Residual volume is the amount of air that remains in a person's lungs after fully exhaling. Health professionals use tests to measure a person's residual air volume to help evaluate how well the lungs are functioning.

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.