A Highly Flammable Liquid (HFL) is a liquid with a flash point below 32 deg C. The flash point of a liquid is the lowest temperature at which the liquid can evaporate to produce vapour in sufficient concentration to form a combustible mixture with air near the surface of the liquid. The flash points of some common laboratory solvents are:
| Solvent Type | Temp (Deg C) | Temp (Deg F) |
| ethanol | +12 | 53.6 |
| toluene | +4 | 39.2 |
| acetone | -19 | -2.2 |
| carbon disulphide | -30 | -22 |
| diethyl ether | -45 | -49 |
As the flash points of all these liquids are below average ambient room temperature the liquids will always constitute a fire and explosion hazard. As an example the last three liquids listed above have flash points below the temperatures usually found in a refrigerator or freezer and will therefore constitute an explosion hazard even when in cold storage.
The Flash point is not the temperature at which a substance will ignite spontaneously. It is an indication of the ease with which a substance will burn if the vapours are exposed to a temperature at or above its auto-ignition temperature.