For transportation and handling safety, liquid fuels are classified according to their fire hazard characteristics. The classifications are based on the flash point of the liquid. Depending on the measured value of the flash point, liquids are grouped as to whether they are highly flammable, flammable or combustible liquids.

• A flammable liquid is a liquid that has a flash point <37.8°C (100°F). When the flash point drops below 32°C, it is classified as highly flammable.
• A combustible liquid is a liquid that has a flash point ≥37.8°C (100°F).
• There are also two categories of highly flammable liquids. These are separated by whether a given liquid has a boiling point above or below 37.8°C (100°F), as shown in Figure 38.

Flash points for some common liquids are shown in Figure 39. The lower the flash point, the easier it is for the liquid to form vapour at room temperature and ambient pressure. If vapour forms above the liquid surface, then it can mix with air and form a mixture within the flammability range. If heat is added (ignition source), the mixture will ignite. Since liquids with lower flash points need less energy added to them to form vapour in the first place, once they are burning they also need less heat input from the fire to keep them burning.