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Cobic-Ex will explain everything you need to know about the lower explosive limit in this blog. We will also cover how the LEL differs between substances and other topics regarding the lower explosion limit.

What is LEL?

For an explosion to occur, three substances must be present: air, combustible gas, and an ignition source. However, the combustible gas must have a specific concentration to contribute to the explosion. So, here is where the lower and upper explosive limits become of interest. Subsequently, we arrive at the question: What does LEL mean?

The lower explosive limit is the lowest concentration of a specific gas or vapor that will burn in the air. This limit is expressed as a percentage and varies between gasses and vapors. For instance: the gas LEL of ethane is approximately 3%, while the LEL of methane is 5%. This means that for ethane to support ignition, a lower concentration is needed than for methane.

From this, we can conclude that there are different windows regarding the substance concentration to support ignition. This is also where the upper explosive limit (UEL) becomes essential. More about the upper explosive limit in the next paragraph.

Lower and upper explosive limit

Next to having a minimal concentration to support ignition, gases and vapors also have a maximum concentration to contribute to an explosion. The upper explosive limit is the maximum concentration of a gas or vapor in the atmosphere that can generate a flash or fire when an ignition source is present.

But what will happen if the concentration of a particular flammable gas is below the lower explosive limit or higher than the upper explosive limit in the presence of an ignition source? Ironically, the answer is nothing. In those cases, the concentration of the flammable gases is not within the window to generate a flash or fire.

In the picture above, you can see the LEL and UEL graphed so that you can better understand the concepts. Next, a gas/air mixture is defined as “too rich” if its concentration exceeds the upper explosive limit. If the concentration of the flammable gas is below the lower explosive limit, we can call the gas/air mixture “too lean.”

The lower explosive limit of different substances

Questions as: What is the gas LEL?, What is the LEL of methane? And what is the LEL hydrogen has? Are often asked when dealing with flammable gases and vapors in a particular environment. Environments with combustible gases and vapors are usually classified as ATEX Zones.

To answer the questions above, we have provided a table in which you can see the lower explosive limit and UEL (in volume) of different flammable gases and vapors.

Type of gas LEL UEL
n-Butane 1.8% 8.4%
Iso-Butane 1.8% 8.4%
Ethane 3% 12.4%
Ethylene 2.7% 36%
Hexane 1.2% 7.4%
Hydrogen 4% 75%
Methane 5% 17%
Pentane 1.5% 7.8%
Propane 2.2% 9.5%


It is interesting to see that hydrogen has a huge window in which it can generate a flash or fire in the presence of an ignition source, so you must work carefully with this substance. Contrary to this, the explosive limits of pentane are much closer to each other, so the window in which it can support ignition is smaller. Therefore, pentane is less dangerous to work with than hydrogen in environments with potential explosions.

How is the lower explosive limit measured?

People work with flammable gases and vapors in many industries, such as gas/oil, chemical production, and transportation. In those working environments, the safety of the workers has to be guaranteed. The concentration of those gases is measured to keep environments safe in which flammable gases and potential ignition sources are present. But how is this concentration measured? Usually, the concentration of gases within an area is measured by sensors, often called “LEL sensors.” These LEL sensors calculate the difference in resistance between two elements from which one has a catalyst. This resistance represents the lower explosive limit.

Next, there are different ways to measure the concentration of certain gases within an area. Safety is not necessarily guaranteed by using LEL sensors, so please take adequate measures to ensure safety in your working environment. For questions, don’t hesitate to get in touch with us at

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