agents, num erous industries require personal protective equipment (PPE) to protect their workers. Although PPE can protec t an individual from dangerous environments, it frequently has hi gh insulating and low moisture permeability properties. Therefore, the use of PPE often in troduces or increases the potential of a heat stress hazard. This is especially true when working in hot environments and under a heavy work load. Guidelines have been de veloped to help employers determine safe working conditions by recommending work -rest cycles based on environmental conditions, degree of worker activity, and the use of PPE.(2) This approach is not always desirable, because it extends the time to complete work, increases the need for more manpower, and can require excessively long rest periods. Additionally, as in the case of an explosive ordinance techni cian, mission requirements can in terfere with taking breaks at the recommended intervals. One approach to this dilemma is to use a personal cooling system. Ideally the cooling system maintain s the body’s heat balance or at least extended endurance time by slowing the physio logical responses to heat st ress. In general there are three types of personal cooling system s: liquid, air, and passive. Liquid Cooling Systems Liquid cooling systems (LCS) ope rate on the principle of conduction. The cooling potential varies by desi gn and is determined by the heat exchange characteristics

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5 of the liquid and by thermal capacity (produc t of mass flow and specific heat). LCS conduct heat from the skin to cooler liquid contained in tubes sewn throughout fabric garments. The liquid then travels by a power ed pump through the garment to a heat sink (usually ice). The style of th e garment can be a vest, suit, or shirt which may or may not include a hood. Studies have shown increa sing the body surface area covered by the LCS; that is, increasing the area of conducti on, increases the heat transfer rate.(3-4) Higher flow rates help to maintain the temperature gr adient between the skin and the liquid. Increasing the flow rate assist s in maximizing cooling by conduction and the rate of heat transfer.(4) Similarly, the temperature gradient is widened and the cooling potential is increased by lowering the inlet temperature of the liquid.(3-4) Since the amount of heat generated in the body is proportional to the workload, the LCS is likely limited by the rate of heat transfer and the capacity of the heat sink.(4) When the air temperature is higher than the liquid coolant, the coolant can gain heat from the air. This reduces the cooling efficiency of the LCS. Clothing has an insulating effect a nd can reduce the heat transfer from the environment to the cooling system.(3-4) Although each LCS’s design can affect the degree of coo ling potential, several studi es found LCS significantly lowered physiological responses and heat storage while increasing endurance time.(5, 6) Constable et al. studied the effects of a LCS vest during the resting phase, and found it significantly reduced heat storage, near ly doubled endurance time, and developed a perceived cooling effect for the participants.(5) Cadarette et al. st udied a shirt and hood configured LCS. The test took place at mode rate metabolic rates, in hot environmental conditions, and during short work and rest periods. The study compared two types of toxicological suits (both similar to a le vel B HAZMAT suit). The newer type suit

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6 weighed 4.5 kg less and used a LCS, the traditio nal type suits used no cooling. The study found that although the metabolic rate was greate r for the newer type suits, the endurance time was twice as long and the physiological responses to heat stress were reduced.(6) Heled et al. also performed a study using a LCS, this time consisting of a vest plus a hood with dry ice as the heat sink. The study compared the eff ects of the LCS to a passive cooling system (see below). The experimental conditions were in a hot environment with a long work period. The study did not compare the results to a control nor did it mention the work load.(7) Harrison et al., studie d continuous cooling from a LCS, but the subjects were in a resting phase and tethered to a stationary cooling system during the entire experiment.(3) Air Cooling Systems Air cooling systems (ACS) operate on th e principle of c onvection and sweat evaporation by using a power source to circul ate air under clothing. The circulating air temperature must be lower then the skin te mperature for cooling to occur by convection. As the temperature gradient between the skin and air increases, the rate of cooling increases. If the skin is wet, evaporativ e cooling can also occur. A vortex is often employed to generate cooler air and assist in the cooling. As the inlet temperature of the circulating air lowers, heat tran sfer improves between the skin and air. This is also true when lowering the water vapor pressure. As the water vapor pressu re gradient between the skin and air increases, th e rate of evaporation increase s leading to enhanced cooling.(4, 8) ACS have been compared to LCS when in a hot environment (50C, 30% RH) with resting metabolic rates. After four hours, both systems significantly reduced physiological responses to heat stress and both had similar core temperatures.(9) Another

