Mouth-to-mouth ventilation has its own costs. Exhaled air contains 17 percent oxygen, less than 21 percent of fresh air, and 4 percent carbon-dioxide, which can inhibit cardiac contraction. Studies have found that from 10 to 35 percent of patients who receive CPR inhale stomach contents, emitted after air is blown into the stomach rather than to the lungs. And time allocated to ventilation, especially if only one rescuer is involved, is subtracted from the efforts to provide chest compressions.