Burning and Blast Trauma

• What is fire?

  •Combustion is an exothermic  chemical  reaction in the presence of oxygen, that releases energy from a system (heat, light, sound)

  •General  Rule: Fire  requires  fuel, heat, oxygen (fire triangle)

  •Ignition of a fuel requires heat to cause vaporisation in the presence of oxygen and an ignition source
• Combustion

  •Combustion is the chemical process of burning, which gives out heat and light, and can exist in a number of different forms:

  •Smouldering combustion:

  –Oxidation of a solid fuel in direct contact with oxygen.

  –It can occur when access to oxygen is limited and does not produce a visible flame.

  –The combustion of the solid is visible to the naked eye as an incandescent glow.

  –If enough oxygen is introduced to the fire, the smouldering combustion can give rise to flaming combustion.
• Combustion

  •Flaming combustion:

  – Most common type are diffusion flames, which occur when gases diffusing from the surface of the fuel into the air burn in the presence of oxygen, producing a flame.

  –Some of the heat produced by the fire is fed back to the fuel, which further propagates the combustion, supporting the flame.

  –Flaming combustion is the most common once initiated, is self sustaining until one component of the fire tetrahedron runs out.
• Combustion

  •Explosive combustion:

  –can occur when the four sides of the fire tetrahedron are premixed and combustion takes place in a very short period of time, causing an explosion.
• Flame temperatures

  –Flame temperatures vary according to the size of the fire and the nature of the fuel burning

  –In laminar flames such as those in candles, temperatures of 1400°C have been observed.

  –The types of flames seen in outdoor burning situations are turbulent flames, and temperatures within these flames vary greatly, depending on the area within the fire.
• Flame temperatures

  •Just above the base of the fire, in the continuous flame region, temperatures are more constant (900°C to 1027°C).

  •Above this, in the intermittent flame region, temperatures are more variable as the flames are moving around, hence, turbulent, with visible flame tip temperatures of approximately 320°C.

  •Above this, the temperature drops in the thermal plume region containing smoke, and in an unconfined space, the smoke will cool and eventually disperse.
• Fuel

  •Fuel has a very important influence on combustion:

  –type of fuel

  –amount of fuel present

  –the surface area of the fuel

  –construction and geometry of the fuel

  –combustibility of the fuel

  •All have an effect on the temperature and duration of the fire.
• Fuel

  •Different fuels burn at different temperatures:

  –wood has a maximum flame temperature of 1027°C

  –kerosene has a maximum flame temperature of 990°C

  –animal fat 800-900°C

  –charcoal 1390°C.

  •Fuel surface area has a significant impact on the duration and temperature of a compartment fire.

  –The higher the surface area of the fuel, the higher the temperature and the shorter the duration of the resulting fire.
• Important terms

  •Heat flux:

  –The rate of heat energy transfer through a surface per unit time (usually taken to be 1 second)

  –It is measured in Watts (W) = Heat Energy (joules) released per unit time (seconds) or Kilowatts per unit area per second.

  –SI Units are kW/m² and can be measured using a variety of heat-flux sensors and meters

  –Heat flux is a critical aspect of fire development and dictates in many cases when materials become involved in a fire
• Important terms

  •Heat release rate (HRR):

  –The rate at which a fire will release energy

  –Fires are characterised b their HRR which is often measured in kilowatts (kW) or megawatts (MW)

  –It can be measured for articular materials using a cone calorimeter. The same mass of different materials will have different HRRs
• Important terms

  •Heat of combustion:

  –The total energy released as heat when a material undergoes complete combustion in the presence of oxygen under standard conditions (1 mole of material combusted under constant pressure)

  –Combustion is an exothermic reaction measured in Joules per  unit mass (J/Kg) – it is NOT a measure of the rate at which heat can be released or the flame temperature that a fuel can produce

  –Heat Transfer

  –The process by which heat energy moves from one body or substance to another by radiation, conduction, convection or a combination thereof.
• •The HRR, heat of combustion and heat flux of a fuel and the location of the fuel will influence the degree of damage caused and have an effect on burning patterns.

  •In addition to this, the body itself will act as fuel; skin, viscera and the most efficient fuel - subcutaneous fat.

  •The Muscles- ‘Pugilistic attitude’

  •The Respiratory Tract- Inhalation of hot gases and/or smoke (CO, CO₂, carbon ash etc.)

