A flammable liquid is a liquid which can be easily ignited in air at ambient temperatures, i.e. it has a flash point at or below nominal threshold temperatures defined by a number of national and international standards organisations. The Occupational Safety and Health Administration (OSHA) of the United States Department of Labor defines a liquid as flammable if it has a flash point at or below 93 °C/199.4 °F. Prior to bringing regulations in line with the United Nations Globally Harmonized System of Classification and Labeling of Chemicals (GHS) in 2012, OSHA considered flammable liquids to be those with a flash point below 37.8 °C/100 °F. Those with flash points above 37.8 °C/100 °F and below 93.3 °C/200 °F were classified as combustible liquids. Studies show that the actual measure of a liquid's flammability, its flash point, is dependent on the local air pressure, meaning that at higher altitudes where the air pressure is lower, the flash point is also lower.

Categorization

Both OSHA and GHS further divide flammable liquids into 4 categories: These categorizations are dependent upon a set altitude and atmospheric pressure, as both boiling point and flash point change with changes in pressure.

Labeling

Both GHS and OSHA require the labeling of flammable liquids, on containers and safety data sheets, as follows: