In physiology, dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it (1) remains in the conducting airways, or (reaches alveoli that are not perfused or poorly perfused. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inspiration which remains in the conducting airways where no gas exchange can occur.

In humans, about a third of every resting breath has no change in O2 and CO2 levels. In adults, it is usually in the range of 150 mL.[1]

Dead space can be increased (and better envisioned) by breathing through a long tube, such as a snorkel. Even though one end of the snorkel is open to the air, when the wearer breathes in, they inhale a significant quantity of air that remained in the snorkel from the previous exhalation. Thus, a snorkel increases the person's dead space by adding even more "airway" that doesn't participate in gas exchange.