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7 study found ACS and passive cooling systems wh en under moderate temperatures (28C, 22C wet bulb) and a high metabolic rate (430 W) provide si milar physiological responses which were both signi ficantly better than no-cooling.(10) Passive Cooling Systems Passive cooling systems (PCS) do not requ ire power. Two PCS designs are the ice vest and water spray suit. Ice vests are the most common type of PCS and operate on the principle of conduction, by placing a heat si nk in direct contact with the body. Body heat is conducted directly to the heat sink (usually water ice). As the su rface area between the skin and heat sink increases so does the rate of heat transfer.(11-12, 4) The metabolic rate is inversely proportional to the service time of th e heat sink. The quicker the metabolic rate increases and generates heat the quicker the heat sink is spent.(13) Also, the heat sink service time is directly proportiona l to its heat absorbing capacity.(11-12) The insulating factor of clothing helps to reduce the loss of c ooling potential to the environment. This is why many vests are insulated. Water spray suits operate on the princi ple of cooling by evaporation. This procedure requires a water evaporative cotton suit to be wetted periodically with water. Unlike the majority of cooling systems, the su it is worn over protective equipment rather then under. The attenuation of heat strain using this method was found to be comparable to the LCS during the first hour of exer cise and better during the second hour.(7) It was suggested the evaporative suit was more eff ective in the second hour because the heat sink may have been exhausted in the LCS.

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8 Intermittent Versus Continuous Cooling Highly mobile jobs make cooling through a stationary cooling system connected by a tether impractical. Therefore, many st udies were conducted on intermittent cooling during the resting phase only. Portable LCS ha ve increased the potential for continuous cooling. Subjects have shown they are better able to maintain thermal equilibrium with continuous cooling.(8) Advantages and Disadvantages of the Different Systems Each cooling system type has inherent advantages and disadvantages. LCS minimizes the potential for a contamination ri sk, because they are a closed loop system and are often portable. On the other ha nd, LCS can weigh more than other cooling systems. ACS will keep users drier and depending on design may reduce facial sweating and eye irritation. Unfortunately, ACS usually do not have a portable unit to cool air and require individuals to connect to a stationa ry unit during rest. PCS are inexpensive, simple, and easy to maintain. The main disadvant age, in the case of the ice vest, is users must doff any clothing over the PCS to switch out the heat sinks. This could be time consuming, especially if decontamination pr ocedures are required before doffing. The water spray PCS requires access to enough water to take periodic 30 second showers. In addition, clothing worn under the evaporativ e suit will affect the cooling potential. Previous Reports There are many reports testing the effectiven ess of the different cooling systems. Since the effectiveness of a cooling system is influenced by many different variables, it is important to know and understand each when co mparing studies. The cooling type must be known, because as discussed above, there are advantages and disadvantages in the

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9 application of each type. In addition to knowing the cooling type, the style should be known. This is needed since it helps determ ine which areas are exposed to the cooling elements. The clothing worn during the expe riments can either as sist in cooling by reducing the loss of the heat sink potential to the environment or hinder cooling by preventing evaporation. For th is reason clothing must be ev aluated. Heat sinks can be exhausted quicker by higher metabolic rates; therefore, it is imp erative to know the metabolic rate along with the length of the test, and lengt h of the work/rest cycles. Obviously, hot environments will require more cooling; therefore, the environmental conditions should be known. Finally, differenc es in physiological re sponses can occur if continuous or intermittent cooling is perfor med. Although Table 1 is not an exhaustive list of studies, it helps comp are the different types of re search conducted on cooling systems by summarizing the study, type and style of cooling systems, the clothing worn, if cooling was continuous or intermittent, wo rk load, length of work-rest cycles, total length of test, and environmental conditions. All of the reports in Table 1 found cooling systems can increase endurance time and reduce physiological responses to heat stress. Need for Further Research The primary purpose of this study was to determine the efficacy of the Med-Eng CardioCOOLTM liquid cooling system in a hot envi ronment while wearing different types of PPE and performing long uninterrupted wo rk. A secondary purpose was to compare the physiological responses a nd endurance time of wearing a LCS and PPE to wearing work clothes only.

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10 Table 1. Reports of Personal Coolin g Types and Experimental Conditions Study Clothing Type Style C/Ia Mb W/Rc Timed Temp/RHe Harrison & Belyavin(3) Flight suit LCS Suit C Resting Rest only 60-240 ------------LCS Speckman et al.(4) CDEf ACS Varied C/I Varied Varied Varied 29/85 to 52/25 Constable et al.(5) CDE LCS Vest I 400/475 30/30 286 38/26wbg 31wbgth Cadarette et al.(6) Army A+Bi LCS Shirt + hood C 222-278 20/10 120 38/30 LCS Vest + hood Heled et al.(7) CDE PCSspray Suit C ---------55/10 125 35/40 45/15 28/22wb Bomalaski et al.(8) CDE ACS Vest C/I ---------30/30 240 38/26wb LCS Vest/ hood ACS Vest/ hood Epstein et al. (9) Coverall, helmet, boots PCS Vest C Resting Rest only 240 50/30 LCS Bishop et al.(10) CDE ACS Vest I 430 45/15 240 28/22wb 26wbgt Konz et al.(11) None/ Jacket PCS Vest C Resting Rest only 100-240 43.5/45 or 55 Kamon et al.(12) Coverall PCS Shirt C 200-300 5/5 135 55/28 a. Intermittent or continuous cooling f. Air Force chemical defense ensemble b. Metabolic Rate in watts g. Wet bulb temperature in degrees Celsius c. Work/Rest cycle in minutes h. Wet bulb global temperature in degrees Celsius d. Time in minutes i. Level A and B hazardous material suits e. Temperature in degrees Celsius a nd relative humidity in percent

