Toluene is a widely used low conductivity flammable liquid. Toluene’s vapor pressure is such that one is assured of a flammable vapor/air mixture at the ambient temperatures found in most facilities. The Lower Explosive Limit (LEL) of toluene is 1% and is reached at a temperature of 40°F (4°C). The Upper Explosive Limit (UEL) is 7% and is reached at a temperature of approximately 98°F (37°C). Therefore, the vapor is ignitable in the vapor space of an open or closed container at any temperature between 40°F and 98°F. In addition, as with most other flammable liquids, the most easily ignitable toluene vapor/air mixture occurs approximately halfway between the LEL and the UEL; the temperature corresponding to this concentration is approximately 65°F. Therefore, at typical ambient conditions and even in a closed vessel, toluene will always be in the flammable range and simultaneously at its most easily ignitable concentration.
Since toluene can be rapidly charged by pouring, agitating, pumping, etc., and since the liquid itself cannot be grounded, it is not surprising that there have been many fire incidents associated with toluene —many of which could be prevented with a better understanding of static electricity and non-conductive flammable liquids!