The proportion of vapor to air mixture that is ignitable is referred to as the flammable range, and is expressed in terms of percentage of vapor in air by volume. The LFL is the minimum concentration of flammable liquid vapor in air that will support the propagation of flame, or spread of flame through the entire volume of vapor-air mixture, upon contact with an ignition source. The UFL is the maximum concentration of vapor in air that will support the propagation of flame. It is important to note that vapor-air mixtures below the LFL may burn at the ignition source without propagating away from the point of ignition. The auto ignition temperature is the minimum temperature at which a vapor-air mixture will spontaneously ignite, without the necessity of a spark of flame.
Vapor density is a measure of a vapor's weight when compared to air.
Air is assigned a value of 1. Heavier, or denser, vapors tend to sink to floor level while lighter, less dense vapors tend to rise to ceiling level. This property must be taken into account when working with flammable or combustible liquids outside of fume hoods. Most flammable liquid vapors are heavier than air. These vapors can travel some distance and encounter ignition sources remote from the workstation.