A liquid that has a flash point below 100ºF, and which has a vapor pressure not exceeding 40 psia.
Flammable liquids are also designated as Class I liquids. Within this Class I category, there are sub-classifications. Class IA liquids are those that have flash points below 73ºF and boiling points below 100ºF. Class IB liquids have flash points below 73ºF and boiling points at or above 100ºF. Class IC liquids have flash points at or above 73ºF and below 100ºF.
These classifications are important, among other reasons, because they control the type of equipment that the fire codes require be used when the classified liquids are stored and handled. If Class I liquids are present in certain locations, for example, electrical devices used at the same location must be explosion-proof.
See also Flash point.