Lean Mixture 2 : A mixture of air and gas that contains too much air for the amount of gas present to cause an explosion and is thus below the lower flammable limit
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}