Dynamic elaboration on the Art & Science of Hazardous Area Protection
LOWER EXPLOSIVE LIMIT (LEL
LOWER EXPLOSIVE LIMIT (LEL)—is the lowest percentage by volume of gas (or vapor) in a gas-air mixture that will form an ignitable concentration. Below that concentration there is insufficient gas or vapor in the mixture and the gas-air mixture is too lean to be ignited.
UPPER EXPLOSIVE LIMIT (UEL)—is the highest percentage by volume of gas or vapor in a gas-air mixture that will form an ignitable concentration. Above that concentration there is too much gas or vapor in the mixture and the gas-air mixture is too rich to ignite.
An interesting example:
Methane (CH4) is highly flammable and when mixed with air it forms a mixture which can easily be ignited. Flammable mixture is one which will burn itself once ignited (Even after removal of the ignition source).
The Lower Explosive Limit of Methane is about 5% (By Volume: Meaning volume of Methane divided by total volume of mixture of air and methane: 5 ml of methane in 100 ml of Mixture) .
The Upper Explosive Limit is about 15%.
That means methane will not explode below 5% and above 15%. (It may burn around the source of ignition or heat)
More information at The working of a Fire Triangle