Burn Injuries and Their Relation to Wild Land Fire Fighting. M.Y. Ackerman Department of Mechanical Engineering University of Alberta

Transcription

1 Burn Injuries and Their Relation to Wild Land Fire Fighting M.Y. Ackerman Department of Mechanical Engineering University of Alberta March 2010

2 Burn Injuries in Relation to Wild Land Fire Fighting What Are Burn Injuries? Skin is the largest organ in the human body, serving to protect a person from external threats as well as helping to maintain the water balance required for the body to function. The skin is made up of a number of layers as indicated in Figure 1. While most diagrams show the makeup of skin as in the figure one must realize that there really are no layers as such but there is a change in the character and purpose of the cells that make up the skin with depth. This is important when understanding things such a burn injuries or burn injury prediction where attempts are made to quantify the degree of damage that has resulted from an injury. On an average person there are approximately 2 m 2 of skin and the areas apportioned to the parts of the body are as shown in Table 1. Table 1 Area of the Skin on a Typical Human Body (Stoll and Chianta) Head 7 Trunk 35 Arms 14 Thighs 19 Legs 13 Hands 5 Feet 7 Total 100% As indicated in Figure 1, the skin is usually assumed to consist of three layers; epidermis, dermis and subcutaneous tissues. The epidermis is a thin layer of dead cells that provides protection to the body from external threats and also serves as a moisture barrier to help contain the body s liquids. The epidermis layer is usually assumed to be micrometers thick but its thickness changes all over the body and it is especially thick on the palms of the hands and soles of the feet.

3 Figure 1 Makeup of Human Skin ( Skin cells are resilient but are biological entities that cannot tolerate extreme conditions. Burn injuries result when the temperature of the skin cells becomes high enough for the material enclosed by the cell walls to undergo a physical change - usually termed protein denaturation. This process, which occurs at temperatures above 44 o C, is similar to the visual changes that take place when an egg is cracked and dropped on a hot surface - the white (albumen) changes color from clear to white indicating that a physical change has taken place in the proteins. The progression of a burn injury is largely related to the amount of time the skin is above 44 o C at any given depth and the rate at which damage occurs is roughly tripled for each degree the cells are above the threshold temperature [1]. Burn injuries are classified roughly as to how deep the skin layers have been damaged. The usual classification that is applied is: First Degree Second Degree Third Degree (Full Thickness) First degree burns are characterized by a reddening of the skin that usually disappears within a short time period. First degree burns are not usually considered life threatening. Second degree burns result in the formation of a blister which may accumulate fluid and eventually the outer layer in the proximity of the injury will be shed. The damage in this case has progressed into the dermis (essentially between 50 and 100 micrometers depth) but has not progressed so deeply to destroy the regenerative layer (basal layer). Serious second degree injuries can require significant periods of hospitalization and relatively long recovery periods. Extensive second degree injuries are considered life threatening. Third degree burns are serious - the injury progresses to the layers containing the skin regenerating cells (basal layer) and the skin can no longer regenerate. This means that in order to heal the skin in the injured areas must be replaced - usually through grafting - before the person can recover from the injury. Serious third degree injuries require extended hospitalization, usually result in the formation of scar tissue and carry the risk of secondary infections. This layer is usually assumed to be between 1000 and 2000 micrometers in thickness but its depth and thickness change depending on the location on the body. Burn injuries can result from a number of conditions; exposure to a hot surface, gas, or exposure to thermal radiation are the most common causes. Exposure to hot gases can take the form of immersion in flames or other hot gases (steam) while contact exposure burns are more commonly seen in structural fire fighters where the person may kneel on a hot surface or grab on to a hot surface for support. Exposure to thermal radiation results from being in proximity to a flame or burning object. The energy exchange takes place via radiation and as such no intimate contact with either a hot gas or surface is necessary. Wild land fires present what appear to be large hot surfaces (even though they are not really surfaces at all) which emit massive quantities of thermal radiation. If a person is at