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11 METHODS Clothing has a large impact on how the body responses to heat stress. Light weight, loose fitting clothing is ideal for cooling by evaporation and conduction, because it permits air to circulate ove r the skin. On the other hand, PPE often has high insulating and impermeable properties. These pr operties not only prevent cooling from environmental air, but can increase the humidity and temperature underneath the PPE. As the temperature and humidity increase, th e physiological responses to heat stress will also increase. This study implemented a LC S to see how it affected the physiological responses and endurance time. It also compar ed heat strain responses between wearing work clothes only, and work clothes plus PPE and the LCS. Subjects Five healthy males completed all seven te sts in the study. The mean standard deviation (SD) age, height, weight and body surface area were 32.5 9.8 years, 179.6 3.6 cm, 91.2 8.1 kg, and 2.1 0.08 m2, respectively. This research project was approved by the Institutional Review Board of the University of S outh Florida according to the guidelines of the National Institutes of H ealth to ensure subject safety. Prior to this experiment each volunteer signed an info rmed consent and underwent a physical examination by a physician.

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12 Experimental Conditions Tests were performed in a controlled envi ronmental chamber with the ambient air temperature of 35C 0.5C, relative humidity of 50% 2% and a target metabolic rate of 300W. Equipment and Materials The PPE selected for this study not only re presents a wide range of industries, but also a wide range of insulating and perm eability properties. Listed below is the equipment worn in the experiments. Undergarments o T-shirt o Athletic shorts o Men’s underwear o Men’s Athletic socks o Athletic Shoes Work clothes o Undergarments o Long sleeve shirt o Pants Protective Clothing Ensembles o Explosive ordnance disposal suit in cluding helmet (Bomb Suits): MedEng Systems Canada, model EOD 8, NATO Stock # 8470-21-920-2137 o Firefighter turnout suit and hat: Morning Pride Manufacturing, model 1430, meets NFPA 1971 (1986 Edition) o Vapor barrier suit: Polyethylene-coated Dupont Tychem QC coverall with hood Cooling System o Cooling vest: Med-Eng CardioCOOLTM o Portable cooling unit: Med-Eng PortaCOOLTM The seven tests performed and the combin ations of clothing and cooling condition are listed in Table 2. Undergarments and work clothes were worn for all tests.

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13 Table 2. Types of Test Perf ormed and Cooling Condition Protective Clothing Cooling Garment Acronym Work Clothes No WC-NC Vapor Barrier No VB-NC Vapor Barrier Yes VB-C Firefighter Turnout No FF-NC Firefighter Turnout Yes FF-C Bomb Suit No BS-NC Bomb Suit Yes BS-C Experimental Protocol Participants completed a heat acclimatiza tion protocol prior to performing tests. The heat acclimatization protocol consisted of walking on a treadmill at 2.5 mph at 0% grade while wearing undergarments for 5 consecutive days. This occurred at approximately the same time each day in an environmental chamber set to 50C and 20% relative humidity. Subjects were allowed to drink water at will. During this protocol, the initial treadmill speed was set to obtain the target 300 W metabolic rate for the experimental tests. As an alternative to acclimatization, one subject performed tests at intervals no more frequent then than every other day. The time lapse was to prevent acclimatizing from the tests and changing th e individual’s response to heat stress. The experimental protocol i nvolved seven tests per particip ant. The first test was WC-NC for four of the five participants. Th e first test allowed participants to become familiar with the testing protocol without the extra burden of the protective gear. The six remaining tests were performed in random orde r. Respirators were not worn during the tests, because they would interfere with equi pment used to measure metabolic rate. Each subject was instructed to avoid m oderate to high-level exercise 24 hours prior to each test. They were also instructed not to take stimulants or diuretics 12 hours

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14 prior to testing or larg e meals 2-3 hours prior to testing. In addition, they were instructed to maintain normal hydration. Individuals were weighed semi-nude (underg arments only) before each test. They were then connected to probes to measure rect al core temperature, heart rate, and skin temperature. Eight skin s ites specified by ISO 9886 (forehea d, right scapula, left upper chest, upper right arm, lower left arm, left hand, right anterior thi gh, and left posterior calf) were measured. If the test included th e cooling garment, the individual selected the best fit size and donned the vest over the t-shir t. Next the subject put on the work pants and the appropriate protective clothing ensemble The subjects were able to select the size of the vapor barrier and fire fighter turnout gear. If the protocol included the cooling garment, the portable cooling unit (pump and heat sink) was attached over the protective garments. This allowed for easy access during the test. The portable cooling unit’s bottle was filled with 1650 – 1800 ml of water and froze n. Just prior to the test, the remaining 2 L volume in the bottle was filled with cool water. The participant was weighed with the cooling garment, cooling unit, and clot hing ensemble to obtain the clothed weight. Next the subject entered the environmen tal chamber and was connected to the monitoring devices. The heart rate and temperatures were noted. The treadmill was set to obtain the target metabolic rate and the individual began exercising. The heart rate (HR), core temperature (Tre), and skin temperatures (Tsk) were recorded every five minutes. The intent was to change the heat sink when the ice completely melted in the bottle. Due to the configuration of th e bottle inside the pouch it was difficult to determine when all the ice had melted. In act uality, the heat sinks were always changed with ice remaining in the bottle. The h eat sink change occurred while the subject

