The Flammable Range (also called Explosive Range) is the concentration range of a gas or vapor that will burn (or explode) if an ignition source is introduced.

Three basic requirements must be met for explosion to take place:

1. flammable substance - fuel
2. oxidizer - oxygen or air
3. source of ignition - spark or high heat
   Below the explosive or flammable range the mixture is too lean to burn and above the upper explosive or flammable limit the mixture is too rich to burn. The limits are commonly called the "Lower Explosive or Flammable Limit" (LEL/LFL) and the "Upper Explosive or Flammable Limit" (UEL/UFL).