Published: 18.06.2026 | Atemstark Blog

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Most athletes are training the wrong gas.

You optimise your VO2 max, your lactate threshold, your protein intake, your sleep. But the variable most likely limiting your performance is one that is underlying, always present and trainable – it’s your tolerance to carbon dioxide. And the two are not the same thing.

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The Problem: We’ve Been Misreading the Signal

Most people – athletes included – treat the urge to breathe as an oxygen alarm. It isn’t. The drive to breathe is triggered primarily by rising CO2 levels in the blood, not by falling oxygen. When CO2 climbs past a certain threshold, chemoreceptors in the brainstem fire and tell you to breathe. If your tolerance for that signal is low, you breathe too soon, too fast, and too much – even when your muscles don’t yet need the extra oxygen.

Chronic over-breathing keeps CO2 artificially low. This matters because CO2 is the primary trigger for the Bohr effect: without adequate CO2 in the blood, haemoglobin holds onto oxygen more tightly and releases less of it to working muscles and tissues. You can have excellent blood oxygen saturation and still be functionally under-delivering oxygen at the cellular level.

For athletes, this plays out as early breathlessness, excessive ventilation during moderate effort, and poor recovery between intervals. For professionals under cognitive load, it shows up as shallow chest breathing, a low-grade sense of tension, and disrupted sleep – because the same over-sensitive CO2 response that drives hard breathing during exercise also fragments sleep architecture through micro-arousals.

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The Mechanism: CO2 Is a Regulator, Not a Waste Product

Carbon dioxide is produced continuously as a by-product of cellular metabolism, but calling it “waste” is a significant oversimplification. CO2 dissolves in blood to form carbonic acid, which dissociates into bicarbonate and hydrogen ions – this is the primary buffer system keeping blood pH stable. CO2 also causes vasodilation: it relaxes smooth muscle in blood vessel walls, increasing blood flow to the brain and periphery. And critically, as noted above, it governs oxygen release from haemoglobin via the Bohr effect (first described by Christian Bohr, 1904).

Your tolerance for CO2 – the point at which rising levels trigger an uncomfortable urge to breathe – is measurable. Patrick McKeown, in The Oxygen Advantage (2015), formalised this as the BOLT score: the Body Oxygen Level Test, measured as the comfortable breath-hold time after a normal exhale. A BOLT score below 20 seconds correlates with dysfunctional breathing patterns, poor stress regulation, and reduced aerobic efficiency. A score above 40 seconds is associated with calm, efficient, nasal-dominant breathing at rest and during moderate exertion.

The BOLT score is not a test of lung capacity or fitness per se. It measures chemoreceptor sensitivity – how quickly your brain panics in response to CO2. Train the tolerance up, and the panic threshold rises. Your breathing becomes slower, lighter, and more nasal without conscious effort, because the signal that was forcing you to over-breathe has been recalibrated.

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The Atemstark Approach: Training Air Hunger Deliberately

The mechanism for improving CO2 tolerance is straightforward: you must repeatedly and safely expose yourself to mild-to-moderate air hunger. This is not breath-holding to the point of dizziness or distress – that serves little purpose and teaches nothing useful. It is the deliberate practice of breathing slightly less than you feel you need to, for sustained periods, until the chemoreceptors adapt.

Nasal breathing is the foundation. The nasal passages produce nitric oxide – a potent vasodilator – which increases oxygen uptake in the lungs by approximately 18% compared to mouth breathing (Lundberg et al., 1996, Acta Physiologica Scandinavica). Nasal breathing also adds resistance, which slows respiration rate and naturally raises CO2 slightly, providing a low-level training stimulus every time you breathe. Switching to nasal breathing during low-to-moderate intensity exercise is one of the most direct ways to begin raising your BOLT score.

Reduced breathing exercises – where you deliberately breathe below your ventilatory demand and sit with the resulting air hunger for 3–5 minutes at a time – are the targeted training tool. The discomfort is the stimulus. The adaptation is a recalibrated chemoreceptor response, a higher BOLT score, and a nervous system that no longer interprets mild CO2 elevation as a threat. For athletes, this translates to a higher anaerobic threshold before breathing becomes laboured. For professionals, it means a more stable stress response, because the same CO2-sensitive system that governs breathing also modulates the autonomic nervous system’s threat detection.

Sleep quality is a downstream beneficiary of the same adaptation. Over-breathing during the day maintains a low CO2 baseline; when that baseline is already low, the small CO2 rises during sleep are enough to trigger micro-arousals. Raising daytime CO2 tolerance through consistent practice means the sleeping brain is less reactive to normal nocturnal CO2 fluctuations – fewer disruptions, deeper slow-wave sleep.

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One Thing You Can Do Today

Measure your BOLT score. Sit quietly for five minutes. Take a normal breath in through the nose, a normal breath out through the nose, then pinch your nose and start a timer. Stop the timer at the first definite urge to breathe – not the maximum you can hold, just the first clear signal. That number, in seconds, is your BOLT score.

If it’s under 20 seconds: your breathing is almost certainly contributing to unnecessary fatigue, elevated stress reactivity, and suboptimal recovery. If it’s between 20 and 30 seconds: there is meaningful room to improve. Above 40 seconds: your CO2 tolerance is in a functional range – the work now is maintaining it under load. Write the number down. Test again in four weeks after consistent nasal breathing and reduced-breathing practice. The number will move.

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Want to Go Further?

If you’re curious about how CO2 tolerance training fits into a structured programme for athletic performance or stress resilience, the Atemstark blog goes deeper on the methods, the physiology, and the practical protocols.

Read the next article →

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Atemstark | Functional Breathing & Cold Exposure Coaching | Zürich

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