Most people assume the urge to breathe is caused by running out of oxygen.
It's a reasonable assumption, but it's wrong. Under normal conditions, the signal to inhale is triggered not by falling oxygen levels but by rising carbon dioxide. Specifically, chemoreceptors in the brainstem and carotid arteries detect elevated CO₂ in the blood and fire an increasingly urgent signal: breathe now.
This is CO₂ tolerance — and it varies enormously between people.
Someone with low CO₂ tolerance will feel breathless, anxious, and panicky at relatively modest CO₂ levels. Their breathing tends to be faster, shallower, and more frequent — the body's attempt to expel CO₂ as quickly as possible. Someone with higher tolerance can sit comfortably with elevated CO₂ for longer, breathing slowly and deeply without distress.
Breathwork recalibrates the threshold.
Practices that involve breath holds, extended exhales, or deliberately slowed breathing expose the body to slightly elevated CO₂ levels in a controlled way. Over time, the chemoreceptors adjust. The panic threshold moves. What felt uncomfortable in week one becomes unremarkable by week four — not because you're tougher, but because the underlying physiology has genuinely shifted.
This explains a lot about anxiety.
Chronic over-breathing — breathing faster and more shallowly than the body requires — is often associated with anxiety. It keeps CO₂ artificially low, which may make the body more sensitive to any slight rise. Research suggests the relationship is likely bidirectional: anxiety drives over-breathing, and chronic over-breathing may maintain or amplify anxiety (Meuret et al., 2003). The result can be a cycle in which any pause in breathing feels uncomfortable, which in turn heightens distress. Respiratory training approaches have been explored as a way to help recalibrate this sensitivity in people who experience frequent anxiety or panic-like symptoms — though anyone experiencing panic disorder should seek guidance from a qualified clinician rather than relying on self-directed breathing practice alone.
Training CO₂ tolerance breaks that cycle.
It's not about willpower or pushing through discomfort. It's about gradually teaching the body's chemoreceptors that a slight rise in CO₂ is safe, normal, and nothing to react to. This may be one reason breathwork feels helpful to many people managing anxiety — by working at the physiological level, not just addressing the psychological experience of it. This is complementary to, not a replacement for, professional support where that's needed.