Residual volume is the amount of air that remains in a person's lungs after fully exhaling. Doctors use tests to measure a person's residual air volume to help check how well the lungs are functioning. It is normal to have some air remain after exhaling to keep the lungs from collapsing.

Residual volume is measured by:

• A gas dilution test. A person breathes from a container containing a documented amount of a gas (either 100% oxygen or a certain amount of helium in air). The test measures how the concentration of the gases in the container changes.
• Body plethysmography. This test measures the total amount of air the lungs can hold (total lung volume). For this test, a person sits inside an airtight booth called a plethysmograph and breathes through a mouthpiece while pressure and air flow measurements are collected.