4 a location where he/she can see the flames then by definition there is a radiation exchange between the flames and the person. The blocking of thermal radiation is the basis for the US Forest Service Fire Shelter, where the outer aluminum layer is highly reflective to thermal radiation and as a result is able to withstand radiation loads that would very rapidly injure an unprotected individual. The limits of tolerance depend on the environment (hot surface contact, hot gas contact or thermal radiation), the amount of energy transfer, the level of protection (bare skin to light fire resistant clothing to structural fire fighting clothing) and the time of the exposure. Predicting Burn Injuries There are a number of studies with in the literature dealing with the prediction of thermal injuries. The most cited four, and the basis for most if not all thermal injury prediction systems (small scale material evaluation to mannequin - protective clothing evaluation systems), are: Stoll and Greene [2] Weaver and Stoll [3] Stoll and Chianta [4] Takata [5] Burn injury prediction can take several forms but perhaps the best known (and most widely used) is based on the work of Stoll. Stoll s results have been used in almost all small scale material evaluation systems; small bench scale tests that are used to evaluate the protective capabilities of materials used in protective clothing. Stoll s work involved exposing volunteers to a series of heat flux time combinations and observing the resulting injury. This involved exposing the forearms of volunteers to a known heat flux for a fixed duration and then observing whether or not a blister formed within a 24 hour period following the exposure. This work has become known as the Stoll Criteria and is widely used for the evaluation of potential burn injury. Two levels are typically cited, pain threshold and time to the onset of a second degree burn. The criteria is used in either tabular form, Table 3, or more commonly in graphical form as in Figure 2 (extracted from Stoll and Chianta, 1969).

5 Figure 2 Time Required to Produce Either Pain or a Thermal Injury (Stoll and Chianta, 1969) Table 2 Response of Skin to Various Energy Pulses at the Onset of Second Degree Burn Injury (Stoll and Chianta, 1969) The information shown in Table 2 is the incident energy in calories/cm 2 -s (1 calorie/cm 2 - s = 42 kw/m 2 ), the amount of energy absorbed and the temperature change at the surface and the basal layer (80 micrometers below the surface). Note that most if not all material/clothing evaluation systems are based on these results. There has been little in the literature in the way of experimental data for burn injuries in humans since Stoll s work. There are a number of studies dealing with excised human skin or porcine skin but little in the way of human trials. Many models have been proposed and there have been discussions over what skin thickness should be used in the

6 models but Stoll s work is used largely unchanged in most, if not all, clothing evaluation systems. Today the experimental points are represented with a curve fit rather than a chart as indicated in Figure 3. Figure 3 Stoll Criteria for the Onset of Second Degree Thermal Injury (based on Stoll and Chianta, 1969) It should be noted that the information contained in Figure 3 relates to exposed skin without any form of external protection. The information in the figure indicates that the onset of second degree thermal injury would occur in 1 second at heat fluxes of 60 kw/m 2 and about 10 seconds at 10 kw/m 2. One must be cautious in extending the prediction below about 10kW/m 2 or above 50 kw/m 2 as this is outside of the range of experimental data presented by Stoll (range shown on figure). At very high exposures the timing of the exposure is a source of uncertainty (and not really of consequence since the injury occurs almost instantaneously) whereas at low heat fluxes the blood flow and sweating provide a mechanism to deal with the incident energy and extend tolerance times. There are a number of uncertainties associated with the prediction of a thermal injury for a given heat flux exposure. These largely deal with the fact that the skin on a person is not uniform in thickness over the body and that the skin is not really a series of distinct layers at all but a continuum where the cell structure varies with depth. Material properties (heat capacity, thermal conductivity, density) change with the composition of the layers and with temperature. Stoll s work was done on the forearms of her subjects, where the skin layers are relatively thin, and this may or may not be representative of the average skin thickness or properties on the rest of the human body. The other large uncertainty deals with the idea of burn injury as usually expressed a percentage of the

