Atelectasis is an area of the lung that is not completely inflated. In atelectasis in newborns (fetal atelectasis), the lungs do not expand normally at birth. Reasons for this happening include premature birth (being born too early), lack of oxygen, and airway blockage.

In adults, atelectasis may be the result of airway blockage or of not breathing deeply (after surgery or because of neuromuscular disease). Because atelectasis is known to occur most commonly as a complication after surgery, deep breathing is often used to prevent or treat it. Mild atelectasis is not usually a problem. But when atelectasis is severe, it can cause pain and/or shortness of breath, and it can lower oxygen levels.

Atelectasis is treated by restoring airflow to the lung and by removing any blockage.