How many percent of oxygen and carbon dioxide in the atmosphere?

The air we breath, at sea level, consists of 21% Oxygen, 78% Nitrogen and the remaining 1% mixed Gases. The human body requires only 4% Oxygen to sustain life, therefore, we breath out approxamately 17% Oxygen and the balance is Carbon Dioxide. =Note: This is why CPR works so well - when ventilating a patient, we are giving that patient 4 times the required Oxygen, and because you're blowing into the lungs, that positive pressure, makes sure that Oxygen creeps into all areas of the lungs.=