Threshold for dangerous air contamination due to flammable gas or vapor?

Threshold for dangerous air contamination due to flammable gas or vapor?

• A. 20%
• B. 15%
• C. 25%
• D. 30%

Answer: (A)

When evaluating flammable gases or vapors in air, the important measure is how close the mixture is to its lower explosive limit (LEL). The LEL is the minimum concentration of a flammable gas in air that can ignite if an ignition source is present. Safety practice expresses contamination as a percentage of that limit. Reaching 20% of the LEL means the air contains a substantial amount of flammable vapor—close enough to ignition risk that it’s considered dangerous. At this threshold, ventilation or removal of the vapor is required before continuing work. The other percentages aren’t the standard threshold used to designate dangerous air contamination in this context, so 20% of the LEL is the best choice.

When evaluating flammable gases or vapors in air, the important measure is how close the mixture is to its lower explosive limit (LEL). The LEL is the minimum concentration of a flammable gas in air that can ignite if an ignition source is present. Safety practice expresses contamination as a percentage of that limit. Reaching 20% of the LEL means the air contains a substantial amount of flammable vapor—close enough to ignition risk that it’s considered dangerous. At this threshold, ventilation or removal of the vapor is required before continuing work. The other percentages aren’t the standard threshold used to designate dangerous air contamination in this context, so 20% of the LEL is the best choice.