The lower explosive limit, or LEL, is the minimum concentration of a combustible gas in air that will ignite and burn. It is the yardstick against which flammable-gas hazards are measured, and it underpins the alarm settings on every combustible gas detector.
Lower Explosive Limit (LEL) in one line: The lower explosive limit (LEL), also called the lower flammable limit, is the lowest concentration of a flammable gas or vapor in air that can ignite. Below the LEL the mixture is too lean to burn; above the upper explosive limit (UEL) it is too rich. Combustible detectors report concentration as a percentage of LEL, and alarms trip well before 100 percent.
A gas-air mixture only burns within a specific window called the flammable or explosive range, bounded below by the lower explosive limit and above by the upper explosive limit (UEL). Below the LEL there is not enough fuel to sustain combustion; above the UEL there is too little oxygen. Methane, for example, has an LEL of about 5 percent and a UEL of about 15 percent by volume in air.
Each gas has its own limits. Hydrogen sulfide is flammable from roughly 4 to 44 percent, propane from about 2.1 to 9.5 percent, and hydrogen across a very wide range. The narrower the gap between LEL and UEL, the smaller the flammable window, but any gas within its range near an ignition source can explode.
Combustible gas detectors do not usually report the absolute volume percent of gas; they report the fraction of the way to the lower explosive limit, expressed as percent LEL. If methane's LEL is 5 percent by volume, then a reading of 20 percent LEL means the air holds one percent methane by volume, one-fifth of the way to a flammable mixture. A reading of 100 percent LEL means the mixture has just reached the flammable threshold.
This scale is convenient because it puts every combustible gas on the same 0-to-100 safety scale regardless of its actual chemistry, and it makes alarm setpoints intuitive. It is also why a combustible reading of, say, 30 percent LEL is already a serious warning even though only a fraction of a percent of gas is present by volume.
Because reaching 100 percent LEL means a flammable atmosphere exists, detectors alarm far earlier. Typical fixed-detector setpoints put a low alarm around 10 to 20 percent LEL and a high alarm around 40 to 60 percent LEL, giving people time to find the source, ventilate, and shut down before the mixture becomes explosive. Confined-space entry limits are often stricter, treating anything above about 10 percent LEL as unacceptable for entry.
Percent-LEL monitoring is central to hot-work permits, confined-space entry, and general area monitoring around tanks, separators, and compressors where hydrocarbon vapor can accumulate. Keeping atmospheres well below the LEL, by ventilation, purging, or isolation, is the fundamental strategy for preventing vapor-cloud fires and explosions.
LEL, the lower explosive limit, is the leanest gas-air mixture that will burn; below it there is too little fuel. UEL, the upper explosive limit, is the richest mixture that will burn; above it there is too little oxygen. A mixture is flammable only between the two.
No. It means the gas has reached the lowest flammable concentration for that specific gas, which is only a small fraction of the air by volume. For methane, 100 percent LEL corresponds to about 5 percent methane by volume, the threshold at which the mixture becomes flammable.
Combustible detectors output a percent-LEL signal that a controller uses to drive low and high alarms and shutdowns. Those readings can also be trended in a cloud SCADA platform like Merobix, pulled over Modbus, DNP3, or OPC UA, so operators see rising combustible-gas levels across a site while the trip logic stays in the local fire-and-gas system.
This page references the standards, specifications, and official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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