Let's start with the conclusion: Follow the protocols, and it almost never happens. The term "lung burst" is an exaggeration; it usually corresponds to what medicine calls pulmonary barotrauma. In freediving, the more common form is the squeeze-type injury, that is, lung squeeze — it is not the lung actually exploding.
How does it happen? At depth, the gas in the lungs is compressed. If equalization of the ears or sinuses fails and you keep descending, if you exhale during the descent, or if you force a depth beyond what your body is currently adapted to, the chest cavity is subjected to negative pressure it should not have to bear, and lung tissue may be injured, showing up as chest tightness or coughing up blood.The key point is that training and discipline can push the risk very low, but no single measure can guarantee it will never happen.
- Don't force depth.If you cannot equalize, or you feel a squeezing sensation in your chest, ascend immediately; build your depth target step by step, and if you are not in good shape that day, call it a day.
- Never get in the water when you are ill.With a cold, a blocked nose, or during an allergy flare-up, equalization and the airways are not normal, and the risk of forcing a dive rises many times over.
- During descent, do not exhale, do not force, and control your ascent speed.Keep these three in check at the same time, and the chance of lung squeeze drops to very low.
In one sentence: What injures the lungs is never freediving itself, but people who do not follow the protocols.If you cough up blood, feel chest tightness, or have persistent discomfort, first exit the water following the surface protocol under your buddy's supervision, then seek medical care immediately after coming ashore — do not try to tough it out on your own; all training must be done under the supervision of an instructor or a buddy with rescue capability.