BOLT score — measure your CO2 tolerance
The Body Oxygen Level Test gives you a repeatable number to track nasal breathing progress.
Key takeaways
- What it is: The Body Oxygen Level Test gives you a repeatable number to track nasal breathing progress.
- Why it works: The BOLT score measures the duration of comfortable breath-hold after a normal exhale — a proxy for CO2 tolerance. A low score (under 20 seconds) correlates with habitual over-breathing and respiratory-related performance limits. Tracking it over weeks turns an invisible physiological parameter into actionable feedback.
- Evidence: Plausible mechanism, limited direct outcome data.
- Avoid: Holding as long as possible rather than stopping at the first urge to breathe — this inflates the number and measures anaerobic tolerance rather than CO2 sensitivity.
Why it works
The BOLT score measures the duration of comfortable breath-hold after a normal exhale — a proxy for CO2 tolerance. A low score (under 20 seconds) correlates with habitual over-breathing and respiratory-related performance limits. Tracking it over weeks turns an invisible physiological parameter into actionable feedback.
How to do it
- 1Sit quietly for 5 minutes. Take a normal breath in, then a relaxed breath out.
- 2Pinch your nose and count seconds until the first definite urge to breathe (not the maximum hold — the first urge).
- 3Release. Breathing should return to normal without gasping; if not, the hold was too long.
- 4Test at the same time of day, at rest. Log weekly.
What the evidence says
MechanisticThe BOLT score is a clinical tool used in Buteyko practice, with some correlation to respiratory chemosensitivity data. It is not validated against gold-standard respiratory measures in large trials.
Honest caveat: Correlation with VO2 max or athletic performance has not been established in independent, large-scale studies. Treat it as a personal progress marker, not a clinical diagnostic.
Common mistake
Holding as long as possible rather than stopping at the first urge to breathe — this inflates the number and measures anaerobic tolerance rather than CO2 sensitivity.
IX Coach prompts a weekly BOLT score check-in and plots the trend line, so you see whether your breathing practices are moving the needle or need recalibration.
Practice this with IX Coach →