An atmosphere is considered hazardous if it contains a hazardous gas in excess of 10 percent of the hazardous material's:

• A. Lower Explosive Limit (LEL).
• B. Upper Explosive Limit (UEL).
• C. Oxygen percentage.
• D. Particulate concentration.

Answer: (A)

The thing being tested is how flammability is measured in the air. The Lower Explosive Limit, or LEL, is the smallest concentration of a flammable gas in air that can ignite if there’s an ignition source present. Safety guidelines use a practical threshold of 10% of the LEL to flag a hazardous atmosphere because, at that level, there’s a real ignition risk even under normal conditions.

So, when a gas in the air reaches or exceeds 10% of its LEL, the environment is considered hazardous due to the possibility of combustion. The upper end of the flammable range, the UEL, is where the mixture becomes too rich to ignite, which is why it’s not used for classifying a hazardous atmosphere in this context. Oxygen percentage matters for combustion and health, but it isn’t described as a percentage of the LEL. Particulate concentration isn’t the factor being asked about here, since the focus is on flammability of gases.