To control the potential hazards posed by handling flammable and combustible liquids, several properties of these materials should be understood. It is the liquid's vapor, rather than the liquid itself, that ignites when mixed in certain proportions with air in the presence of an ignition source. Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated. 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. The 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. An important characteristic of any flammable or combustible liquid is its flashpoint. Flashpoint is the minimum temperature at which the vapor concentration near the surface of the liquid is high enough to form an ignitable mixture.