During the first phase of a fire, the oxygen concentration within the room contains approximately ______ percent oxygen.

• A. 10
• B. 16
• C. 21
• D. 79

Answer: (C)

In the early phase, the room’s oxygen level stays near what’s found in normal air because the fire is just starting and hasn’t consumed a large amount of oxygen yet. Normal air is about twenty percent oxygen, and during the incipient stage that amount remains essentially present. Only as the fire grows and more oxygen is used up—along with the buildup of heat and combustion products—does the concentration drop noticeably. So the approximation for the first phase is around twenty percent. The other options would require far greater oxygen depletion than typically occurs at this stage, and the much higher figure reflects the fact that most air is nitrogen, not oxygen.