Air trapping, also called gas trapping, is an abnormal retention of air in the lungs where it is difficult to exhale completely.^[1] It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive pulmonary diseases such as emphysema and chronic bronchitis.

Air trapping is not a diagnosis but is a presentation of an illness, and can be a guide to the appropriate differential diagnosis.

Exhaled volumes are measured by a pulmonary function test or simple spirometry, leading to an elevated residual volume and a measurement of forced expiratory volume. Air trapping is often incidentally diagnosed on computed tomography (CT) scanning. On expiratory films, retained hyperlucent gas will be visualised in cases of air trapping.^[2]

Air trapping represents poorly aerated lung, but on its own is clinically benign. It is a common problem for smokers who dive. On diving the lung volume collapses and pushes air into the poorly aerated regions. On arising from a deep depth, these air-trapped areas of lung expand. This places great pressure on the lung tissue which can rupture.^[3]