This refers to the application of scientific methods & techniques to determine the origin and cause of fire once firefighters have extinguished the fire.
The practice is similar to the examination of crime scenes in that the scene must be preserved and evidence collected and analyzed, but with numerous additional difficulties and dangers. The investigation will include closely surveying the damaged scene to establish the origin of the fire and eventually establishing the cause.
However in order to effectively examine and evaluate a fire scene, it is imperative that the investigator has a detailed knowledge of the chemistry and behavior of fire and its effects.
Understanding The Nature & Chemistry of Fire
Fire occurs due to the exothermic reaction of combustion (burning), producing heat and light. In order for a fire to occur, three vital components must be present: a fuel source, an oxidant (O2) and a sufficient amount of energy in the form of heat. Together these make up the fire triangle. A fourth factor can also be described – a self-sustaining chemical chain reaction – to produce the fire tetrahedron. The absence of any of these conditions will result in a fire not starting or extinguishing through smothering (oxygen removal), cooling (heat removal) or starving (fuel removal).
Solid and liquid materials do not actually combust, but the process of heating causes them to produce vapors which can burn. This is the process of pyrolysis. Through this pyrolysis products will be formed, flammable and volatile substances of low molecular weight caused through the decomposition of materials by fire.
Heat produced by a fire can spread in one of three ways; convection, conduction and radiation. Convection is the transfer of heat through air circulation, and only occurs in liquids and gases. An example of convection is the heat from a fire rising and heating the ceiling of a room. Conduction is the transfer of heat through a medium by direct contact, such as a fire heating a metal beam which transfers the heat elsewhere. Radiation is the emission of heat as infrared radiation without a medium, such as a fire heating and igniting a nearby sofa.
The primary purposes of a fire investigation is to establish the origin (seat) of the fire, determine the likely cause, and thus conclude whether the incident was accidental, natural or deliberate. It is vital to establish the cause to ensure similar events do not occur (in the case of natural or accidental) or to allow a legal investigation to be conducted (in the case of deliberate fires).
The initial concern with regards to a fire incident scene is safety. Such a scene has an increased risk factor with possible hazards including heated materials, structural collapse, damaged electricity and gas mains, debris, asbestos, dangerous combustion products and other toxic substances. A dynamic risk assessment should be conducted, the scene must be declared safe and all individuals entering the scene should wear appropriate protective clothing such as hardhats, fire-resistant overalls, steel-capped boots, thick gloves and, in some cases, a face mask. Supplies of gas and electricity should be switched off before the investigation begins.
Establishing the Origin
A vital aspect of the forensic fire investigation is to establish the point of origin of the fire, also known as the seat of fire. There are numerous indicators that can be used to determine the possible origin. The region in which a fire started will generally burn for a longer amount of time, thus will be an area with the worst damage. Fires tend to burn upwards, therefore the seat of the fire is likely to be found at a lower point of burn damage. However this is not always reliable as fires can spread downwards, particularly in the presence of certain fuel sources.
Fire effects on certain materials can indicate the direction of fire. As fire burns upwards and outwards, V-shaped smoke/burn patterns may be found on surfaces adjacent to the fire, with the end of the V pointing towards the point of ignition. However ventilation can affect the path or shape of V-shaped patterns. Smoke deposits of object surfaces can suggest the direction from which the fire originated, and glass and plastics tend to melt in the direction of fire, thus distortion of such materials can act as directional indicators.
Structural damage to the building can also be used to locate the seat of the fire. In some instances buildings may collapse in such a way that the area first weakened by the fire is clear, suggesting this is where fire damage first occurred and thus is the origin. Similarly, windows and ceiling structures are likely to fail in areas close to the seat of the fire first. However this is by no means an accurate method of locating the seat of the fire, as the collapse and damage of a building is affected by numerous factors, not just the fire itself.
Establishing the Cause
Determining the cause of the fire is often greatly aided by locating the seat of fire, at which point investigators can identify characteristics or artefacts associated with ignition. The investigator will aim to establish whether the cause of the fire was accidental, natural, deliberate or undetermined. Accidental fires generally involve no malicious human contact, with examples including the malfunction of an electrical appliance or an unattended candle. Natural fires include “acts of God”, such as lightning strikes. Deliberate fires are those ignited purposely by individuals, often with malicious intent, in an act known as arson. Finally, if the cause of the fire cannot be ascertained due to lack of evidence, it may be classed as undetermined.
Evidence directly linked to the fire may be found at the point of origin, such as fuel sources, incendiary devices, electrical appliances or pools of accelerant. In addition to examining the artefacts present at the scene, the lifestyle of individuals living or working in the building should be taken into consideration. For example, factors such as whether individuals were smokers, used candles or kept large amounts of possible fuel packages such as newspapers and magazines may be relevant.
Article by Peace Okrogu scsp HSE Lead.
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