Breathing, explained
General background on breath training, the BOLT score, and the Buteyko method.
Why train your breathing?
Regular breath training can increase tolerance to carbon dioxide, contribute to calmer everyday breathing, and lower perceived exertion during physical activity. Many techniques originate from endurance and apnea sports, but are now also used for general wellbeing.
Why CO2 matters for the body
Carbon dioxide is often seen as just a waste product of breathing — but in reality it plays a central regulatory role in the body. It isn't a lack of oxygen that triggers the urge to breathe, but primarily the rising CO2 level in the blood: when holding your breath, it's this rise that eventually creates the sensation of needing to breathe again. Carbon dioxide also affects how easily oxygen is released from the blood to the cells (the Bohr effect) — so a certain CO2 level actually improves oxygen delivery to tissue and brain rather than hindering it. Training your CO2 tolerance and gradually raising it teaches you to breathe less and more calmly in everyday life, too. And that has real effects: calmer, more efficient breathing can settle the nervous system, improve oxygen delivery, support concentration, and lower perceived exertion under load.
Overbreathing in everyday life
Overbreathing doesn't only happen in acute stress situations — it often occurs unnoticed in everyday life: many people constantly breathe a bit faster or deeper than the body actually needs, often through the mouth and into the chest rather than the belly. This mild, chronic form of overbreathing also lowers the CO2 level in the blood. And a low CO2 level isn't without consequences: it narrows the blood vessels in the brain and, at the same time, shifts hemoglobin's oxygen-binding curve so that oxygen is released to tissue less readily. The paradox is the same as with acute hyperventilation: even though it seems like you're breathing "more," the cells may actually have less oxygen available. This is exactly where breath training comes in. Learning to breathe more calmly, more slowly, and through the nose gradually raises your CO2 tolerance — and as a result, your everyday breathing automatically becomes more economical. Slower, calmer breathing also activates the calming part of the nervous system (the parasympathetic nervous system) and can help lower stress and inner tension. A healthy relationship with CO2 is therefore far more than a detail for athletes: it's a foundation for calm, relaxed, and efficient breathing.
Which breathing exercises calm you down — and why
Whether breathing has a calming effect depends less on how much you breathe than on how you breathe. The most important lever is the ratio of inhale to exhale: during inhalation, the calming influence of the vagus nerve is briefly dampened and heart rate rises slightly — during exhalation, this influence returns and heart rate drops again. The exhale is therefore the phase that activates the parasympathetic, calming part of the nervous system. Making the exhale longer than the inhale (for example, 4 seconds in, 6 seconds out) deliberately extends this calming phase. Studies show that an exhale that's longer relative to the inhale can increase heart rate variability (HRV) — a sign of increased vagus nerve activity. This is supported by an overall calm breathing pace: at around six breaths per minute, heart rate and breathing fall into a kind of resonance that raises HRV especially strongly. Diaphragmatic breathing is key here — breathing where the movement happens low down, at the diaphragm and in the belly, rather than in the shoulders and chest. Importantly, the goal isn't to inhale a large volume of air, but to breathe calmly, evenly, and economically — because too much air would unnecessarily exhale CO2 and work against the actual goal.
What is the BOLT score?
BOLT stands for Body Oxygen Level Test. After a normal exhale, you time how long it takes until the first noticeable urge to breathe — deliberately not the maximum time you can hold. The score is a proxy for CO2 tolerance and is most meaningful as a trend over several weeks, not as a single reading.
The Buteyko method, briefly
The Buteyko method is based on the idea that many people unknowingly breathe more than the body needs (overbreathing). The goal of training is to gradually raise CO2 tolerance through structured breath-hold exercises — reflected in a rising BOLT score and often calmer, less noticeable everyday breathing.
Training CO2 tolerance
Structured training tables with gradually increasing intervals (e.g. CO2, O2, or pyramid tables) aim to raise CO2 tolerance deliberately and traceably, rather than repeating isolated maximal breath-holds.
This content is for general information only and does not replace medical advice. If you have respiratory or cardiovascular conditions, are pregnant, or are unsure, consult a doctor first.