7 body area and the degree of damage. It is very difficult to estimate the extent of a burn injury except in a relatively coarse way. Physicians usually estimate the damage visually, knowing that an area the size of the palm of the hand is roughly 1% of the body area. Physicians see numerous thermal injuries but rarely, if ever, know the exact nature of the exposure that caused the injury. Thermal injury (or burn injury) can result from a number of causal factors; thermal radiation emitted by a hot body or flames and received by the person, contact with a hot object or immersion in a hot gas (flames or combustion products). The amount of energy transferred is approximately proportional to the temperature of the object or gas and the amount of contact time. Everyone has been at a birthday party and seen someone pass their hand over top of the candles on the cake with no apparent ill effects. This is a result of insufficient time for the energy to transfer in sufficient quantity to elevate the skin temperature to either a pain threshold or worse. Everyone knows that holding your hand over the candles for an extended period will result in pain and ultimately a burn. For the most part the prediction of thermal injury is a two part exercise. The first part deals with the incident heat flux (be it hot surface contact, hot gas contact or thermal radiation) and how this energy is transmitted into the skin. Most models treat skin (for a while) as a semi infinite solid. What this means is that the temperature at some depth beneath the skin surface remains constant over the duration of the exposure. This is only true for perhaps a few tens or seconds or a minute before the body s physiological responses (increased blood flow, sweating) come into play. With a given incident heat flux and fixed thermal properties of skin the surface of the skin will rise in temperature according to (1). (1) Where α is the thermal diffusivity of the skin (k/ρc) Q is the incident heat flux (Watt/m 2 ) t is time (seconds) k is the thermal conductivity of the skin (Watt/m-Kelvin) ρ is the density of the skin (kg/m 3 ) C is the volumetric heat capacity of the skin (J/kg-Kelvin) T is the temperature rise at the surface of the skin (Kelvin) The energy absorbed at the surface of the skin is then transferred towards the core via conduction (rate dependent on temperature and skin thermal properties). At depths below the surface the prediction of temperature in response to an incident heat flux is a bit more complicated. Equation 2 shows the usual model employed to predict the temperature at any depth below the skin surface.

8 (2) In equation 2, erfc is the complementary error function which can be determined from tabulated values or numerically. If x, depth, is set to zero then equation 2 reverts to equation 1. These relationships allow the calculation of temperature as a function of time at any depth below the surface as long as the properties of the skin are known. The literature shows a number of studies in which the thermal properties of skin (human alive, human excised and porcine) have been measured. There is no general agreement on the properties but the values shown in Table 3 are used in several standard test methods (ASTM 1930 [6], ISO [7]) that are used to predict thermal injury in response to a set hazard (flame exposure). Table 3 Skin Properties Used in Prediction of Thermal Injury (from ASTM 1930) The thermal models allow prediction of the temperature at any time and depth below the skin surface and the burn injury prediction is based on work by Henriques and Moritz [8]. In this work the skin is assumed to be undamaged as long as the temperature remains below 44 o C and the rate of damage increases exponentially with temperature at temperatures above 44 o C. This relationship is shown below in equation 3. (3) Where Ω is a measure of burn damage at the basal layer or at any depth within the dermis P = a frequency factor [s -1 ] E = activation energy for the skin [J/mol] R = universal gas constant [8.314 J/kmol-K] T = absolute temperature [K] t = total time for which T is above 44 o C [ K]

9 There have been many discussions about what are appropriate skin properties for use in the calculation but perhaps the most widely used are those found in ASTM It has been agreed that perhaps the best values to use are those of Stoll for injuries within the epidermis layer and those of Takata [5] within the dermis or subcutaneous tissues. The values of P and E developed by Stoll are: For T<50 o C For T>50 o C P = x s -1 and E = K P = x s -1 and E = K and those of Takata are: For T<50 o C For T>50 o C P = 4.32 x s -1 and E = K P = 9.39 x s -1 and E = K Inhalation Injuries Inhalation injury is associated with damage to the upper airway and is seen in a good number of serious burn injury cases. There are documented cases of fatalities associated with inhalation injury but little in the literature on the accident conditions that resulted in the injury observed. The human upper airway is a marvelous heat exchanger and as a result gases are dramatically cooled before they travel very far into the air tract. There are few studies of damage to airways as a result of hot gases, steam or combustion products as these tests are very difficult to do and typically involve animal trials. Perhaps the most cited study dealt with the effects of hot gas, steam and flames on the air tract of anesthetized dogs (Moritz et.al 1944 [9]). In this study 18 dogs were placed under anesthetic and subjected to various temperatures and quantities of heated air, steam or flaming combustion products. The authors were quite amazed that in all cases, with the exception of superheated steam, the injuries produced were insufficient to cause mortality in the test subjects. Figure 4, extracted from Moritz et.al. shows the general experimental setup used to expose the animals. Figure 4. Animal Tests use to determine the effects of heated gases on the upper airway.

