As discussed previously, for gaseous fuels to burn, they must mix with air such that the concentration of fuel is within the flammable range. A fuel with a higher vapour pressure generates more fuel vapour that can mix with air and potentially form a flammable volume. The vapour pressure of a given liquid also increases with temperature, which means that as fuels heat up in a fire, it can become easier for fuel vapour to form and for the fire to spread.

If heat is added to a mixture, once the fuel concentration is within the flammable range, it is possible for it to ignite. A fire may then begin to grow and spread wherever a flammable mixture is present.