An atmosphere containing a flammable gas is ignitable when the concentration of the gas in the air is between its lower flammable limit (LFL) and upper flammable limit (UFL). The LFL is the lowest concentration of gas in air that can ignite and sustain a flame, while the UFL is the highest concentration of gas in air that can ignite and sustain a flame. Below the LFL, the mixture is too lean to burn, and above the UFL, the mixture is too rich to burn.