10 Table 4 show the range of conditions that were used in the study and the resulting injury observed at each condition. In all cases the authors observed injury to the upper trachea but the injury rapidly lessened with depth into the air tract and with the exception of steam there were no cases of lung injury observed. In retrospect that authors pointed out that this should have been an expected result as the energy content of hot gases that do not undergo a phase change was quite small and relatively easily absorbed into the lining of the air way without a sufficient temperature rise to cause damage. Table 4 Inhalation Injury Study by Moritz et.al (1944). What is PPE and what does it do? Based on NFPA 1977[10], Standard on Protective Clothing and Equipment for Wildland Fire Fighting, Personal Protective Equipment or PPE, The goal of this standard was to provide thermal protection for the wildland fire fighter against external heat sources with flame-resistant clothing and equipment while not inducing an extraordinary internal heat stress load Examination of this quote, extracted from NFPA 1977, indicates that the purpose of PPE is to protect the worker from external heat sources, such as fires and hot surfaces, while at the same time allowing the body to rid itself of excess heat generated during high activity levels in a warm or hot environment. In general these two ideas are contradictory in that protection levels from an external source of energy such as a wildfire are maximized by placing as much insulation material between the wearer and the source while maximum

11 cooling is obtained by removing as much of the insulating material from the wearer as possible (ignoring the possibility of actively cooling the person). This illustrates the dilemma that has been plaguing manufacturers and users alike when dealing with occupations like wild land fire fighting. The operational thermal environment alone is enough to produce large internal heat loads and yet the perceived hazard is the fire. Examination of the literature relating to workplace injuries in wild land operations really points to a different hazard - that of preventing the body from cooling itself during normal operations. That is not to say that protection is not needed. A review of the fatalities associated with wild land fire operations shows that there is obviously an element of risk associated with this occupation. Over the period, there were 918 fatalities associated with wild land fire fighting operations in the United States. [11] More than one half of these were unrelated to burn injury but rather were the result of auto accidents, aircraft accidents, heart attacks and a host of others. In recent times, , there were 133 fatalities in the United States (Mangan [12] Wild Land Fire Fatalities in the United States). These fatalities were grouped into six categories as follows: Heart attacks; Burnovers; Falling snags; Vehicles, including single- or multiple-vehicle collisions, as well as individuals struck by a moving vehicle; Aircraft accidents, both fixed and rotary wing, as well as aircraft-related accidents on the ground; Miscellaneous causes, such as training, medical, suicide, drowning, and so forth. Of the 133 fatalities over the time period 29% were related to entrapment and burnover and the remaining 71% were attributed to other causes. What are the hazards associated with wild land fire fighting? Wild land fire fighters face a number of hazards when dealing with the various stages of a wildfire. All of the duties from, initial attack to mop up, carry some risk. This is perhaps best illustrated by the Eighteen Watch Out Situations [13], all of which have been associated with a fatality. WATCH OUT SITUATIONS 1. Fire not scouted and sized up. 2. In country not seen in daylight. 3. Safety zones and escape routes not identified. 4. Unfamiliar with weather and local factors influencing fire behavior. 5. Uniformed on strategy, tactics, and hazards. 6. Instructions and assignments not clear.

12 7. No communication with crew members or supervisor. 8. Constructing fireline without safe anchor point. 9. Building fireline downhill with fire below. 10. Attempting frontal assault on fire. 11. Unburned fuel between you and the fire. 12. Cannot see main fire, not in contact with anyone who can. 13. On a hillside where rolling material can ignite fuel below. 14. Weather is getting hotter and drier. 15. Wind increases and / or changes direction. 16. Getting frequent spot fires across line. 17. Terrain and fuels make escape to safety zones difficult. 18. Taking a nap near the fireline. While not all of these situations have resulted in burn injuries they serve to illustrate the large number of possibilities for injury or worse. In keeping with the theme of burn injuries we can examine the mechanisms that contribute to the injury and what PPE does or does not do to counter the energy transfer. Hot Surface Contact When a person comes into contact with a hot object the energy transfer is largely via conduction from the object to the person. The rate at which the energy transfer occurs is a function of the temperature of the object and the thermal resistance between the person s skin and the object. Materials used in PPE are largely thin (the exceptions are turnout gear used by structural fire fighters), perhaps 1 mm, or less in thickness. The energy transfer in total is the time integral of the heat flow as shown in equation 5 below. (5) This relationship in essence says that the energy transfer is directly proportional to temperature difference, T, and inversely proportional to thermal resistance, R. What makes this calculation a little more complicated is that as the skin heats the difference in temperature that is the driving force for the energy transfer decreases. As a result is this it is expected that the energy transfer will decrease over time as the injury progresses. In general it is also true that the thicker the garment or the greater the number of material layers the slower the rate of heat transfer and resulting burn injury. Hot Gas Contact Immersion in a hot gas or combustion products produces energy transfer that is proportional to the temperature of the gas (assuming skin temperature remains constant for a while) and a convective heat transfer coefficient that is largely a function of gas velocity. As with hot surface contact the energy transferred to the skin is a function of the time spent in contact with the gas as illustrated in equation 6.

