8 min.The most important thing first: Most people breathe too much – and that makes them more stressed, sleep-deprived, and less capable. CO2 tolerance training is the antidote: get more out of every breath by taking fewer of them, stay calmer, and sleep better. This article explains why – and how you can start today.
The CO2 Paradox: More Oxygen Through Less Breathing
Intuitively this sounds wrong: if you breathe more, you get more oxygen, right? Not quite. The key mechanism is called the Bohr effect: hemoglobin – the molecule in your red blood cells that transports oxygen – only releases oxygen efficiently to tissues and cells when CO2 levels in the blood are high enough.
People who chronically over-breathe (everyday hyperventilation) have a persistently low CO2 level. The blood is saturated with oxygen – but the cells can't properly absorb it. The result: fatigue, difficulty concentrating, anxiety, shallow sleep.
What Exactly Is CO2 Tolerance?
CO2 tolerance is the ability of your body – more precisely, your nervous system – to tolerate elevated CO2 levels in the blood without a panic response. When you stop breathing, CO2 rises. At some point this triggers the breathing reflex – the strong urge to inhale again.
People with low CO2 tolerance experience this reflex early and intensely. The system is set to be sensitive. Under stress, exertion, or sleep deprivation it overreacts – which can lead to breathlessness, a racing heart, and anxiety. High CO2 tolerance means the system is more stable, calmer, and more resilient.
You can measure this – with the so-called BOLT test.
The BOLT Test: How to Measure Your CO2 Tolerance
BOLT stands for Body Oxygen Level Test, developed by Irish breathing researcher Patrick McKeown. Here's how it works:
- Breathe normally in and out (through your nose)
- After the exhale: pinch your nose shut
- Start the timer – until you feel the first impulse to breathe again (not until you can no longer hold it)
- That is your BOLT score in seconds
Results: Under 20 sec = low CO2 tolerance, needs improvement. 20–40 sec = average. Over 40 sec = very good. Competitive athletes and experienced breath trainers often reach 60+ seconds.
Do the test now – and again in 4 weeks after the training below.
How Does This Connect to the Wim Hof Method?
Good question – and a misconception I regularly clear up in my workshops: Wim Hof breathing and CO2 tolerance training are not opposites, they are complements.
- Wim Hof breathing deliberately creates a temporary CO2 drop (hyperventilation) followed by a breath hold – training your ability to handle extreme states and activating the sympathetic nervous system.
- CO2 tolerance training raises the resting threshold – meaning the stability of your system in everyday life, during sport, and during sleep.
Together, both approaches give you full control in both directions: activation (WHM) and calm (CO2 training). More about the core WHM technique: Wim Hof Breathing – explained simply.
3 Exercises for CO2 Tolerance Training
1. Nasal Breathing in Daily Life
The simplest and most effective measure: breathe through your nose for everything you do – when walking, during exercise (up to a certain intensity), when sleeping. The nose filters, humidifies, and warms the air, produces more nitric oxide (NO), and naturally slows the breathing pattern. Mouth breathing has led many people to a chronically elevated breathing rate – nasal breathing reverses that.
2. Box Breathing (4-4-4-4)
Inhale for 4 sec – hold for 4 sec – exhale for 4 sec – hold for 4 sec. Repeat for 5–10 minutes. This technique, well known from Navy SEAL training, increases CO2 tolerance, activates the parasympathetic nervous system, and is scientifically proven as a stress regulator. Ideal before sleep or after a WHM session.
3. Cadence Breathing During Exercise
When running or cycling: breathe exclusively through your nose and match your pace to nasal breathing – not the other way around. If your nose can't keep up, slow down. After 2–4 weeks you'll notice that you can run faster at the same heart rate. Your BOLT score typically improves by 5–10 seconds per month.
When Do Results Show?
The first effects – calmer breathing, less stress – are often noticeable within one week. Measurable improvements in your BOLT score (5–10 sec) after 3–4 weeks of consistent practice. Deeper changes in sleep quality and endurance after 6–8 weeks.
How breathwork and cold training reinforce each other is explained in the article What cold showers do to your body. The scientific background on the power of breath is here: The Power of Breath – what the latest research reveals.
FAQ – CO2 Tolerance Training
What is CO2 tolerance and why does it matter?
CO2 tolerance is your nervous system's ability to tolerate elevated CO2 without panic. People with high CO2 tolerance breathe more efficiently, stay calmer under stress, and sleep more deeply. Most people over-breathe – which paradoxically means less oxygen in the cells (Bohr effect).
How do I train my CO2 tolerance?
Nasal breathing in daily life, Box Breathing, Cadence Breathing during exercise, and regular BOLT tests to track progress. Typical improvements: +5–10 sec BOLT score per month with consistent practice.
How does CO2 training fit with the Wim Hof Method?
WHM trains the conscious handling of low CO2 (hyperventilation + retention). CO2 tolerance training raises the resting threshold. Together both techniques give you full control over your nervous system in both directions.
What is the BOLT test?
Exhale normally, pinch your nose, time yourself until the first breathing impulse (not the limit). Under 20 sec = low tolerance. 20–40 sec = normal. Over 40 sec = very good.
Bottom line: Sometimes less is more – including with breathing. CO2 tolerance training is the quiet superpower among breathing techniques. Combined with the Wim Hof Method you get a complete toolkit for self-regulation. If you want to experience both live, join one of my workshops.
