Presentation on theme: "REFERENCES 1-4 NFPA Fire Protection Handbook, 17th ed, Table A-6 5 SFPE Predicting 1st and 2nd Degree Skin Burns from thermal Radiation, Bethesda, MD,"— Presentation transcript:
REFERENCES 1-4 NFPA Fire Protection Handbook, 17th ed, Table A-6 5 SFPE Predicting 1st and 2nd Degree Skin Burns from thermal Radiation, Bethesda, MD, 2000, p13 6 Fire Protection Handbook, op cit, A-6 ACKNOWLEDGMENTS Major Ron McCardle, Florida State Fire Marshal Carl Chasteen, Chief, Forensic Lab, Florida State Fire Marshal, and Dr. Said Nurbakhsh, California Bureau of Home Furnishings for their extensive support in conducting these experiments. COMBUSTION PROPERTIES OF HUMAN AND LARGE ANIMAL REMAINS John D DeHaan, PhD Fire-Ex Forensics, Inc. and Elayne Pope, MA University of Arkansas INTRODUCTION: The bodies of humans and other large mammals are often involved in fires in structures, vehicles and wild land environments. They may constitute a minor portion of the fuel load for the fire and only be affected by the later stages of a prolonged, fully-developed fire. In other cases, they represent a significant contribution to the fire s ignition, development, heat release rate and duration. Fire investigators should be aware of the fuel properties and fire behavior of such unusual fuel packages. OBJECTIVES: Presents heat release rates and fire behavior data from both cone and furniture calorimeter tests on small samples of isolated materials and from room calorimeter tests on intact remains. Human and large animal bodies were also burned under a variety of conditions and processes of combustion observed directly. Data relevant to fire investigators and fire engineers involved in reconstruction of such events were assessed. Skin: proteinaceous material, varying moisture content. ΔH c (gross) = 18.2 – 19.8 kJ/g for leather (presumably tanned) 1 Fur/hair: proteinaceous material. ΔHc (gross) = 20.7 – 26.6 kJ/g 2 Animal fat: mixture of low melting point (< 50°C) fats, water and loose connective tissue. ΔHc (gross) = 39.8 kJ/g 3 Lard (rendered fat): ΔHc (gross) = 40.1 kJ/g 4 Muscle and tendon: proteinaceous material, moisture content ~40%. ΔHc unknown Body fluids: 70-98% water content. Non-combustible Internal organs: torso: proteinaceous materials, 50-60% moisture content, fat content variable Brain: high fat content, high moisture content. ΔH c unknown. Bone: mineral and collagen (proteinaceous). Marrow contains fat and fluids Reference Physical and Combustion Properties of Fuels from Bodies OBSERVATIONS: Fat only burned where it had been rendered and absorbed into carpet, towel, blanket, clothing or charred wood. Figure 6: Charring of carpet or wood flooring supports the wick effect necessary to sustain combustion of rendered body fat. Figure 7: Extensive consumption of the body (60 kg) occurred over an extended period of time (4 hours) even without involvement of other fuels in the room. Figure 8: Extensive floor damage (charring & penetration) occurred where rendered fat burned on the charring floor, not where petrol pooled. BODY FAT Figure 9: Burning in the vicinity of the body was observed long after the fire in other combustibles has self-extinguished. Flame exposures from body fat were sufficient to calcine even large bones (given long enough duration) to the point they were friable to the touch. 1 Hour 3 ½ Hours 4 ½ Hours 2 ½ Hours Figure 2: Heat release rate for human fat in cone calorimeter Figure 1: Mammalian tissue is complex with multiple layers of different cells. Cross-section of human skin (from DeHaan, with permission) Figure 3: Effective heat of combustion from cone calorimeter for pork fat Figure 4: Heat of combustion: human fat Figure 5: Rendering fat burning on adjacent plywood floor PROCEDURE: Bench-Scale Tests: Stanton-Redcroft Cone Calorimeter used to measure the HRR and ΔH eff of various fats at heat fluxes of 35 and 50 kW/m2 Lab-Scale Test: 1-2 kg samples of fresh pork fat wrapped in cotton on a carpeted wood floor and ignited with petrol. HRR measured in the California BHF room calorimeter, with load cell. Full-Scale Tests: Fresh, intact pig carcasses of various weights wrapped in cotton cloth or blanket, on a carpeted wood floor and ignited with petrol. HRR data collected via room calorimeter. Full-scale tests: Intact, unembalmed, clothed human bodies were burned, one in a room and two in vehicles. Temperature data were collected via thermocouples, and direct observations made of flame spread and behavior. Of all the tissues of the body, the subcutaneous fat is the best fuel. It behaves like a low melting point wax or thermoplastic when tested in a cone calorimeter. At 35 kW/m2 radiant heat exposure, it melted readily and off-gassed but