Flash point

At the flash point a flame or spark can ignite the vapor, and the flash may go out again. The liquid itself does not burn steadily until it is slightly warmer. What matters for safety is that the vapor, not the liquid, is what catches fire, and a liquid with a low flash point is giving off vapor at ordinary room temperatures.

Flash point is measured in a laboratory using either a closed-cup or an open-cup test, and the two methods can give different numbers for the same liquid. The SDS should say which method was used. Closed-cup values are generally lower and are the ones most often used for classification.

Flash point drives several rules. OSHA and GHS sort flammable liquids into Categories 1 to 4 using flash point together with initial boiling point. DOT Class 3 flammable liquids have a flash point of not more than 60 C (140 F), and DOT combustible liquids have a flash point above 60 C and below 93 C (200 F). Fire codes use the same idea to set limits on how much of a liquid may be stored in a room or cabinet.

A high flash point does not make a liquid safe. Heated above its flash point, or sprayed as a fine mist, even a combustible liquid can ignite. Mixtures also behave differently from pure liquids, so check the figure for the product you actually have.