CO₂ tolerance, and why breath holds work
The urge to breathe is a carbon dioxide alarm, not an oxygen one. Almost everything about breath-hold training follows from that.
3 min read · Updated 2026-09-26
The alarm is measuring the wrong gas
Hold your breath and the discomfort that builds is almost entirely a response to rising carbon dioxide, not to falling oxygen. Central chemoreceptors in the medulla respond to the pH change that dissolved CO₂ produces in cerebrospinal fluid, and peripheral chemoreceptors in the carotid bodies add a faster contribution. Oxygen does have a receptor — also in the carotid bodies — but it barely contributes until saturation falls a long way below normal.
This is a sensible design for ordinary life, because CO₂ rises before O₂ meaningfully falls, so it is the earlier warning. It has one dangerous consequence, which the safety section below is about.
It also explains the training target. When people talk about CO₂ tolerance they mean reducing the intensity of that alarm at a given CO₂ level — not increasing lung capacity, not storing more oxygen.
What actually adapts
Repeated, controlled exposure to mildly elevated CO₂ appears to blunt the ventilatory response to it: the same blood gas produces less air hunger and less drive to breathe. The practical effects are a longer comfortable breath hold, less breathlessness at a given workload, and — for people who habitually over-breathe — a more settled resting pattern.
The endurance-sport version of this is hypoventilation training, usually done by reducing breathing frequency during exercise, often at low lung volume. The research base, largely from Woorons and colleagues, shows changes in muscle oxygenation and buffering capacity and some performance improvements in repeated-sprint work [1]. It is a real method with a real literature, and it is also a smaller and less consistent effect than its enthusiasts suggest.
The clinical version is breathing retraining for people who over-breathe. A large trial of physiotherapy-led breathing retraining in asthma found improvements in asthma-related quality of life without changes in underlying airway inflammation or lung function [2]. That distinction matters: retraining changed how people breathed and how they felt, not the disease. For anyone with asthma, this is an addition to treatment, not a replacement for it.
The BOLT score, and what it is worth
The Body Oxygen Level Test asks you to exhale normally and time how long until the first definite urge to breathe. It is widely promoted, usually with a set of target numbers attached.
Taken as a rough, repeatable self-measure it is fine. Taken as a validated physiological index it is not: the targets in circulation are not well supported, the measurement depends heavily on how the instruction is phrased and how motivated you are, and comparing your number to someone else's tells you very little.
Used sensibly, it tracks your own change over weeks. That is all it does, and it is enough.
Doing it without cheating
The commonest error in breath-hold training is hyperventilating first. Several fast deep breaths before a hold lower arterial CO₂ below normal, which delays the alarm and lengthens the hold dramatically. It feels like progress. It is the opposite: you have reduced the stimulus rather than adapted to it, and the number improves while the adaptation does not.
Hold from a normal breath, or from a normal exhale. Keep the throat open rather than sealing it. Stop at a clear urge, not at a struggle — the training stimulus is in the mild-to-moderate range, and pushing to the limit adds risk without adding much adaptation. Progress by lengthening the hold, not by shortening the recovery.
The safety rule that is not negotiable
Never do breath holds in water. Not in a pool, not in the bath, not in open water, not with a friend watching from the side.
The reason is the gas asymmetry at the top of this guide. Hyperventilating before a hold suppresses the CO₂ alarm, but it does not add a meaningful oxygen reserve. Oxygen therefore keeps falling with nothing to signal it, and consciousness can be lost without any warning sensation at all. In water that is drowning, and it kills competent, fit, experienced swimmers every year. The mechanism is called shallow water blackout.
This is why the breath-hold work on this site — the CO₂ tolerance hold and the hypoxic swim ladder — is written as dry-land practice. The adaptation transfers to the water. The risk does not have to.
Breath holds are also not appropriate during pregnancy, or with uncontrolled high blood pressure or a heart condition. Techniques carrying those restrictions are filtered out of the relevant pages on this site rather than flagged with a warning.
References
- Woorons X, Billaut F, Lamberto C, et al. Physiological responses to repeated running sprints performed with reduced breathing frequency. Various, 2016–2021 (review of the voluntary hypoventilation literature).
- Bruton A, Lee A, Yardley L, et al. Physiotherapy breathing retraining for asthma: a randomised controlled trial. The Lancet Respiratory Medicine, 2018. Link
Techniques mentioned
General information about breathing practices, not medical advice. How this is written and sourced.