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15 continued to walk. If the HR exceeded 95% of the age predicted maximum or if the Tre exceeded 38.5C before 90 minutes, the exerci se phase was terminated. It was also stopped if the subject reported excessive fatigue faintness, headache, disorientation, or if the subject requested to stop. The metabolic rate was measured at 15 mi nutes into the exercise phase and then every 30 minutes thereafter. The subject’s metabolic rate was cal culated by capturing and measuring the exhaled air over approxi mately two and a half minutes using the Douglas Bag method.(14) After the termination of the ex ercise, there was a 30 minut e recovery phase. The recovery phase took place while sitting in side the environmental chamber. The individuals undid zippers, opened the protective suit, and removed protectiv e head gear to assist in cooling. If the test consisted of cooling, the subject continued to wear the LCS during the resting phase. The HR, Tre, and Tsk were still measured and recorded every five minutes. If the HR exceeded 95% of the age predicted value or if the Tre rose above 39.0C, the test was terminated. The metabo lic rate was measured midway through the recovery phase. The individual was allowed to drink up to 350 ml of cool water with no ice. Theoretical amount consumed was recorded. The participant was weighed to get the posttest clothed weight. The subject then doffed the protective gear, work clothes, and probes to obtain the post semi-nude weight.

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16 RESULTS Three major factors influence heat stre ss: environmental conditions, workload, and clothing. In this study, environmental conditions remained constant. This ensured significant changes in physiological responses to heat stress between tests could not be contributed to the environment. In an atte mpt to control for the workload, the treadmill speed was set for a target 300 W metabolic rate. With similar metabolic rates, differences in physiological responses betw een tests were not lik ely a result of the workload. The clothing ensembles were quite different between protocols. The vapor barrier suit had a much higher evaporative resistance, the firefighter turnout suit was more insulating, the 75 lbs bomb suit was heavie r, and the work clothes was the least of all these properties. The different propertie s of the clothing could affect the metabolic rate and in turn the level of heat strai n. To compare among clothing types the metabolic rates had to be similar. Therefore knowing the environment, workload, and clothing did not significantly contribute to changes in physiological responses to heat stress, the changes could then be contributed to the use of the cooling garment. The workload was determined by measuring the metabolic rate(14) and dividing it by the subject’s body surface area (MSA). A three way analysis of variance (ANOVA) was performed with cooling condition, clothi ng type, and subject identification as the three independent variables. An = 0.05 level of significance wa s selected. The analysis found the MSA was not significantly different when comparing cooling status (p=0.64),

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17 but significantly different when comparing cl othing type (p=0.002) a nd subject (p<0.001). A Tukey’s hsd analysis was performed for th e MSA when looking at clothing type. The analysis found work clothes, the vapor barrier su it, and the firefighter gear to have similar MSA’s, but not similar to the bomb suit. Therefore, knowin g the environmental conditions did not change and there was no st atistically significant change in workload between cooling conditions, any significant difference in the subject’s response to heat stress could then be contributed to the cooling unit when comparing similar clothing types (work clothes, vapor barrier suit, and firefighter turnout). The mean Tsk (mean skin temperature) was calculated using the ISO 9886 Standard(15): Tsk = 0.7 Tforehead + 0.175 Tchest + 0.05 Thand + 0.19 Tthigh + 0.175 Tscapula + 0.2 Tcalf + 0.07 Tarm + 0.07 Tforearm (2) Due to malfunction of the skin probes in so me trials, data were missing for one site during six tests and two sites during one test. When there were missing data during a nocooling test, values were a ssigned to the missing data by ta king the sum of the recorded values times the respective weighting factor and dividing the sum by the total of the weighting factors. During tests using the LCS, all of the missing data were in areas not in contact with the LCS. In these cases, valu es were assigned to the missing data by taking the sum of the recorded values not in contac t with the LCS (i.e. excluding the chest and scapula) times the respective weighting fact or and dividing the su m by the total of the weighting factors for the no-cooled sites. Af ter values were assigned to the missing data, the Tsk, using ISO equation was calculated.

