Biohacking · Breathing physiology

CO₂ tolerance: why breathing training does more than just relax you

3 Min. read YUTHI Redaktion

Most breathing exercises are associated with relaxation and stress relief. But there is a second, less well-known effect: CO₂ tolerance. What this concept is about and why it can influence your performance and resting breathing.

What CO₂ tolerance means

Your breathing drive is not primarily triggered by too little oxygen, but by too much CO₂. When you feel the urge to breathe, your body is responding to a rise in carbon dioxide levels in the blood — not to a lack of oxygen. CO₂ tolerance describes the ability to tolerate elevated CO₂ levels in the blood without experiencing a strong urge to inhale.

People with low CO₂ tolerance tend to over-breathe: they breathe too frequently and too deeply, which paradoxically results in less oxygen reaching the tissues — because CO₂ is needed as a vasodilator and for the Bohr effect. The Bohr effect describes how haemoglobin releases oxygen into the tissues more readily when CO₂ and acid are present.

How over-breathing develops and what it triggers

Chronic mouth breathing and shallow chest breathing are common causes of a tendency towards hyperventilated breathing in everyday life. Stress, anxiety and sedentary work reinforce the pattern. The consequences can include tingling in the hands and feet, shortness of breath, mild dizziness and muscle tension — symptoms that arise from changes in blood pH.

Patrick McKeown, one of the best-known proponents of functional breathing training, has developed a test protocol based on the Buteyko method: the BOLT score (Body Oxygen Level Test). You breathe out normally and measure how long you can comfortably pause before the first urge to breathe arises. Under 20 seconds is considered an indication of heightened breathing sensitivity.

What breathing training achieves at the CO₂ level

Regular exercises that allow CO₂ levels to rise moderately — for example controlled breath holds or reduced breathing volume — gradually raise the tolerance threshold. Mechanistically, the chemoreceptor system adapts: the breathing drive is only triggered at a higher CO₂ level.

Sports studies have shown that a higher BOLT score correlates with better endurance performance and lower oxygen consumption at submaximal effort. Whether this is causal or a marker of an overall better state of training is difficult to disentangle.

Nasal breathing as the basic protocol

The simplest and best-supported entry point into CO₂ training is consistent nasal breathing — including during exercise. The nose filters, humidifies and warms the air you breathe, produces nitric oxide, and through its narrower cross-section offers a natural breathing resistance that slightly raises CO₂ in the blood.

Studies show that nasal breathing at moderate effort does not worsen endurance performance, even if it feels that way at first — because the perceived breathlessness is related to low CO₂ tolerance and improves with training.

Tracking CO₂ tolerance in the YUTHI app

Do the BOLT test once a week and enter the result in the YUTHI app as a biomarker. After four to eight weeks of daily nasal breathing training and short breath-hold exercises, you will likely see a measurable improvement.

Combine this with observing your heart rate during submaximal efforts and your subjective breathing effort. If both decrease, that is a valid signal that your respiratory system has adapted.

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