What is the approximate oxygen level at the start of a fire in a typical room?

• A. 15%
• B. 21%
• C. 25%
• D. 30%

Answer: (B)

The starting concept is that room air has a standard oxygen level, which is about 21 percent by volume. Before a fire has consumed much oxygen, the atmosphere in a typical room is essentially the same as outdoor air, so the approximate level is 21%. The other numbers would indicate atmospheres that are either already depleted of oxygen (below about 21%) or enriched with oxygen (above 21%), which doesn’t occur in a normal room at the start of a fire. As combustion begins and grows, oxygen is consumed and carbon dioxide increases, so the oxygen level gradually drops from that baseline as the fire develops, especially in enclosed spaces.