That concentration of a combustible material in air below which ignition will not occur. Also known as the Lower Explosive Limit (LEL). Mixtures below this limit are said to be "too lean"
NFPA 91 Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Noncombustible Particulate Solids, 2004 ed. http://iepi.neu.edu.cn/Download/Standards/NFPA/NFP A%2091-2010%20Standard%20for%20Exhaust%20Systems%2 0for%20Air%20Conveying%20of%20Vapors,%20Gases,%20M ists,%20and%20Noncombustible%20Particulate%20Solid s.pdf, (Std: Exhaust Sys for Air Conveying of Vapors, Gas, Mists, and Noncomb Particulate Solids - Amer) {237 NFC}