Most people believe the urge to breathe comes from low oxygen. This is incorrect. The primary driver of the breathing reflex is rising carbon dioxide in the blood, detected by central chemoreceptors in the medulla oblongata. Oxygen rarely drops to levels that trigger the reflex under normal conditions. Carbon dioxide is the critical variable. Adults who have chronically breathed too fast, often as a result of stress, sedentary lifestyle, or habitual mouth breathing, develop a low CO2 tolerance: they feel the urge to breathe at lower and lower CO2 concentrations, creating a self-reinforcing pattern of over-breathing.
The Consequences of Low CO2 Tolerance
Mild hypocapnia, chronically low CO2, has several physiological consequences that are well documented but rarely discussed in clinical settings. CO2 is a powerful vasodilator. Low CO2 causes cerebral vasoconstriction, reducing blood flow to the brain and contributing to brain fog, anxiety, poor concentration, and dizziness. Low CO2 also shifts the oxygen-haemoglobin dissociation curve to the left through the Bohr effect, meaning that haemoglobin holds onto oxygen more tightly and delivers less of it to tissues. Adults breathing too fast can paradoxically have lower tissue oxygen delivery than those breathing slowly.
The Bohr Effect and Oxygen Delivery
The Bohr effect describes how CO2 concentration in the blood determines how readily haemoglobin releases its oxygen to tissues. When CO2 is low, the affinity of haemoglobin for oxygen increases and less oxygen is offloaded to cells. When CO2 is at appropriate levels, oxygen is released efficiently from haemoglobin into the surrounding tissue. This is the physiological basis of the observation that endurance athletes who train to breathe through their nose, accepting higher CO2 during exercise, often show better recovery and performance despite appearing to take in less air.
Building CO2 Tolerance
CO2 tolerance is trainable. The most studied approach is the Buteyko method, a series of exercises involving reduced breathing, breath holds, and nasal breathing that progressively increase the CO2 concentration at which the breathing reflex is triggered. A Cochrane review of Buteyko training in asthma found significant reductions in bronchodilator use and improvements in quality of life, with the mechanism proposed as improved CO2 regulation reducing airway hypersensitivity.
- Practise nasal breathing exclusively during low-intensity exercise for four to six weeks
- Add brief breath holds after a normal exhale: hold until mild air hunger, then breathe normally
- Reduce your breathing rate consciously: try five breaths per minute for ten minutes daily
- If you experience anxiety, check whether it correlates with periods of faster breathing or sighing
- Avoid aggressive deep breathing techniques during anxiety, as they worsen hypocapnia
Sources
- Courtney, R. (2009). The functions of breathing and its dysfunctions and their relationship to breathing therapy. International Journal of Osteopathic Medicine, 12(3), 78-85.
- Cooper, S., et al. (2003). Effect of two breathing exercises on asthma. Thorax, 58(8), 674-679.
- McKeown, P. (2015). The Oxygen Advantage. HarperCollins.