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18 The average heat storage (S) in W m-2 for each clothing type and cooling condition was calculated using the formula(16): S = [(mb cd)/AD] *( Tb/ t) (3) Where mb is the mean body weight [kg]; cd is the specific heat constant 0.965 [W h-1 C-1 kg-1]; AD is the DuBois surface area [m2]; Tb is the change in mean body temperature [C] where Tb = 0.2 Tsk + 0.8 Tre; and t is the elapsed time [h]. The null hypothesis of this wo rk is that the LCS does not reduce heat strain. It was tested by checking for significant differences in the subjects’ endurance time and physiological responses to heat stress when wearing the LCS as compared to no-cooling for each clothing type. The physiological respon ses evaluated were mean heat storage (S) and mean rate of rise in heart rate (rrHR), core temperature (rrTre), and skin temperature (rrTsk). The rates of rise in the physiological responses were calculated by measuring the response at the termination of the exercise a nd subtracting the respons e recorded after the initial five minutes then di viding the difference by the elap sed time. The physiological responses at five minutes were used to a llow a physiological steady state due to work rather than heat stress. In reviewing the results, a univariate analysis was performed on the data to check for frequency consistency and to identify ex treme outliers. The frequencies were as expected and no outliers were identified. Next, the physiological responses and endurance time were checked for interactions of clothing (3 leve ls: vapor barrier, firefighter, and bomb suit) by cooling (2 leve ls: No and Yes) using an Analysis of Variance (ANOVA). Work clothe s were not included in the in teraction analysis, because there was only a no-cooling trial. There we re no significant interac tions. The lack of

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19 significance showed there were no synergistic effect s between clothing type and cooling condition. Once again a three way ANOVA was performed, this time using the work clothes as the control in the clothing types. This analysis was performed to determine if there was a significant difference in the endura nce time and physiological responses when looking at the cooling status, cl othing type, and subject. The results are listed in Table 3. Table 3. Results of a 3 Way ANOVA for C ooling, Clothing, and Subject with Respective p-Values Response Cooling Clothing Subject rrHR <0.0001a <0.0001 0.0365 rrTre <0.0001 <0.0001 0.0223 rrTsk <0.0001 <0.0001 0.0648 S <0.0001 <0.0001 0.1814 Time 0.0010 <0.0001 0.1183 a. Shaded areas are significant p-values There were statistically significan t differences in the rrHR, rrTre, rrTsk, and endurance time in the cooling condition and clot hing type. From this, it appears using the cooling ensemble affects the rrHR, rrTre, rrTsk, S, and time. To further evaluate the significance of th ese differences, a paired t-test on the a priori comparisons of interest was performed. The paired t-test analyzed each clothing type against cooling condition, checking for significant differences in physiological responses and endurance time. Th e results are listed in Table 4. Table 4. Results of an A Priori Paired tTests for Each Clothing Type Against Cooling Condition with Respective p-Values Response Vapor Barrier Firefighter Bomb Suit rrHR 0.0005a 0.0458 0.2956 rrTre 0.0051 0.0111 0.1929 rrTsk 0.0281 0.0135 0.0634 S 0.0142 0.0080 0.1133 Time 0.0714 0.3739 0.0143 a. Shaded areas are significant p-values

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20 The results revealed that the cooling en semble significantly changed the rate of rise of heart rate, core temp erature, and skin temperature for the firefighter and vapor barrier suits. It only had a significant impact on the endurance time for the bomb suit. Therefore, the null hypothesis was rejected for the firefight er and vapor barrier suits’ physiological responses. The null hypothesis was accepted for the bomb suit’s physiological responses, but was re jected for the response time. The mean one standard deviation we re calculated for each of the above physiological response and depict ed in Figures 1– 5. The fi gures show there was a large variation among the subjects for the enduran ce time and physiological responses while wearing the bomb suit. The large standard deviation among subjects while wearing the bomb suit may have contributed to the lack of significant betw een cooling statuses. Figure 1: Mean Rate of Rise in Heart Rate (rrHR) When Co mparing Each Clothing Ensemble Without Cooling to Cooling. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 WCVB ***FF *BSClothing TypesMean rrHR (bpm/min) Cooling No CoolingNote: (0.01 < p < 0.05); ** (0.01 < p < 0.001); *** (p < 0.001)

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21 Figure 2: Mean Rate of Rise in Core Temperature (rrTre) When Comparing Each Clothing Ensemble Without Cooling to Cooling. 0.000 0.005 0.010 0.015 0.020 0.025 0.030 WCVB **FF *BSClothing TypeMean rrTre (C/min) No Cooling Cooling Note: (0.01 < p < 0.05); ** (0.01 < p < 0.001) Figure 3: Mean Rate of Rise in Skin Temperature (rrTsk) When Comparing Each Clothing Ensemble Without Cooling to Cooling. -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10 WCVB *FF *BSClothing TypeMean rrTsk(C/min) No Cooling Cooling Note: (0.01 < p < 0.05)

