An accelerant is any substance intentionally used to start a fire or make it spread faster. The term comes from fire science and criminal investigation, and it applies to liquids, solids, and gases. What makes something an accelerant isn’t its chemistry but its purpose: gasoline sitting in your car’s tank is fuel, but gasoline poured across a floor to start a fire is an accelerant.
How Accelerants Work
Fire needs three things: heat, oxygen, and fuel. An accelerant works by adding a highly flammable fuel source that ignites easily and burns intensely, giving the fire enough energy to catch surrounding materials like wood, carpet, or furniture. Without an accelerant, a fire has to slowly heat solid materials until they break down and release flammable gases on their own. An accelerant shortcuts that process.
Research on gasoline-soaked wood chips shows exactly how this works. Gasoline lowers the energy needed to start the chemical breakdown of wood, affecting two critical stages: the initial drying phase (where moisture evaporates) and the main decomposition phase (where the wood itself starts to break apart and release combustible gases). In practical terms, the fire gets hotter faster and spreads to surrounding materials that would otherwise take much longer to ignite on their own.
Common Types of Accelerants
Gasoline is by far the most commonly used accelerant in deliberately set fires. It’s cheap, widely available, and extremely flammable. But accelerants span all three states of matter:
- Liquids: Gasoline, kerosene, diesel fuel, lighter fluid, paint thinner, and alcohol. These are the most frequently encountered in arson cases because they can be spread over a wide area before ignition.
- Solids: Paper, road flares, fireworks, and black powder. These tend to burn in a more contained area but can be effective at starting fires in specific locations.
- Gases: Butane (lighter fuel), propane, and natural gas. These are particularly tricky from an investigation standpoint because they burn cleanly and leave little to no chemical residue behind.
Why the Term Matters in Arson Investigation
The word “accelerant” carries legal weight. The National Fire Protection Association’s NFPA 921 guide, which is the standard reference for fire investigations in the United States, formally defines an accelerant as “a fuel or oxidizer, often an ignitable liquid, intentionally used to initiate a fire or increase the rate of growth or spread of fire.” That word “intentionally” is key. Finding gasoline residue in a garage where someone stores a lawnmower means nothing. Finding gasoline residue soaked into a living room floor is a different story entirely.
Fire investigators look for physical clues that an accelerant was used: unusual burn patterns on floors, “pour patterns” where liquid was spread before ignition, fires that burned hotter or faster than the available fuel (furniture, building materials) would normally allow, and multiple points of origin in the same fire. These patterns help distinguish an accidental fire from one that was deliberately set.
How Investigators Detect Accelerant Residue
Even after a fire burns out, traces of liquid accelerants often survive in debris, absorbed into flooring, soil, or charred materials. Investigators collect samples from suspicious areas and send them to a forensic laboratory, where a technique called gas chromatography/mass spectrometry separates and identifies the individual chemicals in the sample. This method can differentiate between various petroleum products and solvents, helping analysts determine exactly what type of accelerant was used.
Before lab work even begins, many fire departments use specially trained dogs to locate accelerant traces at a fire scene. These dogs have remarkable sensitivity. In testing, some canines detected gasoline at volumes as small as 0.005 microliters, which is a fraction of a single drop, in a standard sample container. That’s below the threshold that lab instruments need to make an identification (about 0.1 microliters of gasoline). However, the dogs’ effectiveness varies significantly based on their training. A study of 42 canine teams found that dogs trained primarily on one type of accelerant (usually gasoline) sometimes missed other types, while dogs trained on a variety of accelerants performed more broadly. Handler skill also played a major role: in several cases, dogs correctly identified positive samples but were pulled away by handlers who didn’t trust the alert.
Gaseous accelerants like propane and natural gas present a unique challenge. Because they disperse into the air and leave no chemical residue, investigators rely almost entirely on physical evidence and witness accounts rather than lab analysis.
What Burns Off and What Stays Behind
When accelerants burn, they produce byproducts just like any other combustion. Gasoline combustion primarily generates carbon dioxide and water vapor under ideal conditions, but incomplete burning also produces carbon monoxide, a colorless, odorless, and potentially lethal gas. Propane burns similarly. Other accelerants produce more complex residues: road flares release nitrogen oxides and sulfur dioxide, while certain gel-based fire starters can produce manganese dioxide and styrene.
In open-air settings like prescribed burns in forestry, these byproducts disperse quickly. In an enclosed building, they accumulate rapidly and create toxic conditions well before the flames themselves reach a person. Carbon monoxide poisoning is the leading cause of death in structure fires, and the presence of an accelerant increases how quickly those dangerous gas levels build up.
Accelerants Outside of Arson
Not every use of an accelerant is criminal. Firefighters and forestry crews use accelerants routinely in controlled burns to manage wildland fire risk. Fusees (road flares), gel fuels, and even ping-pong balls filled with reactive chemicals are standard tools for prescribed fire operations. Charcoal lighter fluid is an everyday household accelerant. The distinction between a legitimate use and an illegal one comes down entirely to intent and context, which is why fire investigators focus not just on whether an accelerant is present but on whether its presence makes sense given the circumstances of the fire.