13 (6) Combustion processes produce hot gases in the range of o C and immersion in these gases, even for a short period, is sufficient to result in serious thermal injury. The intent of fire resistant clothing is to provide some time for escape and to not catch fire and contribute to the injury. Because the clothing is also intended to allow metabolic energy to escape it is usually breathable ; that is it does not contain any sort of barrier material that would prevent moisture removal from the individual (the exception would be structural fire fighters turnout gear, which contains membranes to ensure that external moisture from fire fighting operations does not soak the materials and degrade the thermal resistance). Because it is permeable to gases and moisture it is also permeable to combustion products to a degree. This does not mean that the combustion products simply flow through the material (there is a barrier on the back side skin) but it does mean that in most cases the material affords limited protection time before the energy (in whatever form it takes) penetrates the material. Thermal Radiation All bodies that are above absolute zero emit thermal radiation. We usually associate thermal radiation with hot objects or flames because, in that case, there is a net energy transfer from the flames or object to the person. Standing next to a cold window in January when the ambient temperature is well below zero also results in a net energy transfer from the person to the window and the person feels cool or cold. This is the same mechanism, albeit the energy transfer is in the opposite direction. The energy transfer is determined using the Stefan-Boltzmann relationship, indicated in equation 7. (7) In equation 7, σ is the Stefan Boltzmann constant, 5.67 x 10-8 W/m 2 -K 4, is the emissivity of the material (usually assumed to be unity for large flames, dimensionless) and T 1 and T 2 are the temperatures of the emitting and receiving objects in degrees Kelvin (Kelvin = Celsius ). The added complication in determining thermal radiation is that the emitting object, be it flames or a hot surface, emits radiation in all directions and only a fraction of the energy leaving the object is actually received by the person or receiving object. The amount of energy that leaves the flames or hot surface and arrives at the receiving object or person is determined by calculating a geometric view factor or shape factor between the emitting and receiving surfaces. While this is not a terribly difficult calculation the view factor change depending on the size of the emitting surface, the orientation of the emitting and receiving surfaces and the distance between the two. Thus it is a calculation that must be done for each combination of emitter size, separation distance and surface orientation.

14 Most people have sat around a campfire and know that sitting too close results in very hot body parts in short order. The solution is usually that the person moves away from the fire to a point where it is more comfortable. What happens here is not that the flames or combustion process has changed but by moving away from the fire the person alters the view factor between the fire and themselves to the point where less thermal radiation is received. The thermal equilibrium that occurs is a balance between the energy received from the fire and that lost back to the environment via convection and re-radiation (the clothing surface becomes warmer than the environment and thus transfers energy back to the environment). The wavelength distribution of an emitting surface is a function of its absolute temperature and is described by Planck s Law, equation 8. The peak emitting wavelength is described by Wien s displacement law, equation 9. This relationship is only a function of temperature and allows determination of the wavelength of electromagnetic radiation that is the highest in the emitted spectrum of wavelengths. This is important in describing energy absorption and thermal injury only when the emitting object is at very high temperature (short wavelengths) where the skin may not be considered to be opaque and the energy can penetrate and be absorbed at depths below the surface. (8) (9) Where λ is the wavelength in micrometers, h is Planck s constant (6.626x10-34 J-s), c is the speed of light (3x10 8 m/s) and k is the Boltzmann constant (1.38x10-23 J/K). For most purposes it can be assumed that incident radiation does not penetrate and is absorbed at the outer surface of the skin. This is because, for the most part, combustion processes such as wild fires or building fires, are rate limited by fuel vaporization and oxygen diffusion and as a result rarely produce temperatures higher than 1200 o C (1473K). At this temperature the peak emitting wavelength is around 1.97µm, relatively far into the infrared portion of the spectrum. Wavelengths of this size do not readily penetrate skin and as a result the energy is absorbed at the surface and penetrates to deeper layers via conduction. Protective FR Clothing and Burn Injuries Fire resistant clothing is intended to minimize thermal injury in the event of an accident and at the same time allow the person wearing to function without undue stress that results from internally generated energy. It is not intended to be a barrier for all severities of hazards and thus it is important to understand its limitations. Fire resistant clothing, whether made from treated natural fibers (such as cotton) or synthetic fibers