  •The Internal Organs-Viscera, Brain
• Burning Progression – Influence of body position

  •On a non-combustible floor for duration of fire

  –Body will burn more severely on the surface that is exposed to the fire, and showing less damage on the side in contact with the non-combustible floor

  •On top of burning items

  –The side of the body in contact with the burning items will burn first, proceeding up around the body.
• Burning Progression – Influence of body position

  •On combustible floor that collapses during fire

  –The floor, if combustible, will act as fuel and burn the side of the body in contact with it. If the floor collapses, remains could be deposited over a wide area.

  •In suspension on metal framework e.g. car seat

  –The car seat or mattress may initially shield the body. If the fire then continues to burn long enough, the fragmented remains may fall through the framework and scatter.

  •Exposed on all sides

  –The body will burn on all sides, and burning will progress in a relatively predictable way.
• •Standardisation model for describing the extent of burn injury to human remains

  •The Crow-Glassman scale (CGS) is divided into five levels depicting increasing destruction to the body relative to burn injury

  
• Effects on the skull

  
• Effects on the body

  
• •Bone fragments, discolours and may change shape due to burning

  •May make some individual bones unrecognisable

  •Complicates analysis of:

  –MNI

  –Age determination

  –Sex determination

  –Stature estimations

  –Ancestry determination

  –Peri-mortem condition (active research area)

  –Nutritional status

  –Trauma (active research area)
• •The pugilistic pose of burned remains is the natural position of thermal induced muscle shrinkage.

  –Despite initial body positioning, the pugilistic posture will influence the subsequent pattern of burning and fracture production.

  –Muscle contraction can dislocate and fracture heat compromised bone and joints.

  •Coupled with pugilistic pose as an influence on bone, is the protective properties of soft tissues and their variable thicknesses.

  •The absence of a burn pattern typical of a body in the pugilistic pose may reflect conditions that restricted or confined the body.
• Analysis of the Burning Process

  •X-ray diffraction has been used to identify the key stages in the burning process

  •Dehydration

  •The hydroxyl-bonds break and both the loosely-bound water (physisorbed) and bonded water (chemisorbed) are lost.

  •Decomposition

  •The organic components of the bone are removed by pyrolysis.

  •Inversion

  •The loss of the carbonates, likely associated with the conversion of

    – hydroxyapatite crystal structure to beta-tricalcium phosphate.

  •Fusion is characterised by the warping and coalescence of the crystal matrix.

  
• 4 Stages of Heat-induced Transformation

  
• Modeling Heat-induced Change

  
• Colour Change in Bone

  1.Unaltered fresh bone (normal bone colour)

  Protected by soft tissue insulation.

  2.Heat line (white line or translucent bone)

  Initial line of contact and heat destruction to bone.

  3.Heat border (brown to white band of variable width)

  Location where organic material (collagen) is permanently altered anddestroyed by heat, which distinguishes it from green bone. This feature follows contours of the preceding heat line.

  
• Colour Change in Bone

  4.Charred (black) Advanced stage of burning.

  Bone is thought to be directly in contact with fire and heat, hence the colour resulting from a reduction atmosphere. Complete loss of organic material and moisture, which compromises the bone structure, resulting in tensile shrinkage fractures that run both parallel and perpendicular to the heat border.

  5.Calcined (grey to white)

  Post-organic destruction and modification of bone mineral content (crystallization of hydroxyapatite in bone). Structures exhibit deformation

  
• Colour Change in Bone

  •Colours relate to:

  –Temperature attained

  –Length of exposure

  –Organic and inorganic composition of body

  •Brown: associated with haemoglobin or soil discolouration

  •Black: carbonisation of burned bone in oxygen starved state

  •Grey and blue: Result of pyrolysis of organic components of bone

  •White: final end stage of calcination

  –Complete loss of organic components

  –Fusion of bone salts

  •Green, yellow, pink, red: presence of copper, bronze and zink in surrounding environment

  –E.g coffin linings, coins, rivets etc.

