What Is CO₂ Tolerance - And How You Perform Under Stress

Most conversations about stress management focus on mindset. Think differently. Reframe. Breathe.

That last one is right – but for the wrong reasons.

Most people are told to breathe deeply when stressed because it “calms you down.” That’s true. But why it calms you down, and what’s actually going wrong when you’re stressed, is a story that almost never gets told.

It starts not with oxygen – but with CO₂.

The part of breathing nobody talks about

When we think about breathing, we think about oxygen. Inhale oxygen, exhale CO₂. Simple.

But here’s what that picture leaves out: CO₂ is not just a waste product to be expelled as fast as possible. It is, in fact, the primary signal your body uses to release oxygen from your blood into your tissues and brain.

This is called the Bohr Effect – a principle of respiratory physiology described over a century ago by Danish scientist Christian Bohr, and still one of the most underappreciated concepts in applied health.

The mechanism works like this: haemoglobin – the protein in your red blood cells that carries oxygen – only releases that oxygen when CO₂ is present at the right level. When CO₂ is too low, haemoglobin holds onto the oxygen. Your blood arrives at the brain carrying oxygen – but doesn’t let it go.

The practical implication: breathing more does not automatically mean getting more oxygen to where it matters.

What stress does to your breathing – and why it backfires

When your brain perceives a threat – a difficult conversation, a packed calendar, a looming deadline – it activates the sympathetic nervous system. Your body prepares for fight or flight.

Part of that preparation is an immediate change in breathing:

Rate increases, you breathe faster
Volume increases, each breath draws in more air
Pattern shifts, breathing moves from the diaphragm up into the chest
Mouth breathing often replaces nasal breathing

This response evolved to serve short, acute threats: sprint away, fight back, survive. In those situations, ventilating rapidly makes sense.

The problem is that modern stress is rarely short or acute. It’s chronic, low-grade, and relentless. And when fast, shallow breathing becomes a habit – when the body stays in that heightened state long after the immediate trigger is gone – CO₂ levels stay persistently low.

The result, via the Bohr Effect: less oxygen released to the prefrontal cortex – exactly the part of the brain responsible for clear thinking, decision-making, and emotional regulation.

This is why chronic stress doesn’t just feel bad. It measurably impairs cognition – not because of willpower or mindset – but because of a physiological cascade that starts with a breathing pattern.

What low CO₂ tolerance looks like in everyday life

CO₂ tolerance describes your body’s ability to remain calm and functional at higher CO₂ levels – in other words, how well you tolerate the natural rise in CO₂ that comes with physical or mental effort, without triggering a stress response.

When CO₂ tolerance is low, even moderate challenges can set off that cascade: faster breathing, rising anxiety, loss of focus. It becomes a self-reinforcing loop.

People with low CO₂ tolerance often recognise these patterns – even if they’ve never connected them to breathing:

A feeling of not being able to get a full, satisfying breath
Frequently sighing or yawning throughout the day
Mind going blank or clouding over under pressure
Waking up tired despite enough hours of sleep
A tendency to over-breathe during exercise – gasping through the mouth even at moderate intensity
Difficulty winding down in the evening

None of these are personality traits or signs of weakness. They are physiological patterns – and physiological patterns can be changed.

See how cell oxygenation and PH levels change depenging on CO2 levels by moving the lever in this interactive diagram:

How to train your CO₂ tolerance

CO₂ tolerance is not fixed. It responds to training – often within weeks of consistent practice.

The core principle is simple: by regularly exposing your body to slightly elevated CO₂ levels in a controlled way, you recalibrate your nervous system’s threshold. Over time, the body learns to remain calm at CO₂ levels that would previously have triggered alarm.

Here are the three most effective approaches, in order of how to build the practice:

1. Switch to nasal breathing – permanently

The nose is the body’s built-in breathing regulator. Nasal breathing slows the breath naturally, filters and humidifies the air, produces nitric oxide (which dilates blood vessels and improves oxygen uptake), and maintains a healthier balance of O₂ and CO₂.

Mouth breathing bypasses all of this.

Start by committing to nasal breathing during low-intensity activity: walking, working at a desk, reading. If you find nasal breathing uncomfortable or restricted, that’s useful information – it indicates your resting breathing volume is already higher than it needs to be.

For those ready to go further: nasal breathing during sleep, assisted by mouth tape if needed, can produce significant improvements in sleep quality and morning energy within one to two weeks.

2. Practise reduced-volume breathing

This is the foundation of the Oxygen Advantage® method. Rather than breathing deeply, you breathe less – gently reducing the volume of each breath until you notice a mild, comfortable air hunger.

That feeling of air hunger is the CO₂ signal. You’re not in danger. You’re training your chemoreceptors – the sensors in your bloodstream that detect CO₂ – to tolerate a slightly higher level before triggering the urge to breathe faster.

A simple starting exercise:

Sit comfortably with a relaxed posture
Breathe in gently through the nose for 2 seconds
Breathe out slowly through the nose for 4-6 seconds
After the exhale, hold for 2-3 seconds before the next inhale
Repeat for 5-10 minutes

The air hunger should remain mild throughout – a 3 out of 10 at most. If it becomes uncomfortable, return to normal breathing and try again with a shorter hold.

3. Add breath-hold exercises

Breath holds are the most direct way to raise CO₂ and train your body’s response to it.

The Control Pause – developed by Konstantin Buteyko and used extensively in the Oxygen Advantage® methodology (here called the BOLT Score) – is a simple measurement and training tool:

Take a normal breath in through the nose
Let it out naturally through the nose
Pinch the nose and hold
Time how long until you feel the first distinct urge to breathe (not the maximum you can hold – the first urge)
Release and breathe normally through the nose

A Control Pause of 25 seconds or less indicates low CO₂ tolerance and a likely breathing pattern disorder.
40 seconds or more indicates healthy breathing function.

Practise this regularly – mornings are ideal – and use it to track your progress. As your CO₂ tolerance improves, your Control Pause will lengthen.

What to expect – and when

Most people notice an initial shift within one to two weeks of consistent nasal breathing practice: calmer mornings, clearer thinking under pressure, less mid-afternoon energy dip.

Meaningful improvement in Control Pause scores – and with them, a noticeable difference in stress resilience, sleep quality, and exercise performance – typically appears within four to eight weeks of daily practice.

This is not a quick fix. It’s a recalibration. But unlike most wellness interventions, the results are measurable, physiologically grounded, and – once established – they hold.

Where to go from here

If this resonates, the next step is simple: start with the nasal breathing commitment for one week. Just one rule – nose only, all day. Notice what comes up.

If you’d like to go deeper – with guidance on technique, personalised baseline testing, and a structured programme – I work with individuals and teams in Zürich and online.

Book a free intro session →

Michael Schnekenburger is an Oxygen Advantage® Instructor based in Zürich. He works with professionals and corporate teams on breathing, stress resilience, and performance.