At what oxygen concentration does combustion usually not occur?

Combustion typically requires an adequate concentration of oxygen to maintain a fire. The general threshold for combustion to occur is at least 16 percent oxygen in the atmosphere. When oxygen concentrations drop to around 10-12 percent, combustion processes are significantly hindered. At these lower levels, the ability of materials to ignite and sustain a fire is greatly diminished.

The range of 14-16 percent may allow for some combustion under specific circumstances but is still insufficient for sustained fire. As oxygen concentration approaches 18-20 percent, combustion becomes more efficient and stable, allowing fires to ignite and burn more vigorously. At concentrations exceeding 20 percent, combustion can occur even more effectively, but this leads to an increased risk of rapid spread and intensity of fire.

Therefore, 14-16 percent oxygen levels are not sufficient for reliable combustion, making this the correct choice as the oxygen concentration at which combustion usually does not occur effectively.