Key values
Source: GESTIS substance database (IFA, October 2026).
Explosive range and ppm values
Hydrogen sulphide can ignite when mixed with air between 3.9 % and 50.2 % 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) | 39,000 | 55.3 |
| 20 %LEL | 7,800 | 11.05 |
| 10 %LEL | 3,900 | 5.53 |
Lighter or heavier than air?
Hydrogen sulphide is heavier than air: relative density 1.18 (air = 1). A release sinks and can collect in pits, trenches, drains and basements and travel a long way along the ground.
Ex equipment selection
Autoignition temperature 270 °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?
Forms in wastewater treatment plants, sewers and pumping stations, biogas plants, refineries and tanneries. It is highly toxic: the poisoning hazard starts far below the explosive limit, and at high concentrations it can no longer be smelled. Gas detection and ventilation are therefore assessed separately.
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: GESTIS substance database (IFA, October 2026).
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, Hydrogen-sulfide-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 hydrogen sulphide in ppm?
3.9 % by volume, that is 39,000 ppm. 10 %LEL corresponds to 3,900 ppm and 20 %LEL to 7,800 ppm.
Is hydrogen sulphide heavier than air?
Yes. Relative density 1.18 (air = 1): a release sinks and can collect in pits, trenches and drains.
Which Ex equipment is suitable for hydrogen sulphide?
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 autoignition temperature of hydrogen sulphide?
270 °C. A mixture touching a surface at this temperature can ignite without a flame or spark, so the surface temperature of equipment must stay below it (at least T3).