What percent of oxygen does mouth-to-mouth rescue breathing provide?

• A. 21%
• B. 16%
• C. 50%
• D. 100%

Answer: (B)

The gas delivered in mouth-to-mouth rescue breaths comes from the rescuer’s lungs and is not pure oxygen. While ambient air is about 21% oxygen, the air you exhale after taking a normal breath has had some oxygen removed by your own body’s tissues. Exhaled air typically contains roughly 16% O2, with the rest being nitrogen and a small amount of CO2. So, when you rescue-breathe, the patient receives about 16% oxygen, not 21% or 100%. If higher oxygen concentrations are needed, supplemental oxygen with a mask or other device would be used, not mouth-to-mouth alone.