  
• Colour Change in Bone

  
• Heat Induced Morphological Changes

  Heat-induced fractures

  •Patina fractures

  –Observed on surface of flat bones and long bones

  –Fine cracks

  –Do not penetrate marrow cavity

  •Longitudinal fractures

  –Follow long axis of bone

  –May penetrate marrow cavity

  –Follows orientation of collagen fibers

  •Curvilinear fractures

  –Circumscribe long bone shaft

  –Can exhibit oblique orientation

  •Transverse fractures

  –Perpendicular to shaft

  –Delamination fracture

  •Peeling or flaking of bone layers

  •Separation of cortex from trabeculae

  
• Heat Induced Morphological Changes

  Shrinkage and deformation

  •Reduction in bone length and width occur during burning

  •Several experiments to investigate shrinkage

  –For review see: Correia. (1997). Fire modification of bone: a review of the literature. In: Haglund and Sorg. Forensic Taphonomy.

  •General consensus 5-20% shrinkage

  
• Heat Induced Morphological Changes

  •Three phases of shrinkage reported

  –  1. 150-300oC

  –2. 750-800oC (most significant)

  –  3. 1000-1200oC

  •Shrinkage dependant on four criteria

  –Distribution of bone types (e.g compact vs trabeculae)

  –Temperature of exposure

  –Bone mineral content

  –Aspects of mineral content

  
• Dimensional Change

  •Warping is clearest form of dimensional change- assertions:

  –Heat-induced warping is more apparent in bone that was fleshed at the time of burning

  •implies that the heat-induced contraction of the muscle fibres pulls and twists the bone away from its natural shape.

  –Heat-induced expansion of air within the medullary cavity also causes dimensional change.

  •The likely manifestation of this is an increase in size of the diaphysis, and particularly the epiphyses.

  
• Dimensional Change

  •Both are speculative statements and not substantiated by quantitative data.

  •More likely that intrinsic properties of bone geometry and anisotropy play a more significant part.

  
• Thompson (2005)

  •Thompson burnt 60 sheep long bones (only 6 sheep)

  –Defleshed

  •11 linear dimensions taken before burning.

  •The bones were heated to 500°C, 700°C or 900°C for 15 or 45 min.

  •After removal from the furnace the bones were allowed to cool for five minutes before being re-measured.

  •Pairwise statistical tests were then used to assess dimensional change.

  •Also assessed:

  –% weight loss

  –Crystal size (using XRD)

  –Density

  –Levels of porosity

  •Multivariate PCA applied to dimensional and structural results
• Thompson (2005)

  •Bi-variable impact of temperature and duration explored

  •Heat-induced bone changes observed:

  –Colour changes

  –Weight loss

  –Reduction in mechanical strength

  –Distinct fracture patterns

  –Alterations in microporosity

  –Alterations in crystalline structure

  –Reduction and expansion in size
• Thompson (2005)

  Heat-induced dimensional changes

  •With increasingly intense burns, more long bones fragmented

  –Could not be measured post burn

  –Reassembly of bone pieces may have strengthened research

  •As specimen cooled bone dimensions changed

  –Shrinkage continued on cooling

  –Heat induced shrinkage is dynamic

  –Expansion observed in some specimens (followed by cooling shrinkage)

  •Low intensity burning phenomenon

  –Trabecular bone more influential on dimensional change than compact bone
• Thompson (2005)

  •Major implications on biological profile

  •Series of equations produced to predict heat induced change

  –Can be applied to correct for burning when producing biological profile

  –Caution advised!
• Pre-existing trauma

  •Differentiating pre-burn trauma from post burn trauma is problematic

  •Study documents survivability of pre-burn trauma through all stages of burning

  •40 cadaver heads burned

  –Some intact, some traumatised

  •Results:

  –Identification of pre-existing trauma is possible

  –Signatures of ballistic, blunt and sharp force trauma survive burning process

  –In intact specimens skull does not explode from internal pressure, but rather fragments due to collapsed debris, extinguishment method and sloughing.
• Pope and Smith (2004)

  
• Pope and Smith (2004)

  •Heat related fracturing

  •Margins outlined in deep black from pressurised venting of organic materials within the vault

  
• Pope and Smith (2004)

  •Linear fractures

  •Difficult to differentiate heat related vs pre-existing

  •Heat related:

  –Occur in early stages of burning due to shrinking and cracking of bone

  –May radiate from charred black areas into buff coloured bone

  –Will never radiate into unburned bone

  •Pre-existing

  –Only pre-existing fractures extend into unburned bone

  
• Pope and Smith (2004)

  • Heat-related fracture

  –Well-defined sharp corresponding margins

  •Pre-existing fracture

  –Eroded, blunt, deformed margins

  –Due to prolonged thermal exposure