In a fire, oxygen must chemically react with a combustible fuel in a rapid oxidation process. For flaming combustion to begin and continue, the concentration of fuel vapour in the burning mixture (air and fuel) must be between the upper and lower flammability limits for that fuel. Figure 26 shows the flammability ranges for natural gas (methane) and acetylene. The flammability range of a gaseous fuel provides a general idea of the ratio of fuel to air required for ignition and continued burning.

When the concentration of fuel in air lies between the lower flammability limit (LFL) and the upper flammability (UFL) limit, there is a danger of the fuel igniting if a flame or another energy source is present.

Mixtures that lie between the LFL and UFL will support flame propagation and are said to lie within the flammability range .

Mixtures that are below the LFL have too little fuel to support combustion. With the addition of energy, combustion can occur, but will not sustain itself. The same will occur in a mixture that is above its UFL.

The peak burn ratio represents the ideal mixture of gaseous fuel in air; the concentration of fuel in air where complete combustion occurs and combustion efficiency is highest, as shown in Figure 26. Therefore, if two gases require similar amounts of oxygen for all the carbon in the fuel to oxidize to CO2 and all the hydrogen to react to H2O, they will have similar peak burn ratios.

The highest flame temperatures, flame speeds, and flame spread rates occur at concentrations that are slightly on the rich (too much fuel) side of the ideal mixture concentration.