Why are LFL and UFL important in hazard analysis of vapor releases?

• A. They define the flammable range; outside this range, flammable mixtures cannot propagate flame, while inside, ignition hazard is present.
• B. They measure vapor density.
• C. They determine toxicity thresholds.
• D. They indicate ignition energy.

Answer: (A)

Lower and upper flammable limits mark the concentration window of a vapor in air where ignition can occur and flame can propagate. If the vapor concentration is below the lower limit, the mixture is too lean to burn; if it’s above the upper limit, the mixture is too rich to burn. Within this range, an ignition source can start and sustain a flame. This makes the limits crucial in hazard analysis: they tell you when a vapor release could become flammable and therefore what controls are needed (such as ventilation to keep concentrations below LFL, or preventing ignition sources). They don’t describe how dense the vapor is, toxicity, or the energy required to ignite, which are separate properties.