Freediving Lung Squeeze: Causes, Symptoms, and Prevention

A lung squeeze is pulmonary barotrauma of descent: injury to the lung tissue caused by the chest being compressed past the point it can safely handle at depth. The medical name is freediving-induced pulmonary syndrome, and the simple description is the one divers use, your lungs get squeezed too hard and fluid and blood are forced into the air spaces where only air belongs.

It is not a fringe concern for record chasers. It is the most common serious injury in depth freediving, and it shows up in divers at surprisingly modest depths when conditions line up badly. Understanding it is part of taking the sport seriously, alongside shallow water blackout and the other risks covered in our overview of whether freediving is dangerous.

To understand why a squeeze happens you need three pieces: Boyle's law, residual volume, and the blood shift. They work together on every deep dive, and a squeeze is what happens when the system runs out of margin.

Boyle's law says gas volume shrinks as pressure rises. At 10 meters the pressure has doubled and your lungs are compressed to roughly half their surface volume. At 30 meters they are around a quarter. The chest keeps getting smaller the deeper you go, and it does so faster near the surface than at depth.

Total lung capacity

The full volume of your lungs at maximum inhale, often around 6 liters in a trained adult. This is what you carry down.

Residual volume

The air that stays in your lungs after a full exhale, typically 20 to 25 percent of total capacity. Below this volume the lung tissue itself starts to bear the compression.

Blood shift

Blood pooling into the chest and pulmonary capillaries as you descend, filling the shrinking space with incompressible fluid so the lungs are not crushed.

Here is the old, naive math: a diver with 6 liters total and 1.5 liters residual volume hits residual volume around 30 meters. On paper that should be the depth where the lungs collapse. Divers go far deeper, and the reason is blood shift. As you descend, capillaries around the alveoli engorge and blood pools into the chest. Schaefer's classic measurements recorded blood shifts of over 1,000 milliliters at around 30 meters. That incompressible fluid takes up space the shrinking air can no longer fill.

A squeeze happens when this protection is overwhelmed. When the lungs are compressed at or beyond residual volume, the pressure inside the alveoli falls below the pressure in the surrounding tissue and blood vessels. That negative pressure differential pulls plasma and blood across the capillary walls into the alveoli and small airways. The result is pulmonary edema, swelling of the small airways, and bleeding into the spaces meant for gas exchange.

A squeeze is rarely one mistake. It is usually a stack of factors that together push the chest past what it can handle on a given day. The same diver can be fine at 35 meters one morning and squeeze at 32 the next because the conditions shifted.

• Pushing depth faster than your body has adapted. The single biggest contributor. Adaptation of the chest and the blood shift response lags behind ambition.
• Poor thoracic and diaphragm flexibility. A stiff ribcage and tight diaphragm cannot accommodate the compression, concentrating stress on the tissue.
• Forceful equalization at depth. Hard Valsalva, pinch and blow, recruits the diaphragm and abdominal muscles, distorting the chest under compression at exactly the worst moment.
• Tension, contractions, and movement at depth. Diaphragm contractions and any straining or sudden movement deep in the dive add dynamic strain to already loaded tissue.
• Cold water and no warm-up. Cold tightens the body, and skipping a progressive warm-up means the blood shift response is not primed.
• Fast descents. Speed gives tissue less time to redistribute and accommodate the changing volume.
• Aggressive packing or reverse packing too deep. Packing raises starting lung volume and stress; reverse packing at depth pulls air from the lungs and can drop you below safe volume.

Equalization is worth singling out because it is both a cause and something you can fix. The Frenzel technique uses air held in the mouth above a closed glottis, so it does not recruit the diaphragm and chest the way a deep Valsalva does. Switching from Valsalva to Frenzel removes one of the most common triggers. Our guide to equalization techniques covers the progression to Frenzel and mouthfill, both of which protect the chest as well as the ears.

Symptoms can appear on the surface immediately or build over minutes to hours after the dive. That delayed onset is dangerous: a diver who feels a minor tickle at the surface can be significantly worse an hour later. Watch for these signs after any deep dive.

• A persistent urge to cough or a tickle in the chest right after surfacing
• Coughing up blood-tinged sputum, or frothy pink mucus
• Chest tightness or pain
• Breathing that sounds wet, crackly, or wheezy
• Shortness of breath and a feeling of not getting a full breath
• Fatigue and weakness out of proportion to the effort of the dive
• Dizziness, and in severe cases blue lips or fingertips from low oxygen

Severity is commonly described in three grades. The grade is a guide for how seriously to respond, not something to self-diagnose in the water, because a mild presentation can still progress.

If you suspect a squeeze in yourself or your buddy, the priorities are stop, support oxygenation, and get medical care. Do not try to dive it off or test whether the next dive feels fine.

1. 01 —Stop diving immediately. No more descents, not even shallow ones. The day is over.
2. 02 —Get out of the water and rest. Keep the diver calm and upright, which helps breathing, and minimize exertion.
3. 03 —Give supplemental oxygen if it is available and someone is trained to use it. Oxygen helps offset the reduced gas exchange from fluid in the lungs.
4. 04 —Assess and escalate. Worsening breathing, frothy pink sputum, blue lips, or confusion is an emergency. Call emergency services and transport to hospital.
5. 05 —See a doctor even for an apparently mild squeeze. Tell them it is suspected pulmonary barotrauma of descent from breath-hold diving so they evaluate the lungs specifically.
6. 06 —Do not return to diving until cleared and fully recovered. Healing tissue squeezes more easily, and repeat injuries compound.

Recovery timelines vary. Mild cases can settle within hours to a few days, while imaging in confirmed barotrauma has shown lung changes resolving around six days and clearing by roughly three weeks. The common practical guidance is several weeks off after a mild squeeze and a clean medical clearance after anything more serious. This is also a good moment to revisit your buddy system, because a squeeze on a solo dive is a far worse situation than the same injury with a trained partner present.

Prevention is almost entirely about patience and preparation. The divers who avoid squeezes are not the ones with magic lungs, they are the ones who progress slowly, stay relaxed, and treat depth as something earned over seasons rather than sessions.

Progress depth gradually

Add small increments and give your body time to adapt between steps. Chasing personal bests on a single good day is how most squeezes happen.

Warm up properly

Do progressive warm-up dives before targeting depth, especially in cold water. A primed blood shift response and a relaxed body tolerate compression far better.

Train thoracic and diaphragm flexibility

Stretch the ribcage and diaphragm so the chest can accommodate compression. Flexibility work is one of the few things that genuinely raises your safe depth ceiling over time.

Fix your equalization

Use Frenzel rather than forceful Valsalva, and learn mouthfill before it becomes the only thing standing between you and a hard equalization at depth.

Stay relaxed and still at depth

Tension, straining, and sudden movement deep in the dive all add strain. Calm freefall and a smooth turn protect the chest.

Respect cold, congestion, and fatigue

Cold water, a chest cold, tightness, or tiredness all lower your margin. On those days, dive shallower than usual or not at all.

Note what is missing from that list: packing early in your development. Lung packing raises lung volume and tissue stress and belongs much later in training, under instruction, not as a shortcut to depth. The same caution applies to reverse packing. If you are still building toward your first deeper dives, the depth fundamentals in how deep you can freedive will keep your expectations grounded.

"Depth is earned across seasons, not stolen in an afternoon. The chest adapts on its own schedule, and the divers who respect that are the ones still diving in ten years."