Soft Breathe · Free Tool

CO₂ Tolerance Test

Your body's response to CO₂ is one of the best windows into your stress baseline. This 60-second test reveals how well your nervous system handles discomfort — and what that means for your breathing practice.

1 Take 3 normal, calm breaths through your nose.

2 On the third exhale, gently pinch your nose.

3 Press and hold the button below.

4 Release the moment you feel the first definite urge to breathe (e.g. swallow reflex or diaphragm twitch).

Get ready

Take a normal breath

Breathe in through your nose… then out slowly and completely. When your lungs feel empty, hold — and press the circle.

3

Starting in...

Press and hold the circle — now
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hold

Release at the first physical urge to breathe (swallow or stomach twitch) — not at your limit.

Your result
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seconds
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Recommended practice

Score Ranges at a Glance

CO2 tolerance improves with consistent practice. All scores below are improvable — the test is a baseline, not a verdict.

Score Range What it typically means Starting point
Under 20s Low tolerance Chronic over-breathing common; sympathetic nervous system running elevated. Found in ~25–30% of adults (McKeown, 2015). Buteyko nasal breathing
20–40s Average range Functional but unoptimised. Occasional breathlessness under stress, variable sleep quality. Most common score for new practitioners. Box breathing (4-4-4-4)
40–60s Good Well-regulated breathing. Consistent with habitual nasal breathing and measurably better autonomic regulation (HRV). Resonance breathing (5.5 BPM)
60s+ Excellent Typical of advanced practitioners, endurance athletes, experienced prānāyāma practitioners. Advanced techniques are safe and productive. Nadi Shodhana (alternate nostril)

3 Techniques to Improve Your Score

Ordered from beginner-accessible to intermediate. All three improve CO2 tolerance by training your body to breathe slightly less air than usual — a principle called reduced-volume breathing.

01

Reduced-Volume Nasal Breathing

10 min · daily

Breathe in and out through the nose only. After each exhale, pause 2–3 seconds before inhaling. Breathe slightly less air than feels natural — a mild, manageable air hunger is the target. Most practitioners see a 5–10 second improvement in BOLT score within 2–3 weeks (McKeown, 2015).

Try Buteyko breathing →
02

Breath Hold Walks

15–20 min · 3× per week

Walk at a comfortable pace. After a normal exhale, pinch your nose and hold for 20–30 steps. Release and breathe normally for 1–2 minutes. Repeat 8–10 times. This technique leverages the body's natural CO2 production during movement to expand tolerance in a functional context.

Try breath hold training →
03

Resonance Breathing with Extended Exhale

5 min · daily

5-second inhale, 7-second exhale (approximately 5 breaths/minute). The extended exhale engages the respiratory sinus arrhythmia reflex, increasing heart rate variability. Lehrer & Gevirtz (2014) demonstrated significant HRV increases within a single 20-minute session in healthy adults.

Try resonance breathing →

Last reviewed April 2026 by Elena Vance, MSc Cognitive Neuroscience, University of Oxford

Frequently Asked Questions

What is the BOLT score? +

The Body Oxygen Level Test (BOLT) is a clinical measurement of your sensitivity to carbon dioxide. It measures the time until your first definite desire to breathe after a normal exhalation. It is not a test of maximum breath-holding capacity, but rather a reflection of your respiratory centre's tolerance to CO2 buildup. A higher BOLT score indicates a calmer, more efficient breathing pattern and greater resilience to stress.

What is a good CO2 tolerance score? +

A CO2 tolerance score of 40 seconds or above is considered good for a healthy adult. Scores of 20–40 seconds are average for sedentary individuals, while scores below 20 seconds suggest chronic over-breathing patterns that may be contributing to anxiety or fatigue. Elite athletes and advanced breathwork practitioners typically score 60 seconds or above. All scores are improvable — the test is a baseline, not a verdict.

Why does CO2 tolerance matter for anxiety? +

Carbon dioxide is the body's primary respiratory stimulant. Low CO2 tolerance means your brain triggers the 'need to breathe' alarm too early. This leads to chronic over-breathing (hyperventilation), which mimics the physiological symptoms of anxiety — rapid heart rate, chest tightness, dizziness. Improving your CO2 tolerance raises this threshold, making your nervous system more resilient to stress. A randomised controlled trial by Meuret et al. (2010, Journal of Consulting and Clinical Psychology) found that CO2-raising respiratory training reduced panic disorder symptoms by 82% over 4 weeks.

How do I improve my CO2 tolerance fast? +

The most effective way to improve CO2 tolerance quickly is daily reduced-volume nasal breathing — breathing slightly less than feels natural for 10 minutes per day, sustaining a mild air hunger sensation. Patrick McKeown (The Oxygen Advantage, 2015) reports an average 8-second improvement in BOLT score after 4 weeks of this practice. Adding breath hold walks 3 times per week accelerates gains further.

Is this test safe for everyone? +

The BOLT score is generally safe because it is a sub-maximal breath hold (you stop at the first urge to breathe, not the maximum limit). However, completely avoid breath-holding tests or exercises if you are pregnant, or have a history of panic disorder, COPD, epilepsy, recent head injury, or severe cardiovascular issues without first consulting a physician.

Can low CO2 tolerance cause anxiety? +

Low CO2 tolerance and anxiety share a reinforcing physiological loop. Anxiety drives faster breathing, which lowers CO2, which triggers chemoreceptor alarm signals, which the brain interprets as threat — worsening anxiety. Raising CO2 tolerance through reduced-volume breathing interrupts this cycle at the physiological root, not just symptomatically.

How often should I retest my CO2 tolerance? +

Retest every 2–4 weeks when actively training. Daily testing is not recommended — it can create performance anxiety and does not reflect meaningful physiological change on that timescale. Use each retest to calibrate your current technique intensity: if your score has plateaued, increase the challenge of your reduced-volume or breath hold sessions.

References

  1. McKeown, P. (2015). The Oxygen Advantage. William Morrow.
  2. Meuret, A. E., et al. (2010). Targeting pCO2 in panic disorder. Journal of Consulting and Clinical Psychology, 78(5), 691–704.
  3. Lehrer, P. M., & Gevirtz, R. (2014). Heart rate variability biofeedback. Frontiers in Psychology, 5, 756.
  4. Bohr, C., Hasselbalch, K., & Krogh, A. (1904). Über einen Einfluss der Kohlensäurespannung auf die Sauerstoffbindung. Skandinavisches Archiv für Physiologie, 16, 402–412.