would not sustain a flame. When a single layer of cotton cloth was placed on top of the sample, the cotton charred (at 35 kW/m2) and the off-gassing vapors were ignitable to support a continuous flame. Under these conditions, the heat release rate for human fat ranged from 150 220 kW/m2 (Figure 2) and that for pork fat ranged from 190-250 kW/m2. (At 50 kW/m2 incident heat flux, the observed heat release rate for pork fat was 180-300 kW/m2.) RESULTS: (The accuracy of the wick adaptation for the cone calorimeter was verified by testing paraffin candle wax under the same conditions as that used for the fat. Those results were: 40-44 MJ/kg, the same as the reference values for paraffin wax.) The effective heat of combustion ranged from 24-32 MJ/kg for pork fat (Figure 3) and 24-34 MJ/kg for human fat (Figure 4). It should be noted that water trapped in these samples boiled out of the liquefied specimens and caused spiking of the calculated effective heat of combustion. The carcass fires were progressive, ignited in the vicinity of the burning gasoline (which burned for 2-3 minutes depending on the size of exposed pool), then spreading along the flanks of the carcass where rendered fat seeped into porous materials charred in advance of the flames. The most sustained fires were in the vicinity of the largest deposits of body fat. Areas around the head, feet and lower legs incurred the least damage. Skin splitting was necessary for the exposure of the subcutaneous fat and this would not occur until after several minutes of continuous flame contact. Gasoline burning by itself on skin did not burn long enough to cause splitting and rendering, but would merely self-extinguish after scorching adjacent skin. Where the clothing or carpet ignited, the longer duration flames would cause skin splitting and rendering of subcutaneous fat. Charring of carpet or wood flooring can create a porous char that will support the wick effect necessary to sustain combustion of rendered body fat, as in Figures 5 and 6. The rendered body fat will not smolder but must burn as a flame supported by the wicking effect. The peak heat release rate of the fat-fueled fire was directly related to the surface area of the burning pool of rendered fat. Although accurate estimates of actual burning surface area were difficult, the modest heat release rates (40-70 kW) observed appear to be commensurate with the 190-250 kW/m2 data from the cone calorimeter. The largest pool included much of the surface of the large (26 kg) pile of pork scraps as it was partially covered with a charred cotton robe and the charred carpet around its margins, for a peak heat release of 130 kW. Extensive consumption of a human body (60 kg) occurred over an extended period of time even without involvement of other fuels in the room. Extensive floor damage (charring and penetration) occurred where the rendered fat burned on the charring floor and not where gasoline accelerant was present. Extensive destruction of a body in a vehicle fire took place whether there was an accelerant present or not. Burning in the vicinity of the body was observed long after the fire in other combustibles has self- extinguished. In each lab- or full-scale test, once the petrol burned off (in <200s), the adjacent carpet or clothing supported external combustion until the skin split and the subcutaneous fat become the predominant fuel. For small wick areas (0.03-0.08m2), the peak heat release rates were 30-50 kW and the mass loss rates of 4- 6g/s. For a very large pile of pork scrap, the peak HRR was 130kW with a mass loss rate of 5g/s. For intact pig carcasses, fires of 40-50kW were sustained (mass loss rate: 2g/s (7.2kg/hr). For the large carcass, a steady HRR of 55-60kW was sustained for 331min, at 5.3kg/hr. The maximum flame temperatures (measured with a hand-held pyrometer): 813°C in the carcass test, 880°C during the cone calorimeter test of the pork fat, 913°C during the cone test of human fat. CONCLUSION: The response of a body to fire is complex due to the variety of fuels presented and the layered arrangement of those fuels. Subcutaneous body fat constitutes the major fuel source, but it must be exposed (via skin splitting) and rendered to liquid form so it can be absorbed into porous wick material. Human body fat has an effective heat of combustion of approximately 34 MJ/kg. Its combustion can produce severe destruction of other less combustible tissue (muscle and organs) and bones. A fire supported only by combustion of rendering body fat is unlikely to have a heat release rate greater than 130 kW and a mass loss rate between 3-10 kg/hr. Pig carcasses (or pork fat) can be used to accurately simulate the combustion properties of human bodies.