Lung squeeze, the mouthfill technique and deep narcosis: what happens to your body past 30 meters, and how pros deliberately guard against it.
The honest answer first: beyond 30 meters, your body changes more than it reads on paper. Your lungs, at full volume at the surface, get compressed by rising pressure — at 40 meters, down to roughly a fifth of their original size. That's not theory, that's Boyle's law happening inside your own chest.
If that pressure becomes too extreme or too abrupt, lung squeeze (pulmonary squeeze) threatens: tissue in the chest compresses unevenly, capillaries rupture, blood enters the airways. The average depth where this has been documented sits around 43 meters — not coincidentally right where many freedivers first hit the limits of their mobility and preparation. The most commonly reported causes are inadequate warm-up and poor movement at depth, not depth alone.
This is exactly where the mouthfill technique comes in: past a certain depth, even Frenzel isn't enough anymore, because barely any residual air is left in the mouth. Advanced freedivers therefore "pack" a larger air reserve into their cheeks and mouth before descending, using it in portions to equalize — like a small air tank in the mouth compensating for the missing lung capacity. This technique demands years of closely supervised practice, because mistakes directly invite pressure injuries.
Mentally, something also shifts at extreme depth: at very great depths, freedivers can experience symptoms reminiscent of nitrogen narcosis in scuba diving — confusion, euphoria, delayed reaction. Exactly such a state contributed to Herbert Nitsch's severe 2012 incident at 253 meters, where he lost consciousness on the way back up. Train Smart. Play Hard. — in deep diving that means: every additional depth increment earns years of preparation, not impatience.
The deeper you go, the more water pressure compresses your lung volume — at 40 meters, to roughly a fifth of its surface size. This compression is the physical reason why deep diving carries a completely different risk profile than 10-meter dives.
Inadequate warm-up is one of the main factors behind lung squeeze. Use dry exercises for chest mobility and several shallower preparation dives before heading to your target depth.
The mouthfill technique builds on perfect Frenzel, not alongside it. Practice it first at moderate depth under direct guidance, before applying it at increasing depth.
Coughing after surfacing, chest tightness, blood in saliva or sputum — these are serious signs of a developing lung squeeze. With any of these symptoms: stop deep diving for the day immediately.
Confusion, unusual euphoria, or slowed reaction at great depth aren't "cool" side effects — they're warning signs. Progression plans for such depths belong exclusively in the hands of experienced coaches with an airtight safety setup.
Lung squeezes are serious injuries ranging from mild symptoms to pulmonary edema and drowning at depth. Any trace of blood in saliva after a deep dive belongs in front of a doctor, not brushed off.
Progression into depth has to be slow, documented, and conducted exclusively under experienced guidance. The Nitsch incident at 253 meters is a stark illustration: even decades of experience don't fully protect against the physiological limits of extreme depth.
Run a short body debrief with yourself or your coach after every deep dive: how did the chest feel, was there coughing, how was mental clarity on the ascent? Small changes across multiple sessions are often the earliest sign of emerging problems, long before an acute symptom shows up.
Documented cases cluster below 30 to 45 meters, with an average incident depth around 43 meters. Inadequate warm-up and poor movement further raise the risk, independent of the raw number.
A method where freedivers "pack" a larger air reserve into their cheeks and mouth before descending, because past a certain depth normal residual air isn't enough for Frenzel equalization anymore. It requires advanced training and is never learned without a solid Frenzel foundation.
Yes, at extreme depths freedivers can experience symptoms reminiscent of nitrogen narcosis — confusion, altered perception, delayed reaction. Such a state played a role in Herbert Nitsch's severe incident at 253 meters.
Typical signs are coughing after surfacing, chest tightness, sputum or blood in saliva. With any of these symptoms: stop deep diving for the day immediately and get checked by a doctor if in doubt.
Freediving: Getting Started and Overview