Lung collapse (pneumothorax or atelectasis) can develop due to various reasons depending on its type.

Causes of Pneumothorax (Collapsed Lung):
Pneumothorax is the partial or complete collapse of the lung due to air entering the pleural space. Its main causes are:
- Spontaneous Pneumothorax: Typically occurs due to the rupture of small air sacs (blebs/bullae) on the lung surface. The resulting air creates pressure on the lung, leading to its collapse. This condition is more common in young, tall, slender men. Sudden changes in pressure, such as at high altitudes, during air travel, or diving, can also trigger bleb rupture.
- Traumatic Pneumothorax: Blunt or penetrating chest injuries (e.g., rib fractures, stab or gunshot wounds) can damage lung tissue and cause air leakage.
- Secondary Pneumothorax: Underlying lung diseases such as COPD, cystic fibrosis, lung cancer, or pneumonia weaken lung tissue, increasing the risk of pneumothorax.
- Iatrogenic Pneumothorax: Can develop as a result of unintended lung injury during medical procedures (e.g., biopsy, central venous catheter insertion).
- Risk Factors: Smoking is a significant risk factor for the development of pneumothorax.

Causes of Atelectasis (Lung Airlessness):
Atelectasis is the non-inflation and collapse of part or all of the lung due to airway obstruction or external compression. Its main causes are:
- Airway Obstruction:
- Foreign bodies blocking the airways.
- Tumors or mucus plugs (sputum accumulation) forming in the airway.
- Insufficient coughing or inability to take deep breaths due to pain in the postoperative period.
- External Compression: External pressure on the lung (e.g., a large tumor, pleural effusion - fluid accumulation between the lung membranes) can prevent the lung from expanding.
- Surfactant Deficiency: In newborns or certain lung diseases, a deficiency of surfactant makes it difficult for alveoli to remain open.