Oxygen is considered deficient enough to not support fire when it falls below:

Oxygen is considered deficient enough to not support fire when it falls below:

• A. 3%
• B. 11%
• C. 14%
• D. 19.5%

Answer: (A)

The main idea is how oxygen concentration affects a flame. Fire needs a minimum amount of oxygen to sustain combustion, and as the atmosphere is diluted from normal air (about 21% O2), flames become smaller and eventually go out. In safety testing, there’s a practical threshold where the oxygen level is so low that combustion can no longer be maintained, and that threshold is around three percent. At or below this level, there isn’t enough oxygen to support the chemical reaction of the fuel with oxygen, so a flame cannot be sustained. That’s why three percent is the correct choice. The higher levels listed would still allow combustion under many conditions, so they don’t represent a point where fire cannot be supported.

The main idea is how oxygen concentration affects a flame. Fire needs a minimum amount of oxygen to sustain combustion, and as the atmosphere is diluted from normal air (about 21% O2), flames become smaller and eventually go out. In safety testing, there’s a practical threshold where the oxygen level is so low that combustion can no longer be maintained, and that threshold is around three percent. At or below this level, there isn’t enough oxygen to support the chemical reaction of the fuel with oxygen, so a flame cannot be sustained. That’s why three percent is the correct choice. The higher levels listed would still allow combustion under many conditions, so they don’t represent a point where fire cannot be supported.