Flammable gases will burn under which concentration of oxygen in air?

Flammable gases will burn in normal concentrations of oxygen, which is typically around 21% in the atmosphere. This concentration is sufficient to support combustion because it provides the necessary oxidizing agent for the fuel to ignite and sustain a flame.

In normal air conditions, various flammable materials have specific flammability limits; however, those limits are often designed around this typical oxygen level. The combustion process requires a balance of fuel and oxidizer, and normal atmospheric oxygen concentration strikes the right balance for many flammable gases to ignite and burn effectively.

While some materials may have specific conditions under which they burn best, the general consensus is that the usual atmospheric level of oxygen, which is not excessively high or low, supports the combustion process for a wide range of flammable gases.