Flammability limit is a specific limit of gas concentration when gas and air mixtures may become flammable (explode).
The lower explosive limit (LEL) identifies the smallest amount of gas in a mixture able to sustain a flame (when ignited). The upper explosive limit (UEL) is the smallest amount of gas in a mixture that can cause spontaneous (with no heat inflow from the outside) propagation of flame (self-ignition).

Here are explosion limits for most widespread gases in air mixtures:

  • H2: LEL=4.0; UEL=94.0% by volume
  • СО: LEL=12,.5; UEL=94.0% by volume
  • CH4: LEL=5.0; UEL=61.0% by volume
  • C3H8: LEL=2.2; UEL=5.5% by volume
  • C3H10: LEL=1.7; UEL=49.0% by volume