Almost every meaningful measure of breathing practice requires hardware. HRV needs a watch or a chest strap. Respiratory rate needs a sensor. Coherence needs a live heart-rate stream. Which leaves anyone without a wearable practising on faith, with no number that moves.
The BOLT score is the exception. It requires a timer and twenty seconds of honesty. It is not a substitute for HRV — it measures something else — and it is not a validated test. But it is a number, it rose with three months of breathing training in a controlled trial, and it is the only one on that list you can run with nothing on your wrist.
The protocol
- Sit still and breathe normally for a few breaths. Through the nose. Do not deepen or slow them; you are settling, not preparing.
- Take a normal breath in and let a normal breath out. Not a big one. Not emptied. Just the end of an ordinary exhale.
- Pinch your nose and start the timer.
- Stop at the first definite urge to breathe. The first involuntary swallow, the first flicker of the diaphragm, the first moment your body asks. Not when you run out. Not when it gets hard.
- Release and breathe through the nose. Your first breath afterwards should be calm and normal. If you are gasping, you held too long and the score is invalid.
Step four is the whole test, and it is where people go wrong. BOLT is not a max breath-hold. Treating it as an endurance challenge produces a number that measures your tolerance for discomfort and your competitiveness, both of which change day to day for reasons unrelated to your breathing. Even done correctly, breath-holding is a noisy measure: Parkes’s review of its physiology calls the duration of a hold, its simplest measure, “highly variable”, and we could not find a published test-retest study of BOLT itself. The consistency has to come from you.
Run it first thing in the morning, before coffee, before exercise, before your first stressful email. Same conditions every time, or the comparison is meaningless.
What the number means
| Score | Label |
|---|---|
| Under 10s | A shorter hold. |
| 10–20s | A moderate hold. |
| 20–30s | A longer hold. |
| 30s+ | A long hold. |
The cut-offs are not ours. They follow the bands published by Oxygen Advantage, Patrick McKeown’s training company, which labels 20–30 seconds “good breathing efficiency” and calls 40 seconds the gold standard. That is practitioner convention. We found no peer-reviewed study that validates any of these cut-offs, which is why the labels above, and the ones in the app, describe the length of the hold and nothing more. The only comparison worth making is against your own score from last month, taken the same way at the same time of day.
What it actually measures — and what it doesn’t
BOLT stands for Body Oxygen Level Test, which is a misleading name. It does not measure your oxygen level, or anything else directly. It is a stopwatch.
The theory behind it is Konstantin Buteyko’s: people who chronically over-breathe keep their carbon dioxide low, become sensitive to it, and feel the urge to breathe early — so a short hold signals over-breathing, and training that raises CO2 tolerance lengthens it. It is a tidy theory. The direct test of it did not support it. Courtney and Cohen timed breath-holds by the Buteyko protocol alongside end-tidal CO2 in 83 adults: the correlation came out negative (r = −0.24), the opposite of what Buteyko claimed, and hold time did not predict resting CO2. What it did track, in people with abnormal lung-function results, was breathing pattern — shorter holds went with chest-dominant breathing.
“Although BHT does not predict resting ETCO2, it does correlate with breathing pattern in subjects with abnormal spirometry.”
The physiology of the urge itself is not settled either. Parkes’s (2006) reviewof breath-holding concludes that the involuntary breath that ends a hold is not caused solely by blood gases, lung volume or the carotid chemoreceptors, and that there is at present no simple explanation for it. CO2 matters — holds get longer when it starts low and shorter when it starts high — but it is not the whole story. And that review is about holds taken to the limit; the first urge BOLT stops at is less studied still.
Now the honest part about the evidence. The BOLT score comes out of the Buteyko tradition, popularized in its current form by Patrick McKeown, and Buteyko-style training has some controlled-trial support in asthma. In Bowler et al. (1998), 39 people with asthma who learned the method cut their reliever (beta-2 agonist) use far more than controls and breathed less air per minute at rest, with no change in airway measurements — and, tellingly for the theory, no change in their end-tidal CO2. In a 2024 trial of 60 adults with asthma, the control pause — the Buteyko hold BOLT is based on — rose over three months of Buteyko training and not with usual care, asthma-control scores improved, and use of relievers and inhaled steroids each fell by about 20 percent. It was a single trial, registered after the fact.
The wider picture is less sharp. The Cochrane review of breathing exercises for adults with asthma (Santino et al., 2020; 22 trials, only two involving Buteyko) concludes they may have some positive effects on quality of life, hyperventilation symptoms and lung function, with the certainty of that evidence ranging from moderate to very low.
The BOLT score itself is on weaker ground than the training. The one study we found that tested BOLT by name against performance, in 49 elite speedskaters, found no relationship with any of its anaerobic or aerobic test results.
“However, it has not yet been scientifically validated or linked with exercise performance in highly-trained individuals.”
We include it anyway, and we would rather say why than hide the caveat. It is free, it can be repeated at home, and in the trial above that made it the primary outcome it moved with three months of practice. That makes it a reasonable personal log — as long as nobody pretends it is a lab test. If you want the better-evidenced number, that is HRV, and it needs a device.
How to raise it
Not by holding your breath longer. Practising the hold itself trains you to sit through the urge, which lengthens the number without telling you anything about how you breathe the rest of the day.
What Buteyko and McKeown teach instead is breathing less, all day. The list below is practitioner advice, not a set of separately tested interventions; the nearest thing to evidence for the principle is Bowler’s finding above that the Buteyko group moved less air per minute at rest.
- Nasal breathing by default— waking, walking, working, and where you can manage it, sleeping.
- Slow paced breathing daily. Five to ten minutes at your resonance rate.
- Light rather than deep. The instinct to take enormous breaths during practice runs against the whole idea of breathing less.
- Nasal breathing during easy aerobic work. Uncomfortable at first.
We won’t give you a timeline. The adult trial that made the control pause its primary outcome ran for three months, in people with asthma, and reported a group effect — not what a given person should expect.
BOLT alongside HRV
The two numbers answer different questions and neither replaces the other. HRV tells you about autonomic state — how recovered you are, how much load you are under this week, whether last night’s drink cost you something. It is noisy day to day and meaningful in trend.
BOLT is a rougher instrument pointed at a different question: how you breathe. Courtney and Cohen’s data tie it to breathing pattern rather than to CO2, and only in people with abnormal lung function. Nobody has published how much it moves from one day to the next. If your HRV is flat and your BOLT has climbed, what you know for certain is that you can now hold a comfortable breath longer. What that means beyond itself is not established.
Track BOLT weekly. Track HRV as a rolling average. Ignore both daily.
The BOLT check in HRV Breathe
The test is built into the app, and it is the progress metric we point people to when they do not own an Apple Watch — a breath-hold timer needs nothing but a phone and a chair.
The guided version handles the parts people get wrong. It walks you through three unhurried settling breaths with a haptic on each, so you are not deciding for yourself when you have settled. It starts the timer at the right moment, after a normal exhale rather than a big one. And you stop it by tapping at the first urge — the instruction is on screen while you hold, because the temptation to push is strongest exactly then. Scores are stored with their dates and shown on the Insights tab so the trend is visible without you keeping a spreadsheet.
When not to do it
Skip breath-hold testing entirely if you are pregnant, or if you have uncontrolled high blood pressure, a heart condition, epilepsy, or a history of fainting. If you have panic disorder, be careful: air hunger is a common panic trigger, and deliberately inviting it is not a neutral act. A twenty-second comfortable hold is a mild thing for most people and a bad idea for some. If you are in the second group you can practise slow nasal breathing perfectly well without ever measuring this.