Glossary / Physical properties
Lower explosive limit (LEL)
Lower explosive limit (LEL) is the lowest concentration of a flammable gas or vapor in air, usually given as percent by volume, that can ignite when an ignition source is present. Below the LEL the mixture is too lean to burn.
Explanation
Also called the lower flammable limit (LFL), the LEL is one end of the Flammable range. A mixture with less fuel than the LEL does not have enough vapor to sustain a flame. Between the LEL and the Upper explosive limit (UEL) it can ignite, and above the UEL it is too rich.
The LEL is the number gas detectors rely on. A combustible gas meter reads as a percentage of the LEL, not as a percentage of gas in air, so a reading of 10 percent LEL means the atmosphere is one tenth of the way to the lowest ignitable concentration. Alarm set points are normally set well below 100 percent LEL to give a margin for error.
Gases and vapors with a low LEL are especially hazardous because only a small amount in air is needed to reach a flammable mixture. A small spill of a volatile solvent in a closed room can reach it quickly, particularly if the vapor is heavier than air and settles in low spots (see Vapor density).
Values vary with temperature, pressure and the oxygen content of the air, and published figures apply to standard conditions. For mixtures, the SDS may give a value for the product or none at all.
What to do and watch for
- Use a calibrated combustible gas meter before hot work or entry into spaces that may hold vapors.
- Ventilate to keep concentrations far below the LEL, not just under it.
- Do not rely on smell; many flammable vapors can reach hazardous levels before they are noticed.
Where it appears on an SDS
- Section 9: Physical and chemical properties. Gives the lower (and usually upper) explosion or flammability limits, normally as percent by volume in air.
- Section 5: Fire-fighting measures. Notes explosion hazards and any risk of vapors forming explosive mixtures with air.
SDS content and wording vary by supplier. This page is general reference and does not replace the current SDS for your product.