The ignition of a product depends on its concentration in the air. It can be produced within a range between two limits:

• lower explosive limit (LEL);
• upper explosive limit (UEL).

The lower explosive limit (LEL) of a gas, vapour or dust in the air is the minimum concentration above which the mixture could ignite.
The upper explosive limit (UEL) of a gas, vapour or dust is the maximum concentration below which the mixture could ignite.