Sufficient ventilation shall be provided to keep concentrations of vapors below what percent of the lower explosive limit?

• A. Five percent
• B. Thirty percent
• C. Ten percent
• D. Twenty percent

Answer: (C)

Ventilation is used to dilute flammable vapors so their concentration stays well below the level that can ignite. The lower explosive limit is the minimum vapor-to-air concentration that can support a flame. Keeping the concentration to one-tenth of the LEL provides a solid safety margin, making ignition unlikely even if an ignition source is present and accounting for measurement or ventilation variations. Among the options, this one represents the standard safeguard: a lower fraction gives a clear buffer, while higher fractions move you closer to the flammable range and raise the risk.