Mouth breathing is the default for most athletes once intensity rises. The instinct makes sense — the mouth is a larger airway and feels easier. But from a gas exchange and vascular efficiency standpoint, mouth breathing at submaximal intensities imposes costs that accumulate across every training session.
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The Nasal Airway: More Than Filtration
The nasal passages do three things the mouth cannot:
1. Nitric oxide production. The nasal sinuses and paranasal sinuses are the primary production site for endogenous nitric oxide (NO) in the respiratory tract. NO is a potent vasodilator and bronchodilator — when inhaled nasally, it dilates the alveolar vasculature, improving ventilation-perfusion matching and increasing oxygen uptake efficiency per breath. Mouth breathing delivers air without this vasodilatory signal.
2. Air conditioning. Nasal turbinates heat and humidify inspired air to near-body temperature and 100% humidity before it reaches the bronchioles. Mouth breathing delivers cool, dry air that triggers airway smooth muscle constriction — the mechanism behind exercise-induced bronchoconstriction in susceptible athletes.
3. Airflow resistance. Nasal breathing generates approximately 50% more airflow resistance than mouth breathing. This higher resistance — counterintuitively — increases the partial pressure of CO₂ in alveoli, improving gas exchange efficiency by the mechanism described below.
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The Bohr Effect: Why CO₂ Tolerance Matters
Oxygen delivery from haemoglobin to working muscle is not simply a function of oxygen saturation — it depends critically on local CO₂ partial pressure.
The Bohr effect describes the rightward shift of the oxygen-haemoglobin dissociation curve as CO₂ rises: higher CO₂ concentration in tissues reduces haemoglobin's oxygen affinity, causing it to release oxygen more readily. Muscle that is producing more CO₂ (working harder) automatically receives more oxygen from passing haemoglobin.
The implication for breathing: CO₂ is the primary drive for ventilation, not oxygen deficit. Athletes who habitually mouth breathe — and hyperventilate relative to metabolic demand — exhale more CO₂ than their metabolism requires. This lowers arterial PCO₂ (hypocapnia), causing vasoconstriction of cerebral and peripheral blood vessels and *reducing* oxygen release to tissues despite adequate oxygen saturation.
This is counterintuitive: taking more breaths can actually reduce oxygen delivery to working muscle, because the CO₂-mediated release mechanism is impaired.
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The BOLT Score and CO₂ Tolerance Testing
The BOLT score (Body Oxygen Level Test) — developed by Patrick McKeown (*The Oxygen Advantage*, 2015) — is a simple field measure of CO₂ tolerance:
1. Breathe normally for several minutes 2. After a normal exhale, pinch the nose and hold 3. Measure seconds until the *first definite urge* to breathe (not complete exhale; not distress)
Interpretation:
- <20 seconds: low CO₂ tolerance; likely habitual mouth breather; significant performance headroom available
- 20–35 seconds: moderate; average trained athlete
- 35–40 seconds: good; consistent nasal breather
- 40+ seconds: high CO₂ tolerance; associated with efficient breathing mechanics at high intensities
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Performance Applications: Where Nasal Breathing Actually Helps
Zone 1–2 training (60–75% HRmax): The primary application. Nasal-only breathing during easy and moderate training sessions acts as a CO₂ tolerance training stimulus. The forced constraint — breathing only through the nose — trains the tolerance to higher alveolar CO₂ levels, raising the BOLT score progressively over weeks. This is analogous to resistance training: the restriction builds the capacity.
Warm-up and cool-down: Nasal breathing during warm-ups ensures NO-mediated vasodilation is established before intensity rises — comparable to a vascular primer.
Sleep and recovery: Mouth breathing during sleep is associated with lower SWS quality, higher sympathetic nervous system tone, and elevated morning HRV-depression. Athletes who tape the mouth during sleep (a Buteyko method technique) report better recovery scores — likely mediated by the parasympathetic-dominant state associated with nasal breathing.
For runners calculating their easy and aerobic pace zones — the zones where nasal-only breathing training is feasible and most beneficial — the pace calculator at winsport.uk/tools/performance/running-pace-calculator converts target heart rate zones into per-kilometre pace ranges. Establishing your nasal breathing threshold pace is a practical starting point: if you cannot hold nasal breathing comfortably, you are above VT1 and out of the aerobic base zone.
Do you currently use nasal breathing as a training tool for your athletes — or is it a consideration only at low intensities?