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

Understanding the Science

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 center's tolerance to CO2 buildup.

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.

How do I improve my BOLT score? +

The foundation of improving CO2 tolerance is strict, 24/7 nasal breathing—including during sleep and light exercise. Beyond that, practicing exercises like 'Light, Slow, Deep' breathing or formal breath-hold training (under guided supervision) helps safely acclimatize your brainstem to higher levels of carbon dioxide.

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, uncontrolled hypertension, or severe cardiovascular issues.