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22 Figure 4: Mean Heat Storage (S) When Comparing Each Clothing Ensemble Without Cooling to Cooling. -20.00 0.00 20.00 40.00 60.00 80.00 100.00 WCVB*FF**BSClothing TypeHeat Storage (W/m^2) No Cooling Cooling Note: (0.01 < p < 0.05); ** (0.01 < p < 0.001) Figure 5: Mean Endurance Time When Co mparing Each Clothing Ensemble Without Cooling to Cooling. -20 0 20 40 60 80 100 WCVB *FF *BSClothing TypeMean Time (minutes) No Coolong Cooling Note: (0.01 < p < 0.05)

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23 This study found the portable cooling vest si gnificantly reduced the subjects’ heat strain by reducing rrHR, rrTre, rrTsk, and S for the vapor barrie r and firefighter suits and significantly increased the enduran ce time for the bomb suit. Next, an interesting comparison was pe rformed between the work clothes and cooling with the firefighter and vapor barrier suits. Since the bomb suit did not have a MSA similar to work clothing it was not include d in this evaluation. A paired t-test was used to perform this analysis. The results are listed in Table 5. Table 5. Results of an A Priori Paired t-Te st Comparing Work Clot hes to PPE Plus LCS with Respective p-Values Response Work Clothes Versus Vapor Barrier Work Clothes Versus Firefighter rrHR 0.3190 0.7150 rrTre 0.1222 0.9593 rrTsk 0.5906 0.0008a S 0.2782 0.0342 Time NA NA a. Shaded areas are significant p-values Except for the rate of rise for skin temp erature, both the firefighter and vapor barrier suits in conjunc tion with cooling were similar to wearing work clothing. That is, the worker had approximately the same phys iological stress weari ng cooling with the firefighter or vapor barrier suit s as if they were only weari ng work clothes. Also, they could work for about as long.

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24 DISCUSSION Engineering and administrative controls ar e the preferred method to eliminate or reduce occupational hazards. Unfortunately, this isn’t always feasible. The age-old hazards of fire and explosion along with the development of OELs and more recently the insurgence of biological agents has creat ed a trend of increasing need for PPE. Traditionally, heat stress may not have been a concern in warm work environments, but the addition of PPE has increased the hazard. The risks of heat stre ss are of particular concern when working in hot environments with PPE. One approach to combat heat stress is the use of a personal cooling garment. The main emphasis of this study was to ev aluate the cooling performance of the Med-Eng CardioCOOLTM liquid cooling system. This was done by comparing subjects’ physiological responses and endurance time to heat stress while wearing various protective clothing, with and w ithout the use of the portable liquid cooling vest. This comparison could be made because there was no significant difference in metabolic rates for each clothing type when comparing cooling to no-cooling. In other words, the work demand was similar between the cooling st atuses. The three protective ensembles evaluated were a vapor barri er suit, firefighter tur nout gear, and bomb suit. Vapor Barrier Suit This LCS significantly reduced the rrHR, rrTre, rrTsk, and S when wearing the vapor barrier suit. This supports previous st udies with LCS, where significant reductions

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25 in Tre were seen when comparing no-cooling to intermittent cooling.(5, 10) These same studies conflicted on the si gnificance of reduction in Tsk at the end of the final exercise phase. The conflict in the si gnificance of reduction in Tsk likely resulted because cooling only occurred during the resting phases of the previous studies and was performed continuously during this study. Although an alternate met hod of personal cooling, air cooling supports the importance of continuous cooling in the signi ficant reduction of Tsk. during the exercise phase.(8) The significant reduction in rrHR was not supported by previous studies.(5, 10) The metabolic rates in the previ ous studies were much higher (over 400 W) than the target 3 00 W used in this study. In this study, a statistical analysis di d not find endurance time was significantly affected by the cooling system. This was to be expected, because the subjects completed the arbitrary 90 minute interval for all of the co oling trials and 2 out of 5 no-cooling trials rather then stopping due to heat strain. Although this study did not find the LCS significantly affected endur ance time, LCS have been found to increases endurance time.(5) A longer exercise phase would be needed to determine the impact of this LCS on endurance time. Firefighter Turnout Gear This LCS significantly reduced the rrHR, rrTre, rrTsk, and S when wearing the firefighter turnout ge ar. This supports previous studi es with LCS, where significant reductions in Tre were seen when comparing no-cooling to intermittent cooling.(5, 10) These same studies conflicted on the significance of reduction in Tsk at the end of the final exercise phase. The conflict in the significance of reduction in Tsk likely resulted because cooling only occurred during the resting phases of the previous studies and was