15 such as meta-aramids is intended to provide some degree of protection and to not contribute to any injury as a result of material degradation (burning). Evaluation of the protective capabilities takes several forms depending on the perceived hazard for the particular operation. Evaluation methods for clothing and materials can take several forms usually delineated by whether the test uses a material sample or entire garments. Material tests (bench scale tests) are usually designed to allow discrimination among materials and as such are designed to produce the widest differentiation between materials and not necessarily to reproduce a particular hazard. Two such tests that are usually applied to materials are known commonly as Thermal Protective Performance, TPP [14] and Radiant Protective Performance, RPP [15]. Both of these test methods use a small sample of material which is exposed to an energy source and the energy transfer through the material is measured. The endpoint of the test is usually either where the energy transfer is predicted to be just sufficient to result in a second degree burn injury (Stoll criteria) or when a fixed amount of energy has been transferred through the material. In the thermal protective performance test (ISO 9151, CAN/CGSB ) the heat flux (incident energy) is fixed at either 80kW/m 2 (ISO 9151) or 84 kw/m 2 (CGSB). In these tests a meker burner is used as an energy source. For the Radiant Protective Performance test (ASTM 1939) the heat flux is usually set at a much lower level (21kW/m 2, NFPA 1977) and there is no direct flame contact with the material under evaluation. In this test a series of quartz lamps are used as an energy source rather than a burner. Figure 5 shows the results of a number of fabrics evaluated using the Radiant Protective Performance test, ASTM 1939, at different incident heat fluxes. These tests were done at the University of Alberta s Protective Clothing and Equipment Research Facility.

16 Figure 5 Performance of Fire Resistant Fabrics Exposed to Thermal Radiation. In each case the material was tested using the RPP apparatus (quartz lamps) so the exposure is purely thermal radiation. The apparatus holds the material at a fixed distance away from the lamps and also holds a heat flux sensor behind the material. Because of the physical test apparatus there is always a small air gap (~ 1mm) between the back side of the material and the front of the heat flux sensor. The materials tested are of different thickness and fabric mass but are typical of what is used in practice. Note that there is quite a variation in protection level at lower heat fluxes (predictions ranging from about 65 seconds to 90 seconds at a heat flux of 5 kw/m 2 ) and there is little apparent difference at heat fluxes of 20 kw/m 2. The apparent differences at low heat flux are largely due to different material thicknesses (increased thermal resistance with thickness). One must be cautious in concluding that one material is better than another at these energy levels as the predictions were made using the relationship described by Stoll and these lower heat fluxes are well outside the range of experimental data that was presented. Figure 6 shows the experimental points overlaid with the Stoll data. What is evident in this figure is that the materials provide significant protection over bare skin (Stoll Curve). Tolerance time increases to about 15 seconds at heat fluxes of 20kW/m 2 (bare skin about 3.5 sec.) and 35 seconds at 10 kw/m 2 (bare skin about 10 sec.) Figure 6 Time to Onset of Second Degree Burn Injury with a Single Layer of Fire Resistant Fabric Full Scale Evaluation of Fire Resistant Clothing

