Watch a group of recreational runners and most are breathing through the mouth within the first kilometre. It feels like the fastest way to get more air, and for most people it quietly caps their performance.

The Problem: More Air Is Not More Oxygen

Mouth breathing during a run feels efficient because it moves a large volume of air quickly. The catch is that volume is not the same as delivery. When you breathe hard through the mouth, you tend to over-ventilate, blowing off carbon dioxide faster than your body makes it.

That matters because CO2 is not simply waste. It is the signal that tells your red blood cells to release oxygen into working muscle, a mechanism called the Bohr effect. Push your CO2 too low and your blood stays well saturated while your legs receive less oxygen than they should.

The Mechanism: CO2, Nitric Oxide, and the Bohr Effect

Nasal breathing changes this in two ways. The narrower nasal airway adds resistance, which slows the breath and keeps CO2 in a more useful range, so oxygen actually unloads where it is needed. The nose also produces nitric oxide, a gas that widens blood vessels and airways and improves oxygen uptake in the lungs, which you do not get breathing through the mouth.

Patrick McKeown, who developed the Oxygen Advantage method, frames nasal breathing during exercise as a way to raise your tolerance to CO2, and that tolerance is what lets you hold a strong pace without the gasping, panicky feeling. James Nestor makes a similar case in Breath, drawing on runners who shifted to nasal breathing and saw the effort at a given pace fall over time.

The Atemstark Approach: Train the Switch, Do Not Force It

The usual mistake is to switch to the nose mid-run, struggle, and decide it does not work. Of course it feels hard at first: your tolerance to CO2 was built around years of over-breathing. The Atemstark approach is to train the change in stages, starting nasal breathing on easy runs where you can still hold a conversation and letting your body adapt over a few weeks before you push the pace. As your BOLT score climbs, the intensity you can sustain through the nose climbs with it, and breathlessness arrives later and milder.

One Thing You Can Do Today

On your next easy run, close your mouth and breathe only through your nose for the first ten minutes. If you cannot manage it, that is useful information: you are running faster than your breathing can currently support, so ease off until the nasal breath feels steady. Hold that honest pace for a couple of weeks. The speed at which you can stay nasal is a clean, daily gauge of your aerobic progress.

Want to Go Further?

If you want a structured way to raise your CO2 tolerance instead of guessing at it, the Atemstark blog and the Foundations programme lay out the steps in order. Start nasal, build slowly, and let the pace come to you.

Start the Foundations programme

Related reading

• CO2 Tolerance – Why Air Hunger is the Key to Better Breathing
• Nasal Breathing for Better Performance | Science & Practical Drill