Flash Point Explained: What It Means and How It Sets Flammability Class

The flash point of a liquid is the lowest temperature at which it gives off enough vapour to form an ignitable mixture with air near its surface. At that temperature, a small ignition source such as a spark will make the vapour flash briefly. It is the single most important number in judging how dangerous a flammable liquid is to store, handle and transport, and it is the value that decides which flammable-liquid category your product belongs to.

Flash point is not the same as ignition temperature

These two are easily confused. Flash point is about vapour reaching a flammable concentration when a flame or spark is present. Autoignition temperature is the temperature at which a substance ignites on its own with no spark at all, and it is usually far higher. Diethyl ether, for example, has a flash point around minus 45 degrees Celsius, meaning it is dangerous even in a freezer, while its autoignition temperature is much higher at around 160 degrees. Section 9 of an SDS lists both, and each answers a different safety question.

Open cup versus closed cup

Flash point is measured in standardised apparatus, and the result depends on the method. In a closed-cup test, such as Pensky-Martens or Abel, the sample is heated in a sealed cup and a flame is introduced through a shutter. In an open-cup test, such as Cleveland, the vapour can escape. Closed-cup values are generally lower, by a few degrees, because vapour is trapped. GHS and transport rules are based on closed-cup values wherever they can be used, so you should state the method on your documents, for example closed cup, 12 degrees Celsius.

The GHS flammable liquid categories

The UN GHS divides flammable liquids into four categories using flash point and initial boiling point.

• Category 1: flash point below 23 degrees Celsius and initial boiling point at or below 35 degrees. Extremely flammable liquid and vapour (H224).
• Category 2: flash point below 23 degrees and initial boiling point above 35 degrees. Highly flammable liquid and vapour (H225).
• Category 3: flash point at or above 23 and up to 60 degrees. Flammable liquid and vapour (H226).
• Category 4: flash point above 60 and up to 93 degrees. Combustible liquid (H227), which some jurisdictions do not adopt.

Acetone and ethanol fall in category 2, many kerosene-type products in category 3, and heavier oils in category 4 or are not classified. Categories 1 and 2 carry the flame pictogram and signal word Danger, while category 3 carries the flame with the signal word Warning. Our acetone guide shows a category 2 example.

Mixtures and solutions

The flash point of a mixture is not the weighted average of its components. Adding a high-flash-point liquid to a volatile one raises the flash point, but not in a simple straight line, and a small fraction of a very volatile solvent can drag the flash point of the blend down sharply. For aqueous alcohol solutions, flash point rises as water content rises, and below roughly 20 to 25 percent alcohol the solution may no longer flash. For blends, measure the flash point of the actual product, or use a validated calculation method, and say which you used. Our isopropanol and ethanol guide covers this for alcohol solutions.

Where the number gets used

Flash point determines the flammable-liquid category on the label, the Class 3 packing group in dangerous goods transport (with the boiling point setting group I, II or III), storage limits in fire codes, requirements for hazardous-area electrical equipment, and insurance conditions for warehouses. It also drives a few routine documents, including the SDS, the shipping declaration and the safety label. If it is wrong, all of these are wrong.

Practical tips

Take the number from a test report or a reliable supplier SDS, not from memory. State the method and units. Re-test when the formulation changes. For a product sold under a trade name, specify whether you quote the flash point of the product or of the main component, and keep the test report on file for audits. See also our toluene and xylene guide for two aromatic solvents with different flash points.