Gas is one of the four states of matter, the others being liquids, solids and plasma. Gas particles move around freely and can disperse easily.
Combustion is a burning process; in chemical terms it is the reaction of a substance (fuel) with oxygen, producing heat and light.
A combustible gas is one of these substances, which when mixed with oxygen and ignited, will burn.
Combustible gases include methane, propane, butane, ethanol, ammonia and hydrogen.
If you have a gas hob in your home, it uses combustible gas as its fuel source, as does any other gas flame.
Any gas that can burn, under varying circumstances, can be labelled combustible. A flammable gas is a combustible gas that will burn in typical conditions. However, the terms combustible and flammable are sometimes used interchangeably. These conditions usually relate to the concentration of gas and the upper and lower limits of combustibility.
LEL and UEL (lower explosive limit and upper explosive limit)
Also known as lower and upper flammability limits (LFL and UFL), this is the percentage of gas in the air (concentration) between which gases will catch fire when exposed to an igniter.