Guide
Flash Point Explained: What It Means on an SDS
The flash point is the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near its surface when an ignition source is present. A lower flash point means a more easily ignited liquid. It is listed in Section 9 of an SDS and drives the GHS flammable liquid category and the transport class.
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What flash point actually measures
A liquid does not burn; its vapor does. At low temperatures few molecules evaporate and the vapor-air mixture above the liquid is too lean to ignite. As the temperature increases, the vapor concentration rises until it reaches the lower flammable limit; the temperature at which this occurs under test conditions is the flash point. Ignition at this point produces a brief flash rather than sustained burning. The glossary gives a short definition.
That is why a cold flammable liquid is still dangerous: if its flash point is below room temperature, the room itself is above the flash point, and vapors are present all the time.
Closed cup versus open cup
Flash point depends on how it is measured. In a closed-cup test, the sample is in a sealed vessel, which traps vapor and normally gives lower flash point values. In an open-cup test, vapor can escape and values tend to be higher. Classification systems for flammable liquids normally rely on closed-cup values. When comparing products, compare like with like, and look for the method on the SDS; if it is not stated, ask the supplier. Section 9 also lists related values, such as flammability limits and autoignition temperature; see Section 9.
How flash point sets the GHS flammable liquid category
| GHS category | Criteria | Signal word |
|---|---|---|
| Category 1 | Flash point below 23 °C and initial boiling point at or below 35 °C | Danger |
| Category 2 | Flash point below 23 °C and initial boiling point above 35 °C | Danger |
| Category 3 | Flash point at or above 23 °C and at or below 60 °C | Warning |
| Category 4 | Flash point above 60 °C and at or below 93 °C | Warning |
These are the UN GHS criteria; the corresponding statements, such as H225 and H226, are in the hazard statement reference. Whether a country adopts all four categories depends on its regulations; check yours. See the GHS overview.
Flash point in transport
For transport, flammable liquids are class 3 in the UN system, and the packing group depends on the flash point and the initial boiling point. A flash point around 60 °C (140 °F) is the general upper limit for being considered a flammable liquid in many transport regulations, though details and exceptions vary. See DOT hazard classes and Section 14.
Flash point is not the same as autoignition or fire point
- Fire point: the temperature at which the vapor continues to burn after ignition, slightly above the flash point.
- Autoignition temperature: the temperature at which the material ignites without any spark or flame. It is usually much higher than the flash point.
- Flammable limits: the range of vapor concentrations in air that can burn. The flash point is linked to the lower limit.
Using flash point in practice
- Control ignition sources, including static electricity, whenever the working temperature is near or above the flash point; ground and bond containers.
- Choose ventilation to keep vapor well below the lower flammable limit.
- Store in approved cabinets or rooms according to fire code; see storage compatibility.
- Remember that heated products, mists and sprays can ignite even if their flash point is high.
- For mixtures, use the measured value for the product, not the lowest ingredient value, if available; see SDSs for mixtures.
- The NFPA 704 flammability rating is also based on flash point and boiling point.
Frequently asked questions
What does flash point mean?
It is the lowest temperature at which a liquid produces enough vapor to form an ignitable mixture with air near its surface in a standard test.
Is a lower flash point more dangerous?
Yes. A lower flash point means the liquid forms ignitable vapor at lower temperatures, so it is easier to ignite.
Where do I find the flash point on an SDS?
In Section 9, Physical and chemical properties, with the test method (closed or open cup).
Is flash point the same as autoignition temperature?
No. Flash point needs an ignition source; autoignition temperature is the temperature at which a material ignites on its own and is usually much higher.