In order for gas to ignite there must be an ignition source, typically a spark (or flame or hot surface), fuel and oxygen.

For ignition to take place the concentration of gas or vapour in air must be at a level such that the ‘fuel’ and oxygen can react chemically, and the energy from one molecule burning is enough to ignite the reaction in the next molecule along, and so on. The power of the explosion depends on the ‘fuel’ and its concentration in the atmosphere. The relationship between fuel/air/ignition is illustrated in the ‘fire triangle’.

The ‘fire tetrahedron’ concept has been introduced more recently to illustrate the risk of fires being sustained due to chemical reaction.

With most types of fire, the original fire triangle model works well – removing one element of the triangle (fuel, oxygen or ignition source) will prevent a fire occurring. However, spacing the molecules out too much means molecules of fuel and oxygen reacting do not impart enough energy to the neighbouring molecules to ignite their reaction.

In some situations, for example when the fire involves burning metals like lithium or magnesium, using water to extinguish the fire could result in it getting hotter or even exploding. This is because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas adding to the fuel which subsequently burns.

Not all concentrations of flammable gas or vapour in air will burn or explode. The Lower Explosive Limit (LEL) is the lowest concentration of ‘fuel’ in air which will burn and for most flammable gases it is less than 5% by volume, with many at 1% volume. At lower concentrations the fuel molecules are too spread out to sustain a flame front. There is a high risk of explosion even when relatively small concentrations of gas or vapour escape into the atmosphere if the conditions allow the fuel concentration to spike in any location. This occurs if the fuel is very heavy or light compared with the surrounding air, or it can happen near the leak if the air is still.

LEL levels are defined in following standards: ISO10156, and IEC60079. The ‘original’ ISO standard lists LELs obtained when the gas is in a static state. LELs listed in the EN, and IEC standards were obtained with a stirred gas mixture; this resulted in lower LEL’s in some cases (i.e., some gas molecules take long enough to burn that they are on average closer to the neighbouring molecules when stirred, and thus able to impart enough energy to sustain a flame front).