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What is arson – in forensic science
admin to Forensic Science  



The act of purposely setting a fire with criminal intent. According to the National Fire Protection Agency (www.NFPA.org), there were approximately 130,000 intentionally set building and vehicle fires in the United States in 2003 resulting in more than 700 deaths and more than $2 billion in property damage. Of these fires, only about 17 percent resulted in an arrest and only 2 percent in convictions. Despite the grim statistics, arson has been declining for several years, although it still results in significant property loss each year. It is the job of the fire investigator to determine whether a fire can be assigned to natural causes, accidents, arson (incendiary), or indeterminate causes. In the case of incendiary fires, the usual motive is profit in the form of insurance fraud. Such fires can be set in structures or in vehicles such as cars, trailers, or boats. Other motives include revenge, vandalism, crime concealment, or psychological disorders such as pyromania.

The role of the forensic scientist in arson investigation focuses on detection of accelerants such as gasoline, explosives, or incendiary devices that might have been used to start and sustain a suspicious fire. If the fire is set to hide evidence of another crime such as a robbery or homicide, then the role expands and becomes much more challenging. Fire scenes that are also crime scenes are by definition disturbed not only by the fire but also by the action of firefighters and rescue personnel. Bodies have been moved or removed from scenes before it was known that a homicide had occurred, and smoke and water damage of the scene are inevitable. Destruction of evidence, either related to the fire or to the person who set it, is often difficult to find or exploit.

One of the key pieces of evidence for differentiating arson from other causes is the point of origin. Fires burn longest and hottest near the point of origin, reflected in the degree of damage observed. Multiple points of origin frequently point to arson but are not alone conclusive. Other suspicious signs at a scene include signs of forced entry and sabotaged detection and fire suppression systems. On the other hand, assigning the cause of a natural fire can be difficult since the possible causes are so varied. Electrical problems, cooking, smoking, candles, lightning, and creosote build-up in chimneys can all cause fires.

Fatal fires, especially those set to conceal a murder, require more forensic services and involvement. Tasks include determining at autopsy whether the person was alive or dead when the fire started. If a person is alive during a fire, there is usually a deposit of soot in the nose, mouth, or throat, indicating that the person was still breathing during the fire. Elevated levels of carbon monoxide in the blood also indicate the person was breathing after the fire was burning. Additionally, most victims of the fire itself (as opposed to foul play beforehand) are found face down, except in bed or on other pieces of furniture. In fire deaths, the body is often found in a pugilistic attitude in which the arms are drawn in and the hands curled up in what looks like a defensive posture of a boxer. This is a result of the heat of the fire, which can cause the large muscles to contract. Absence of the pugilistic attitude is not alone conclusive but suggests the possibility that the victim had been dead long enough for rigor mortis to set in, thus preventing or lessening the development of the pugilistic attitude.

If the death is a homicide, then evidence such as weapons and bloodstain patterns may be left at the scene. Although fire alters the appearance of blood and bloodstain evidence, it can survive and be used as evidence. However, the extreme temperatures of the fire can make forensic examinations of the blood and other body fluids difficult or impossible. Despite widespread publicity of a few incidents, suicide by fire (self-immolation) is rare in the United States. See also combustion.