CO₂ Tolerance Test vs BOLT Test: What's the Difference, and Which Should You Use?

Auralize ships two CO₂ tolerance assessments: the BOLT test (Body Oxygen Level Test) and the CO₂ tolerance test (also called the slow-exhale or CO₂ tolerance breath assessment). They reach the same underlying trait — how comfortably your nervous system handles rising CO₂ — through different protocols, and both unlock the same single-phase, 10-session box-breathing program. Picking between them is mostly preference, not quality. You can take the free CO2 tolerance test directly in your browser — no signup required.

What the BOLT test measures

The BOLT score was popularised by Patrick McKeown in The Oxygen Advantage as a simple clinical proxy for chronic over-breathing and CO₂ sensitivity. ^[1] The protocol is short:

Sit quietly for a few minutes. Take a normal breath in through the nose, then a normal breath out through the nose. Pinch your nose closed at the end of the exhale. Start the timer. Stop the timer at the first definite urge to breathe — not when you can't hold any longer, but when your body first signals it wants air (a swallow, a contraction of the diaphragm, a clear tug in the chest). That number, in seconds, is your BOLT score.

Oxygen Advantage-style benchmarks often treat 25 seconds as a working baseline, 40+ as comfortably healthy, and 60+ as athletic-elite. ^[1] Auralize uses its own training ladder so BOLT maps to the same box-breathing unlocks as the CO₂ tolerance score: under 14s starts at Breathfinder, 14–23s reaches Airbearer, 24–36s reaches Oxyguide, 37–49s reaches Skywarden, and 50s+ reaches Highkeeper.

What the CO₂ tolerance test measures

The CO₂ tolerance test (sometimes called the slow-exhale test or the CO₂ tolerance breath assessment) was developed in the human-performance world and is described in detail by Brian Mackenzie and adapted for general use on the Huberman Lab podcast. ^[3] Unlike BOLT, it does not involve a breath hold. Instead, it measures your ability to sustain a long, controlled, soft exhale after a guided warm-up.

The protocol: take five guided coherence breaths at 5.5 seconds in, 5.5 seconds out. On the final breath, take one full inhale, then begin exhaling as slowly and softly as possible through the nose or pursed lips. The clock starts on the exhale and stops when you must inhale again. Your score is the number of seconds of sustained exhale. If you want to run it yourself before using the guided version, the DIY CO2 tolerance test covers the same protocol with stopwatch-only instructions.

The protocol-level differences

Breath hold vs. sustained exhale

BOLT scores a breath hold after a normal exhale. The CO₂ tolerance test scores an active, sustained exhale. The mechanical difference is the main reason to pick one over the other: a breath hold lets you sit in static air and time the rising signal; a sustained exhale keeps the diaphragm and respiratory muscles working throughout. Some practitioners find the hold cleaner to stop honestly; others find the slow exhale calmer and more meditative. Both produce a useful baseline.

What ends the score

BOLT ends at the first definite urge to breathe — a swallow, throat or diaphragm contraction, a clear "I need air now" signal. The CO₂ tolerance test ends when you must inhale to continue. Both stop signals are subjective in slightly different ways: BOLT relies on noticing the first urge instead of pushing through it; the slow-exhale test relies on letting the inhale come when it comes rather than forcing the exhale longer. Honest self-reporting matters for either format.

How long the test takes

Both Auralize tests take about a minute end-to-end. They share the same five-breath coherence warm-up at 5.5 seconds in, 5.5 seconds out — the warm-up is what standardizes your starting CO₂ so the score is comparable to your own past results.

Reproducibility across retests

Because both Auralize protocols share the same guided coherence warm-up, both stabilize the starting CO₂ level. External BOLT measurements (without a standardized warm-up) tend to drift more across retests; if you've been tracking BOLT scores outside Auralize and want a cleaner trend line, retest inside the app and use those numbers going forward.

Which score is "better"?

Neither. They answer the same underlying question — how comfortably your nervous system handles rising CO₂ — and Auralize maps both to the same training prescription. Pick by feel: if the idea of a single breath hold is less appealing than letting an exhale linger, take the slow-exhale test. If the slow-exhale test feels too long to control honestly, take BOLT. Most practitioners eventually try both and keep one as their default.

The CO₂ tolerance test gives you a number you can use to calibrate the length of each side of a box-breathing protocol. The interval that matches your current tolerance is the one that produces the best training adaptation. ^[3]

How scores map to the training ladder

Auralize routes both tests through the same five-level ladder. The CO₂ tolerance level names (Wayfarer → Summiteer) and BOLT level names (Breathfinder → Highkeeper) describe the same training level and unlock the same box-breathing interval:

Wayfarer / Breathfinder — 3-second box sides (CO₂ score under 20s; BOLT under 14s).
Pathmaker / Airbearer — 4–6-second box sides (CO₂ 20–39s; BOLT 14–23s).
Ridgewalker / Oxyguide — 7–10-second box sides (CO₂ 40–59s; BOLT 24–36s).
Cloudclimber / Skywarden — 11–14-second box sides (CO₂ 60–79s; BOLT 37–49s).
Summiteer / Highkeeper — 15–19-second box sides (CO₂ 80s+; BOLT 50s+).

Whichever test you take, your level becomes the input to the single-phase, 10-session CO₂ Capacity Builder program. Retesting with the other format mid-program is fine; if the new level differs, the program updates to match the latest baseline.

How CO₂ tolerance training improves both scores

Both tests respond to the same training stimuli: slow, paced breathing, nasal breathing as a default, and gradual extended-exhale work that teaches the chemoreflex that mildly elevated CO₂ is not an emergency. ^[2] The Bohr effect — the mechanism by which higher blood CO₂ improves oxygen delivery — is the physiological lever that responds to consistent practice. ^[2]

Auralize uses your level (from either test) to prescribe a single-phase, 10-session CO₂ Capacity Builder program with box-breathing intervals matched to your baseline. After the block, retest with either format — the score updates, and the prescription updates with it.

The bottom line

Take whichever fits the moment. Both retest in under a minute, both share the coherence warm-up that stabilizes the starting CO₂, and both produce the same single-phase, 10-session CO2 tolerance training protocol. The only thing that meaningfully changes between them is whether your last act is a held breath or a long, slow exhale — and that's down to which feels easier to do honestly today.