What is the highest concentration in air that will burn or explode when an ignition source is present?

• A. Lower explosive limit
• B. Upper explosive limit
• C. Ignition point
• D. Flashpoint

Answer: (B)

The highest concentration in air that will burn or explode with an ignition source is the upper explosive limit. This marks the upper end of the flammable range for a gas in air; within the range between the lower explosive limit and the upper explosive limit, the mixture is capable of ignition. If the concentration is above this upper limit, the mixture becomes too rich to ignite, even with an ignition source. If it’s below the lower limit, the mixture is too lean to ignite.

For context, the lower explosive limit is the minimum concentration needed for ignition, and the upper explosive limit is the maximum concentration that can still support combustion. The other terms differ: flashpoint is the temperature at which vapors from a liquid can ignite, and ignition point (often autoignition temperature) is the temperature at which a substance will ignite without an external flame. For example, methane has a flammable range roughly from 5% to 15% in air; above about 15%, it won’t ignite under normal conditions.