The lower explosive limit (LEL), also called the lower flammable limit (LFL), is the lowest concentration of a flammable gas or vapor in air that can ignite and burn when it meets an ignition source. Below the LEL, the mixture is too lean to burn. Above the upper explosive limit (UEL), it is too rich. Between the two is the flammable range.
The LEL matters because flammable atmospheres can form quickly in tanks, pits, sewers, process areas and anywhere fuels or solvents are used. Gas detectors measure flammable gases as a percentage of the LEL, and OSHA uses LEL-based limits to define when an atmosphere is hazardous for confined space entry.
What Is the Lower Explosive Limit?
Each flammable gas or vapor has its own LEL, expressed as a percentage of the gas by volume in air. For example, methane has an LEL of about 5% by volume. A mixture of 5% methane in air can ignite; a mixture of 3% generally cannot.
LEL values are usually measured at normal temperature and pressure. Higher temperatures, higher pressures and oxygen-enriched atmospheres can widen the flammable range and make ignition easier, so actual conditions in a process can differ from published values.
LEL vs. UEL
- LEL: the minimum concentration that can ignite.
- UEL: the maximum concentration that can ignite. Above it, there is not enough oxygen for combustion.
- Flammable range: the concentrations between the LEL and UEL.
An atmosphere above the UEL is not safe. Ventilation or a leak can dilute it back into the flammable range, and it is likely to be oxygen-deficient and toxic as well.
Common LEL Values
Approximate published LEL and UEL values at normal conditions include:
- Methane: LEL 5.0%, UEL 15%
- Propane: LEL 2.1%, UEL 9.5%
- Hydrogen: LEL 4.0%, UEL 75%
- Hydrogen sulfide: LEL 4.0%, UEL 44%
- Gasoline: LEL about 1.4%, UEL about 7.6%
Check the chemical's safety data sheet or the NIOSH Pocket Guide for the specific substances you handle.
How %LEL Readings Work
Portable gas monitors usually display flammable gas as %LEL, not % by volume. A reading of 100% LEL means the atmosphere has reached the lower explosive limit. A reading of 10% LEL means the gas is at one-tenth of the LEL.
For methane, 100% LEL equals about 5% by volume, or 50,000 ppm. So 10% LEL of methane is about 0.5% by volume, or 5,000 ppm.
Points to keep in mind:
- Combustible gas sensors are calibrated to one gas, often methane or pentane. Other gases give different responses, so manufacturers publish correction factors.
- Catalytic bead sensors need oxygen to work, and can read low in oxygen-deficient air.
- Silicones, lead compounds and some other substances can poison catalytic sensors.
- Toxic gases can be dangerous far below 10% LEL. For example, hydrogen sulfide is immediately dangerous at concentrations a %LEL sensor would barely register, so toxic sensors are also needed.
Detectors should be bump tested before each day's use and calibrated according to the manufacturer's instructions.
OSHA's 10% LEL Limit for Confined Spaces
Under the permit-required confined space standard, 29 CFR 1910.146(b), a hazardous atmosphere includes flammable gas, vapor or mist in excess of 10% of its lower flammable limit. The construction confined space standard, 29 CFR 1926 Subpart AA, uses the same 10% threshold. Combustible dust at or above its LFL, oxygen below 19.5% or above 23.5%, and toxic concentrations above exposure limits are also hazardous atmospheres. Learn more in our permit-required confined spaces guide.
Typical Gas Monitor Alarm Settings
Many portable multi-gas monitors ship with a low alarm at 10% LEL and a high alarm at 20% LEL, but factory defaults vary by manufacturer and model. Employers should set alarms based on the applicable regulations, the hazards and their own procedures. Many sites require work to stop and workers to leave at or before 10% LEL.
How to Monitor for Flammable Atmospheres Step by Step
- Know the hazard. Identify which flammable gases or vapors could be present and their LELs.
- Check the instrument. Confirm calibration is current and bump test the detector before use.
- Test in the right order. Test oxygen first, then flammable gases, then toxic gases.
- Test at multiple levels. Check the top, middle and bottom of a space, since gases can be lighter or heavier than air.
- Apply correction factors. Adjust readings when the gas present differs from the calibration gas.
- Compare to limits. Do not enter or begin hot work if readings exceed your permit limit, such as 10% LEL.
- Monitor continuously. Keep monitoring during the work and evacuate when alarms sound.
- Record results. Document readings, times, instrument ID and tester on the permit.
Common Mistakes to Avoid
- Confusing %LEL with % by volume.
- Assuming an atmosphere above the UEL is safe because it will not ignite.
- Ignoring sensor cross-sensitivity and correction factors.
- Using a catalytic sensor in oxygen-deficient air and trusting a low reading.
- Testing only at the entry point rather than throughout the space.
How SMS360 Helps
SMS360's Work Permits module helps teams record atmospheric test results, including %LEL readings, on confined space and hot work permits before work is approved. Daily detector bump tests can be logged as checklists in Audits & Inspections, and flammable gas data from safety data sheets can be kept in Safety Data Sheets.
Frequently Asked Questions
What is the difference between LEL and LFL?
None in practice. Lower explosive limit and lower flammable limit are used interchangeably. OSHA's confined space standards use the term lower flammable limit.
What does 10% LEL mean?
It means the flammable gas concentration is one-tenth of the amount needed to ignite. OSHA treats flammable gas above 10% of the LFL as a hazardous atmosphere in confined spaces.
Why is 10% LEL commonly used as a limit?
It leaves a large safety margin below the point where ignition is possible. Gas concentrations can vary across a space and change quickly, and instruments have measurement tolerances, so stopping work well below 100% LEL protects against pockets of higher concentration.
Is 0% LEL always safe?
Not necessarily. A 0% LEL reading says nothing about oxygen levels or toxic gases, which must be tested separately, and a reading can be low if the sensor is poisoned or oxygen is deficient.
