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An accelerant is any substance which speeds a process, but most people use the term specifically to refer to substances which contribute to the spread of fires. Fire accelerants include things like hydrocarbons, paper, plastics, and other materials which can cause a fire to spread more quickly or burn more fiercely than it would otherwise. In arson investigation, the detection of accelerants is especially important, as the presence of such a substance will need to be proved in order to classify the fire as an arson.
When an accelerant is present at a fire site, it contributes significantly to the speed at which the fire moves and how hot the fire burns. Without an accelerant, most fires burn at relatively cool temperatures; while these temperatures are high enough to cause severe damage and kill any living organisms caught in the fire, they are typically not hot enough to do things like melt hard metals. In the presence of an accelerant, however, a fire has what is known as a higher heat release rate, which means that the fire burns very hot, causing substantially more damage to the site.
An accelerant can also cause an explosion, typically through a buildup of vapors which ignites at the heat of the fire reaches it. The explosive nature of accelerants has sometimes literally blown up in the faces of arsonists, as a fire may ignite explosively before someone is fully prepared for it; the bodies of arsonists are sometimes found at the fires they set, as a result. The explosions characteristic of some accelerant are sometimes a key clue for fire inspectors, who may be able to tell that a fire is an arson while it is still burning, thanks to the explosions, high heat, and rapid spread.
Arson investigators can use a number of tools to identify accelerants at a site. Accelerants leave distinct chemical signatures which can be detected by the nose of an arson dog, for example, and sometimes obvious physical clues can show that an accelerant is present. Once the use of an accelerant is proved, a fire can be classified as an arson, opening an investigation to determine who set the fire, and why.
In addition to being deliberately introduced to a site with the intention of causing a fire, an accelerant can also occur naturally, or be brought in by accident. Historically, some plastics acted as accelerants, by offgassing vapors which could ignite dangerously, and some very severe fires were caused by the presence of such vapors in homes, hotels, and businesses. Some glues, solvents, and paints can also act as accelerants, which is why it is important to ensure that such things dry thoroughly when they are applied.