Key values
Source: compilation of the IEC 60079-20-1:2010 data table and GESTIS (IFA, October 2026); the conservative end of the range (lowest LEL, flash point and AIT, highest UEL); gas group from GESTIS.
Explosive range and ppm values
Tetrahydrofuran can ignite when mixed with air between 1.5 % and 12.4 % by volume. Below the LEL the mixture is too lean to ignite, above the UEL too rich. A rich mixture re-enters the range when it is diluted with air, so concentrations above the upper limit are not safe either.
The lower explosive limit in ppm and g/m³. Useful to compare a gas detector's %LEL reading with ppm values in a measurement report.
| Concentration | ppm | g/m³ (20 °C) |
|---|---|---|
| 1 LEL (100 %LEL) | 15,000 | 45.0 |
| 20 %LEL | 3,000 | 8.99 |
| 10 %LEL | 1,500 | 4.50 |
Lighter or heavier than air?
Tetrahydrofuran vapour is heavier than air: relative density 2.49 (air = 1). A release sinks and can collect in pits, trenches, drains and basements and travel a long way along the ground.
Flash point and liquid category
The flash point of tetrahydrofuran is −20 °C, the boiling point 66 °C. As room temperature (20 °C) is above the flash point, an ignitable vapour can form above the surface of an open container or a spill.
With a flash point below 23 °C and boiling point above 35 °C, it is a Category 2 flammable liquid under the CLP Regulation: H225 “Highly flammable liquid and vapour”, signal word “Danger”.
For another liquid, use the flammable liquid category calculator. The product label and safety data sheet apply; mixtures have different values.
Ex equipment selection
Autoignition temperature 230 °C. The maximum surface temperature of equipment in the zone must stay below it: T3, T4, T5 and T6 are suitable; T1 and T2 are not, because the limit is not below the autoignition temperature.
- T1450 °C not suitable
- T2300 °C not suitable
- T3200 °C suitable
- T4135 °C suitable
- T5100 °C suitable
- T685 °C suitable
- IIA not suitable
- IIB suitable
- IIC suitable
Gas group IIB: equipment marked IIB or IIC may be used. The equipment protection level (EPL: Ga, Gb, Gc) depends on the zone, not on the substance; the zone is decided in the explosion protection document from the release and the ventilation. Specific conditions in the certificate (X) and the ambient temperature range are checked separately.
Where is it found?
Used as a solvent in PVC pipe cements, polymer and coating solutions, and pharmaceutical and chemical synthesis. As it can form explosive peroxides after long contact with air, its storage time is also monitored. Commonly called THF.
Where these values sit in the explosion protection document
- Substance properties. The first step of the document is to record these values, with their source, for every flammable substance on site.
- Hazardous area classification. The lower explosive limit and the density are inputs to the release calculation and to the direction in which the zone extends.
- Equipment selection. The temperature class and the gas group decide whether Ex equipment in the zone is suitable.
This page does not decide zones. The type and extent of a zone are calculated from where, how often and how much is released, and from the ventilation.
Sources
Explosive limits, autoignition temperature, gas group, flash and boiling point: compilation of the IEC 60079-20-1:2010 data table and GESTIS (IFA, October 2026); the conservative end of the range (lowest LEL, flash point and AIT, highest UEL); gas group from GESTIS.
The relative density is calculated from the molar mass (M / 28.96). The ppm and g/m³ values apply at 20 °C and 1 atm (molar volume 24.055 L/mol). The temperature class follows the rule that the surface temperature must stay below the autoignition temperature.
Molecule model: Wikimedia Commons, Tetrahydrofuran-3D-balls.png (public domain).
This page is for information. For the substance on your site, the current safety data sheet (section 9) and the relevant standards apply. All substances are in the explosive limits table.
Frequently asked questions
What is the LEL of tetrahydrofuran in ppm?
1.5 % by volume, that is 15,000 ppm. 10 %LEL corresponds to 1,500 ppm and 20 %LEL to 3,000 ppm.
Is tetrahydrofuran heavier than air?
Yes, its vapour is heavier than air. Relative density 2.49 (air = 1): a release sinks and can collect in pits, trenches and drains.
Which Ex equipment is suitable for tetrahydrofuran?
Temperature class: T3, T4, T5 or T6 (T1 and T2 not suitable). Gas group: equipment marked IIB or IIC. The protection level (EPL) depends on the zone decided in the explosion protection document, not on the substance.
What is the flash point of tetrahydrofuran?
−20 °C. As room temperature is above it, an ignitable vapour can form above an open container or a spill. Under CLP it is a Category 2 flammable liquid (H225).