PAGE 33

26 performed continuously during this study. A dditionally in this study, as depicted in Figure 4, the LCS actually had a negative rrTsk for the firefighter suit. This means the mean Tsk was actually lower at the end of the ex ercise phase then in the initial five minutes of the exercise. The reduction in Tsk is mostly likely due to the firefighter suit’s insulation. The insulation reduced the loss of the heat sink potential to the environment.(3) The negative rrTsk for the firefighter suit contributed to the differences seen between the protocols. The significant reduction in rrH R was not supported by previous studies but the metabolic rates in the previous studies we re much higher (over 400 W) than the target 300 W used in this study.(5, 10) In this study, a statistical analysis di d not find endurance time was significantly affected by the cooling system. This was to be expected, because the subjects completed the arbitrary 90 minute interval for all of the cooling trials and all but one no-cooling trial rather then stopping due to heat strain. Although this study did not find the LCS significantly affected endur ance time, LCS have been found to increases endurance time.(5) A longer exercise phase would be needed to determine the impact of this LCS on endurance time. Bomb Suit This LCS did not signifi cantly reduce the rrHR, rrTre, rrTsk, and S when wearing the bomb suit. This was not supported by prev ious studies with LCS, where significant reductions in Tre were seen when comparing no-cooling to intermittent cooling.(5, 10) These same studies conflicted on the significance of reduction in Tsk at the end of the final exercise phase. The bomb suit is very di fferent from the types of protective clothing tested in other studies. It weighs 75 lbs and is quite cumbersome. Unfamiliarity of

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27 donning and wearing this unique suit added to th e subject’s metabolic rate. On average the donning time was 15 minutes longer then the other protocols. That along with the heavy weight of the suit likely elevated th e subject’s physiological response even before starting the exercise phase. The bomb suit incr eased the metabolic rate such that the LCS alone could not keep the body from experien cing excessive heat strain. The large deviation of each physiological response be tween subjects may have impacted the significance of physiological respons es for this LCS. The lack of significant in reduction of rrHR was supported by previous studies.(5, 10) The metabolic rates in the previous studies were much higher (over 400 W) and were closer to the actual metabolic rate that occurred for the bomb suit. The endurance time for the bomb suit wa s significantly affected by the cooling system. This was found with the bomb suit unl ike the vapor barrier and firefighter suits, because the burden of the bomb suit prevente d subjects from completing the arbitrary 90 minute exercise interval in al l trials except two of the cooling trials. Had the exercise phase been longer the significance for increased endurance time might ha ve been stronger. As expected, the LCS has been found to increases endurance time.(5) Comparing Work Clothes to PPE with Cooling Since the mean normalized metabolic rates of the work clothes, vapor barrier and firefighter suits were similar, comparisons c ould be made between these test protocols. This allowed for an interesting analysis co mparing the subjects’ physiological responses to heat stress of the VB-C and FF-C to the WC-NC. Endurance time was not evaluated, because subjects completed the 90 minute exercise phase for all of the WC-NC, VB-C, and FF-C trials.

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28 The rrHR, rrTre, rrTsk, and S were not significantl y different when comparing work clothes to the vapor barrier suit while wearing this LCS. These results are quite unlike the results found in a study compari ng a carbon impinged chemical protective garment used by the military.(5) That study found HR, Tre, Tsk, and S were significantly different when comparing work clothes to th e chemical protective garment while wearing a LCS. The previous study’s higher metabolic rates and intermittent cooling versus this study’s continuous cooling could have caused the conflict in results. The rrHR, rrTre, and S were not significantly di fferent when comparing work clothes to the firefighter tur nout gear while wearing this LC S. These results are quite unlike the results found in a study compari ng a carbon impinged chemical protective garment used by the military.(5) That study found HR, Tre, and S were significantly different when comparing work clothes to th e chemical protective garment while wearing a LCS. The previous study’s higher metabolic rates and intermittent cooling versus this study’s continuous cooling could have caused the conflict in results. This study found the rrTsk was significantly different when comparing the WCNC to FF-C. The LCS actually had a negative rrTsk for the firefighter suit. This means the mean Tsk was actually lower at the end of the ex ercise phase then in the initial five minutes of the exercise. The reduction in Tsk is mostly likely due to the firefighter suit’s insulation. The insulation reduced the loss of the heat sink potential to the environment.(3) As expected the reduction in Tsk was similar to the previous study.(5) Conclusions In summary, the Med-Eng CardioCOOLTM liquid cooling system effectively reduced subjects’ heat strain while in the va por barrier and firefighter suits. Since each

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29 individual can have a different response to heat stress, it is important to reduce heat strain. The cooling system had limited effectivene ss in reducing physiologi cal responses with high metabolic rates such as those that o ccurred with the bomb suit. Although the LCS was not effective in significantly improvi ng the body’s heat balance when experiencing high metabolic rates, it did increase enduran ce time. Increasing the endurance time will aide in lengthening the work phase of a work -rest cycle. Increasing the work phase will reduce manpower needs, production time, and cost s. Increasing endurance time is crucial for explosive ordinance technicians as well as other workers who may be unable to take scheduled breaks due to mission requirements. The study also found the Med-Eng CardioCOOLTM liquid cooling system reduced physiological response to heat stress when wearing vapor barr ier and firefighter suits to that if only wearing work clothes. Employers and supervisors can often relate more with the affects of heat stress while in work clothes rather than in PPE. Therefore, being able to make this comparison can help employers more easily gauge workers’ heat strain and more appropriately schedule necessary breaks. This in turn could help reduce the number of heat related injuries.

