Every breath-hold, whether it lasts fifteen seconds on a couch or three minutes at the bottom of the ocean, sets off a chain of events inside the body. A new study, published in the Journal of Applied Physiology, has just given researchers their most detailed look yet at that chain, by threading arterial catheters into three elite freedivers and drawing blood samples at depths of 20, 40, 60 and 80 metres. The results reshape some long-held assumptions about what happens to oxygen and carbon dioxide once the breath is held, and they have direct relevance for anyone who practices structured hyperventilation before apnea.
Why the body treats breath-holding as an emergency
The moment air stops moving, the diving reflex takes over. Heart rate drops, blood vessels in the limbs constrict to protect the core, and blood shifts toward the chest to help the lungs withstand pressure. This response evolved to conserve oxygen for the brain and heart during submersion, and it activates to some degree even during dry breath-holds, triggered largely by facial cooling and breath-hold duration rather than water itself.
What hyperventilation actually does before a breath-hold
The freedivers in this study followed a standardised pre-dive breathing routine before every descent: slow, deep breathing followed by glossopharyngeal insufflation, a technique that pushes extra air into the lungs beyond a normal full breath. The measured effect on their blood was striking. Resting arterial carbon dioxide averaged around 4.6 kilopascals, but after the routine it dropped to roughly 2.5 kilopascals across all three divers, a textbook hyperventilation response.
That drop matters because carbon dioxide, not oxygen, is what drives the urge to breathe. Lowering it before a breath-hold buys extra time before the body signals distress, but it does almost nothing to increase how much oxygen is actually stored in the blood or lungs. The result is a longer, more comfortable breath-hold that can mask oxygen levels quietly falling toward dangerous territory, a mismatch that sits at the centre of most breath-hold accidents.
What eighty metres of freediving revealed about the lungs
As the divers descended, water pressure compressed their lungs, and both oxygen and carbon dioxide spiked in their arterial blood, a pattern researchers had previously reported at shallower depths. What surprised the team was where that spike peaked. Rather than climbing steadily with depth, oxygen levels reached their highest point at 40 metres in all three participants, then plateaued or fell on deeper dives. Carbon dioxide followed a similar pattern, rising to 40 metres, holding steady to 60 metres, then climbing again at 80 metres.
The likely explanation is Boyle’s law. At 40 metres, ambient pressure is five times that at the surface, enough to compress the lungs to somewhere near residual volume, the point beyond which they can barely shrink further. Once that threshold is reached, extra depth adds little further compression, so the pressure-driven spike in blood gases levels off. Beyond that point, the numbers are governed less by physics and more by ordinary metabolism: oxygen falls as the body consumes it, and carbon dioxide climbs as it accumulates, at a rate of roughly one kilopascal per minute based on the resting apnea measurements taken during the study.
Could carbon dioxide explain the narcosis some divers describe?
Elite breath-hold divers have long described dysphoria and narcotic-like symptoms on very deep dives, and this study offers a plausible mechanism. Extrapolating from the measured rate of carbon dioxide accumulation, the researchers estimate that dives to around 120 metres could push arterial carbon dioxide close to levels known to impair cognition in compressed-gas divers. It is an extrapolation rather than a direct measurement, but it lines up with diver reports that the mental fog tends to peak around the middle of the ascent, exactly when carbon dioxide would still be near its highest and oxygen would already be falling fast.
The one rule that never has an exception
The same physiology that makes deep freediving demanding also explains its single greatest danger: shallow water blackout. Hyperventilation suppresses the carbon dioxide signal that would normally warn a person to breathe, but it leaves the oxygen curve untouched. As the diver ascends and the lungs re-expand, arterial oxygen can collapse within seconds, sometimes falling from well above normal to hypoxic levels in under a minute, with no warning sensation at all because the urge to breathe never arrived in time. Consciousness can be lost silently, often in the last few metres before the surface.
This is precisely why the Wim Hof Method protocol is unambiguous on this point: the breathing exercises and any breath retention should never be practiced in water or near water without direct, attentive supervision. The mechanism behind shallow water blackout in elite freedivers is the same mechanism at work in anyone who hyperventilates and then holds their breath, regardless of skill level or experience. The setting is different, but the physiology, and the risk, is exactly the same.