Air trapping (or gas trapping) is an abnormal retention of air in the lungs after expiration. It is observed in obstructive lung diseases such as asthma, and chronic obstructive pulmonary disease. The cause is obstruction such that the patient is unable to expel air completely.

Air trapping is a cause of obstructive pattern spirometry results, leading to an elevated residual volume. Air trapping is often incidentally diagnosed on computed tomography (CT) scanning. The only way to absolutely differentiate air trapping from emphysema is by taking expiratory images. On expiratory films, retained hyperlucent gas will be visualised in cases of air trapping.

Air trapping represents poorly aerated lung, but on its own is clinically benign. It is 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.