Flammable Liquid Properties


Volatility is the tendency or ability of a liquid to vaporize.

Vapor pressure is a measure of a liquid’s volatility. A high vapor pressure usually is an indication of a volatile liquid, or one that readily vaporizes.

Boiling point is the temperature at which the vapor pressure equals atmospheric pressure, such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form. In general, a low boiling point indicates a high vapor pressure and, possibly, an increased fire hazard.

Flashpoint is the minimum temperature at which the vapor concentration near the surface of the liquid is high enough to form an ignitable mixture. Any liquid with a flashpoint less than 100oF is considered to be a flammable liquid. A liquid with a flashpoint between 100oF and 200oF is combustible. In general, the relative hazard of a flammable liquid increase as the flashpoint decreases.

Flammable range is the proportion of vapor to air mixture that is ignitable and is expressed in terms of percentage of vapor in air by volume. The flammable range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). 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.

Auto ignition temperature is the minimum temperature at which a vapor-air mixture will spontaneously ignite, without the necessity of a spark or 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.


Stanley Howell
Sr. Program Manager
Chemical Safety

Steve Elwood
Associate Director for Laboratory Safety