A lung collapses when air leaks into the space between the lung and the chest wall, breaking the seal that keeps the lung inflated. Under normal conditions, this space (called the pleural space) has no air in it and maintains a slight vacuum. That vacuum is what holds your lungs open. When it’s disrupted, the lung’s own elastic tissue pulls it inward, like a stretched rubber band snapping back to its resting shape.
What Keeps Your Lungs Inflated
Your lungs don’t inflate on their own. They’re passive, stretchy organs that rely on the chest wall and a thin layer of fluid between the lung and the ribcage to stay expanded. The chest wall naturally wants to spring outward, while the lung tissue naturally wants to recoil inward. These two opposing forces create negative pressure in the pleural space, a pressure slightly lower than the atmosphere around you. That pressure difference is what pulls air into your lungs with each breath and keeps them from deflating between breaths.
Think of it like two wet glass slides pressed together. They’re hard to pull apart because the thin film of liquid between them creates a strong seal. The fluid in your pleural space works the same way, coupling the lung to the chest wall so the lung follows the ribcage’s movement during breathing. If air gets into that space, the seal breaks. The negative pressure disappears, and the lung’s elastic tissue does exactly what it’s been trying to do the whole time: recoil inward and collapse.
How Air Gets Into the Pleural Space
There are several ways the seal can break, and the cause determines what type of collapsed lung (pneumothorax) you have.
Spontaneous Collapse in Healthy People
The most puzzling type happens without any injury or known lung disease. Small air-filled sacs called blebs can form on the surface of the lung, and when one ruptures, it creates a hole that lets air escape into the pleural space. Changes in air pressure or a sudden deep breath can trigger a rupture. Tall, thin young men are at the highest risk, likely because rapid growth of the chest during growth spurts stretches the lung tissue in ways that make blebs more likely to develop. In some people, a genetic mutation affecting a protein called folliculin may trigger low-grade inflammation that weakens lung tissue and promotes bleb formation.
Collapse From Existing Lung Disease
When someone already has damaged or weakened lungs, the risk of collapse rises significantly. Conditions like COPD, emphysema, cystic fibrosis, asthma, pneumonia, tuberculosis, lung cancer, and pulmonary fibrosis can all create weak spots or bulging areas (bullae) in the lung tissue that are prone to bursting. This type tends to be more serious because the lungs are already compromised, so even a partial collapse can cause severe breathing difficulty.
Traumatic Collapse
A broken rib, a stab wound, a car accident, or even a medical procedure like a lung biopsy can puncture the lung or the chest wall, allowing air to rush into the pleural space. The mechanism is the same: air enters, the vacuum is lost, and the lung recoils.
What a Tension Pneumothorax Does
Most collapsed lungs are dangerous but manageable. A tension pneumothorax is a medical emergency. It happens when the injury acts like a one-way valve: air flows into the pleural space with each breath but can’t escape. Pressure builds continuously. The expanding pocket of trapped air compresses the collapsed lung further, then starts pushing the heart and major blood vessels toward the opposite side of the chest. It also compresses the other lung. Eventually, the pressure inside the chest gets high enough to squeeze the large veins that return blood to the heart, cutting off circulation and causing shock. Without immediate treatment, it can be fatal within minutes.
Collapse From Blockage Is Different
Not every lung collapse involves air in the pleural space. When an airway gets blocked, whether by a mucus plug, a tumor, or an inhaled object, the air trapped in the lung tissue beyond the blockage slowly absorbs into the bloodstream. With no fresh air coming in to replace it, that section of lung deflates. This is called atelectasis, and it typically affects only part of the lung rather than the whole thing. The mechanism is essentially the opposite of a pneumothorax: instead of air getting where it shouldn’t be, air stops reaching where it should.
What It Feels Like
The hallmark symptoms are sudden, sharp chest pain on the affected side and shortness of breath that comes on quickly. The pain often gets worse with breathing. A small collapse might feel like a sharp twinge that gradually improves, while a large one can leave you unable to catch your breath, with a rapid heartbeat and a sense that something is seriously wrong. In a tension pneumothorax, symptoms escalate rapidly: the neck veins may bulge visibly, blood pressure drops, and the skin may turn bluish from lack of oxygen.
How It’s Diagnosed
A chest X-ray is the standard tool. On the image, the collapsed lung appears as a visible line where the lung edge has pulled away from the chest wall, with empty black space (air) between them. Ultrasound is sometimes used in emergency settings because it’s faster and portable, but its sensitivity for detecting pneumothorax is low. One study found a false-negative rate of 72%, meaning it missed nearly three out of four cases. For that reason, X-ray or CT scan remains the more reliable option when a collapsed lung is suspected.
How the Body Heals
A small pneumothorax can resolve on its own. Your body slowly reabsorbs the leaked air from the pleural space at a rate of about 1.25% of the pneumothorax volume per day. That means a small collapse might take a few weeks to fully resolve without any intervention. During that time, you’d be monitored with repeat imaging to make sure the air pocket is shrinking rather than growing.
For larger collapses, the air needs to be removed actively. This usually means inserting a tube or needle through the chest wall to let the trapped air escape, restoring the negative pressure so the lung can re-expand. In a tension pneumothorax, this decompression is done immediately, sometimes even before imaging, because the situation is life-threatening.
If the lung keeps collapsing repeatedly, a surgical procedure can seal the leak or remove the blebs causing the problem. Recurrence is a real concern: roughly 29% of people who experience a primary spontaneous pneumothorax will have another episode within the first year, and the overall recurrence rate sits around 32%. The risk is highest in people who smoke, are very tall and thin, or had a large initial collapse.