QEHSHealth
Lower Explosive LimitLEL
The minimum concentration of a gas in air at which ignition can occur.
Below the LEL, the mixture is too lean to ignite. Above the UEL (Upper Explosive Limit), it is too rich. Gas detectors typically alarm at 10% LEL (action level) and 25% LEL (evacuation). Oxygen deficiency (<19.5%) can suppress combustion while still being immediately dangerous to life.