17 Small scale evaluation systems are used to rank fabrics based on a small sample of a material. Fire resistant clothing, made from fire resistant textiles is intended to provide the person wearing a degree of protection and at the same time allow the individual to perform required duties. As a result the garments designed is intended to allow freedom of movement and will contain areas that contact the skin as well as areas that are separated from the skin by an air gap or other garments. The presence of both air gaps and other clothing layers will increase the level of protection provided as long as the undergarments do not catch fire or melt during the exposure. A recent study carried out at the Protective Clothing and Equipment Research Facility used an instrumented mannequin to evaluate the protection provided by clothing when exposed to thermal radiation from flames. The facility is usually used to evaluate clothing systems when immersed in flames (intended to mimic a hydrocarbon flash fire) but can also be used to provide a limited range of exposure conditions where the energy transfer mechanism is thermal radiation (a more likely scenario for wild land fire fighters unless they must pass through a flame front). The instrumented mannequin, shown in Figure 7, is fitted with 110 sensors to measure energy transfer to the surface. The mannequin is usually dressed in fire resistant clothing and placed within a circle of propane burners and exposed to hydrocarbon diffusion flames for periods ranging between 3 and 10 seconds. The energy transferred through the material is measured and subsequently used to predict the level of injury a person wearing the garments might receive. Figure 7 Thermal Mannequin Used to Evaluate Fire Resistant Clothing

18 In a wild land firefighting situation it is more likely that people will be exposed to a high radiant heat flux, away from the fire, than to be engulfed in flames. Mannequin tests were performed with the mannequin placed outside of the fire in order to evaluate the effects of an intense, primarily radiant, exposure. With the mannequin s back toward the flames a heat flux of 40 kw/m 2 (~1 cal/cm 2 -s) was measured across the back of the mannequin s torso. In order to effectively evaluate the clothing system 10 seconds was chosen as the exposure duration for these tests. Figure 8 shows photographs taken during a typical exposure. This particular series of tests looked only at the sensors that were covered by the garment and in all cases there was a secondary underwear layer that would provide additional protection. Cotton underwear with an aramid shirt weight outer layer provided sufficient protection that only 4% of the covered 11% body area was predicted to receive a burn injury of second degree or worse. Note that at this heat flux the Stoll curve prediction would be a second degree burn in less than 2 seconds.

19 Figure 8 Evaluation of Fire Resistant Clothing Exposed to Thermal Radiation from Propane Diffusion Flames (frames approximately at 2 second intervals) References 1. Hymes I, The Physiological and Pathological Effects of Thermal Radiation, United Kingdom Atomic Energy Authority, SRD/HSE R275, Stoll A.M., Greene L.C., Relationship Between Pain and Tissue Damage Due to Thermal Radiation, Journal of Applied Physiology 14: , 1959

20 3. Stoll A.M., Chianta M.A., Method and Rating System for Evaluation of Thermal Protection, Aerospace Medicine, Weaver J.A., Stoll A.M., Mathematical Model of Skin Exposed to Thermal Radiation, Aerospace Medicine, Takata A, Development of Criterion for Skin Burns, Aerospace Medicine, ASTM F , Standard Test Method for Evaluation of Flame Resistant Clothing for Protection Against Flash Fire Simulations Using an Instrumented Manikin, ASTM International 7. ISO 13506:2008(E) Protective clothing against heat and flame - Test method for complete garments - Prediction of burn injury using an instrumented manikin, International Standards Organization 8. Henriques F.C., Moritz A.R., Studies of Thermal Injury: The Conduction of Heat to and Through Skin and the Temperatures Attained Therein. A Theoretical and Experimental Investigation, American Journal of Pathology, Moritz A.R., Frederick M.D., Henriques C., Regina M., The Effects of Inhaled Heat on the Air Passages and Lungs, American Journal of Pathology, Vol 21, NFPA 1977, Standard on Protective Clothing and Equipment for Firefighters, National Fire Protection Association. 11. National Interagency Fire Center Mangan R., Wild Land Fire Fatalities in the United States , United States Department of Agriculture Forest Service Technology & Development Program 5100 Fire March MTDC 13. United States Department of Agriculture, Forest Service, IS0 9151:1995(E) Protective clothing against heat and flame - Determination of heat transmission on exposure to Flame, International Standards Organization. 15. ASTM F , Standard Test Method for Radiant Heat Resistance of Flame Resistant Clothing Materials with Continuous Heating, ASTM International.