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30 REFERENCES 1. Salvatore R. DiNardi, The Occupational Environment – Its Evaluation and Control (AIHA Press, 1998), p. 666. 2. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices (ACGIH, 2003), p. 173. 3. M. H. Harrison and A. J. Belyavin, Operational Characteristics of LiquidConditioned Suits (Aviation, Space, and Environmental Medicine, August 1978), p. 9941002. 4. K. L. Speckman, Perspectives in Microclimate Cooling Involving Protective Clothing in Hot Environments (International Journal of Industrial Ergonomics, 1998), p. 121-147. 5. S. H. Constable, P. A. Bishop, S. A. Nunneley, and T. Chen, Intermittent Microclimate Cooling During Rest Increase s Work Capacity and Reduces Heat Stress (Ergonomics, 1994), p. 227-285. 6. Bruce S. Cadarette, Leslie Levine, Ja net E. Staab, Margaret A. Kolka, Matthew M. Correa, Matthew Whipple, and Michael N. Sawka, Upper Body Cooling During Exercise-Heat Stress Wearing the Improved Toxicological Agent Protective System for HAZMAT Operations (AIHA Journal, July-August 2003), p. 510-515. 7. Yuval Heled, Yoram Epstein, and Daniel S. Moran, Heat Strain Attenuation While Wearing NBC Clothing: Dry-I ce Vest Compared to Water Spray (Aviation, Space, and Environmental Medicine, May 2004), p. 391-396. 8. S. H. Bomalaski, Y. T. Chen, and S. H. Constable, Continuous and Intermittent Personal Microclimate Cooling Strategies (Aviation, Space, and Environmental Medicine, August 1195), p. 745-750. 9. Yoram Epstein, Yair Shapiro, and Shai Brill, Comparison Between Different Auxiliary Cooling Devices in a Severe Hot/Dry Climate (Ergonomics, 1986), p. 41-48. 10. Phillip A. Bishop, Sarah A. Nunneley, and Stefan H. Constable, Comparisons of Air and Liquid Personal Cooling for In termittent Heavy Work in Moderate Temperatures (AIHA, 1991), p. 393-397.

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31 11. S. Konz, C. Hwang, R. Perkins, and S. Borell, Personal Cooling with Dry Ice (AIHA, 1974), p. 137-147. 12. E. Kamon, W. L. Kenney, N. S. Deno, K. I. Soto, and A. J. Carpenter, Readdressing Personal Cooling with Ice (AIHA, 1986), p. 293-298. 13. F. Tayyari, C. L. Burford, and J. D. Ramsey, Evaluation of a Prototype Microclimate Cooling System (AIHA 1989), p. 229-234. 14. William D. McArdel Frank I. Katch and Victor L. Katch, Exercise Physiology, 4th Edition (Williams and Wikins, 1996), p. 763. 15. Ken Parsons, Human Thermal Environments 2nd Edition (Taylor and Francis Group, 1993), p. 112-114. 16. R. R. Gonzalez, Biophysics of Heat Transfer and Cl othing Considerations (Benchmark Press, 1998), p. 45-95.


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Physiological responses of men during the continuous use of a portable liquid cooling vest
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ABSTRACT: Heat stress is a well documented hazard across industries. The combination of environmental conditions, work demands, and clothing contribute to heat strain. Left unchecked, heat strain causes changes in an individual's physiological state that can lead to serious and fatal conditions with little warning. Although engineering and administrative controls are the first choice to abate this hazard, they frequently are not feasible. In these cases, personal cooling is often employed. There are three main types of personal cooling: liquid, air, and passive. Each has its own advantages and disadvantages. This study focuses on continuous cooling using a portable liquid cooling system (LCS). The LCS used a vest with tubes circulating water from an ice heat sink. The experiment consisted of five males each completing seven tests in random order. The subjects wore work clothes as the control then in conjunction with a firefighter, vapor barrier, and bomb suits. Each suit was tested with and without the benefit of the LCS. All of the tests took place at 35oC dry bulb and 50% relative humidity while attempting to walk 90 minutes on a treadmill at a 300 W metabolic rate. The study found continuous use of the LCS significantly reduced heat storage (S) and the rate of rise of heart rate (rrHR), core temperature (rrTre), and mean skin temperature (rrTsk) for the firefighter and vapor barrier suits as compared to no-cooling. Although the LCS didn't significantly affect the rate of rise for physiological responses with the bomb suit, it did however, significantly increase the endurance time. Interestingly, the study also found when wearing either the vapor barrier or firefighter suits in conjunction with the LCS that the rrHR and rrTre were not significantly different from only wearing work clothes.
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