The lower explosive limit (LEL) is the minimum concentration of a specific combustible gas required to generate its combustion when in contact with oxygen (air). If the concentration of the gas is below the LEL value, the mix between the gas itself and the air is too weak to spark. The upper explosive limit (UEL) is the maximum level of concentration of the gas that will burn when mixed with oxygen; when the gas concentration is above the UEL value for the gas/vapor, the mix is too “fat” to ignite or explode. Note that UEL is sometimes also called UFL (upper flammable limit).


Note that LEL is sometimes also called “LFL” (lower flammable limit). Examples of LEL definition for two common explosive gas:

  • LEL for Hydrogen: 4.0
  • LEL for Methane: 5.0

The range between the lower and the upper explosive limit (LEL / UEL %) is defined as the flammable range of a specific explosive and flammable gas.

Flammable range meaning (LEL)



The risk of explosion of combustible gases has to be carefully managed. Field personnel operating in environments subject to such risk should know under which conditions specific gases may explode when in contact with air and an ignition element.

To fire an explosion, three conditions should occur at the same time:

  1. The presence of a combustible gas, the fueling element, in a specific concentration
  2. Presence of oxygen
  3. The existence of a sparking element (that ignites the two elements)

The proportion of fuel and the oxygen needed to generate an explosion depends on the type of combustible gas (each one has, indeed, a specific ratio). Gases will ignite only when mixed with air within a specific concentration range. If the gas is mixed with oxygen with too low or too high concentrations, the gas will not ignite and explode.

The lower and the upper explosion values define the required level of concentration by type of gas. Explosions will occur for gas concentrations within the LEL and the UEL value, not above or below, and the maximum explosive power will be for concentration at the midpoint of the flammable range.



(Note: LEL / UEL values are based on room temperature and atmospheric pressure, ignition fired by a tube of 2-inch diameter. As the temperature, the pressure and the ignition increase, the explosive limits by gas vary. The values are determined empirically and may change depending on the source of the information). The lower and the upper explosive limits by gas are:

Acetone 2.6 13.0
Acetylene 2.5 100.0
Acrylonitrile 3.0 17
Allene 1.5 11.5
Ammonia 15.0 28.0
Benzene 1.3 7.9
1,3-Butadiene 2.0 12.0
Butane 1.8 8.4
n-Butanol 1.7 12.0
1-Butene 1.6 10.0
Cis-2-Butene 1.7 9.7
Trans-2-Butene 1.7 9.7
Butyl Acetate 1.4 8.0
Carbon Monoxide 12.5 74.0
Carbonyl Sulfide 12.0 29.0
Chlorotrifluoroethylene 8.4 38.7
Cumene 0.9 6.5
Cyanogen 6.6 32.0
Cyclohexane 1.3 7.8
Cyclopropane 2.4 10.4
Deuterium 4.9 75.0
Diborane 0.8 88.0
Dichlorosilane 4.1 98.8
Diethylbenzene 0.8

1,1-Difluoro-1-Chloroethane 9.0 14.8
1,1-Difluoroethane 5.1 17.1
1,1-Difluoroethylene 5.5 21.3
Dimethylamine 2.8 14.4
Dimethyl Ether 3.4 27.0
2,2-Dimethylpropane 1.4 7.5
Ethane 3.0 12.4
Ethanol 3.3 19.0
Ethyl Acetate 2.2 11.0
Ethyl Benzene 1.0 6.7
Ethyl Chloride 3.8 15.4
Ethylene 2.7 36.0
Ethylene Oxide 3.6 100.0
Gasoline 1.2 7.1


Heptane 1.1 6.7
Hexane 1.2 7.4
Hydrogen 4.0 75.0
Hydrogen Cyanide 5.6 40.0
Hydrogen Sulfide 4.0 44.0
Isobutane 1.8 8.4
Isobutylene 1.8 9.6
Isopropanol 2.2

Methane 5.0 15.0
Methanol 6.7 36.0
Methylacetylene 1.7 11.7
Methyl Bromide 10.0 15.0
3-Methyl-1-Butene 1.5 9.1
Methyl Cellosolve 2.5 20.0
Methyl Chloride 7.0 17.4
Methyl Ethyl Ketone 1.9 10.0
Methyl Mercaptan 3.9 21.8
Methyl Vinyl Ether 2.6 39.0
Monoethylamine 3.5 14.0
Monomethylamine 4.9 20.7
Nickel Carbonyl 2.0

Pentane 1.4 7.8
Picoline 1.4

Propane 2.1 9.5
Propylene 2.4 11.0
Propylene Oxide 2.8 37.0
Styrene 1.1

Tetrafluoroethylene 4.0 43.0
Tetrahydrofuran 2.0

Toluene 1.2 7.1
Trichloroethylene 12.0 40.0
Trimethylamine 2.0 12.0
Turpentine 0.7

Vinyl Acetate 2.6

Vinyl Bromide 9.0 14.0
Vinyl Chloride 4.0 22.0
Vinyl Fluoride 2.6 21.7
Xylene 1.1 6.6



To operate safely in hazardous environments, i.e. closed spaces with combustible gases present, the concentration of the gas should be monitored closely. As the concentration of the gas exceeds 20% of the gas LEL, is considered unsafe.

To monitor gas concentration value in closed and hazardous environments, operators may use LEL meters (also called, LEL meters/detectors) which are designed with catalytic bead and infrared sensing elements to measure the lower explosive limit of gases. These gas detectors give warnings to the operators whenever the combustible gas is present in the environment at levels around 10%. LEL meters are rather sophisticated devices, that feature microprocessors based modular design with self-calibration and digital display of the information.

PID meter LEL UEL combustion

The most used LEL meter is the Wheatstone bridge type, which is effective for most applications and environments. However, the Wheatstone bridge LEL detector may not be effective for specific conditions, or gases, that require higher sensitivity sensors. The PID detectors (“Photoionization detectors”) are an option when a more accurate LEL measurement is required in hazardous environments. PID can measure the concentration of inflammable gases and other toxic gases even at very low levels (from ppb, i.e. parts per billion, up to 10k ppm, i.e. 1%). PIDs are way more sensitive tools then common LEL meters and are generally more expensive. PIDs are suited to measure the following organic compounds:

  • Alcohol
  • Aromatics
  • Amines & Amides
  • Chlorinated hydrocarbons
  • Ketones & Aldehydes
  • Sulfur compounds
  • Unsaturated hydrocarbons
  • Saturated hydrocarbons – like butane and octane

The inorganic compounds that can be measured by photoionization detectors are:

  • Ammonia
  • Bromine
  • Iodine
  • Hydrogen sulfide
  • Nitric Oxide
  • Semiconductor gases