Key values
Source: compilation of GESTIS, NIOSH and ISO/IEC 80079-20-1 data; the conservative end of the range between sources (lowest LEL and AIT, highest UEL).
Explosive range and ppm values
Hydrogen can ignite when mixed with air between 4 % and 77 % 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) | 40,000 | 3.4 |
| 20 %LEL | 8,000 | 0.67 |
| 10 %LEL | 4,000 | 0.34 |
Lighter or heavier than air?
Hydrogen is lighter than air: relative density 0.07 (air = 1). A release rises and can collect under the ceiling, between beams and in enclosed upper spaces, so the position of a ventilation opening matters as much as its area.
The relative density applies to the substance at room temperature. Very cold releases, for example from an evaporating liquefied gas, can behave as heavier than air for a while.
Ex equipment selection
Autoignition temperature 500 °C. The maximum surface temperature of equipment in the zone must stay below it: all temperature classes (T1–T6) are suitable.
- T1450 °C suitable
- T2300 °C suitable
- T3200 °C suitable
- T4135 °C suitable
- T5100 °C suitable
- T685 °C suitable
- IIA not suitable
- IIB not suitable
- IIC suitable
Gas group IIC: equipment marked IIC may be used. Equipment marked IIB + H2 may also be used for hydrogen. 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?
Released while lead-acid batteries are charged: it is the flammable gas in battery charging areas. Also found in laboratories, chemical plants and fuel cell systems.
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 GESTIS, NIOSH and ISO/IEC 80079-20-1 data; the conservative end of the range between sources (lowest LEL and AIT, highest UEL).
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.
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 in ppm?
4 % by volume, that is 40,000 ppm. 10 %LEL corresponds to 4,000 ppm and 20 %LEL to 8,000 ppm.
Is hydrogen heavier than air?
No. Relative density 0.07 (air = 1): a release rises and can collect under the ceiling.
Which Ex equipment is suitable for hydrogen?
Temperature class: any class from T1 to T6. Gas group: equipment marked IIC. Equipment marked IIB + H2 may also be used for hydrogen. 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?
500 °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 T1).