Flammability or Flammability Limits are volume percentages of vapor –air mixtures that can just be ignited. The mixture will not burn if the composition is lower than the lower flammable limit (LFL) nor higher than the higher flammable limit (HFL). LFL and HFL are also called the lower and upper explosive limits, (LEL and UEL).Reference: Cowl and Louvar, Chemical process safety, 3rd edition.