How to Test Your CO₂ Tolerance at Home (In 60 Seconds)

Testing your CO₂ tolerance at home takes about a minute and requires no equipment. The protocol — popularised by Brian Mackenzie and Andrew Huberman — gives you a number you can track over weeks of training and use to calibrate your breathing practice. ^[1] Here's how to do it correctly, what your score means, and what to do with it.

Before you start

For the score to be useful as a trend, take the test under similar conditions each time:

Seated, calm, and rested — not within thirty minutes of vigorous exercise. Not within an hour of caffeine or a heavy meal. Same time of day if you can — morning tests and evening tests can differ by several seconds. Through the nose for the warm-up and the test exhale (mouth breathing changes the protocol). Stop the moment you must inhale — not when it's uncomfortable, not when you could power through, but the actual moment your body says it's time.

The protocol, step by step

Step 1 — Settle (30 seconds)

Sit upright. Relax shoulders. Place hands wherever is comfortable. Take a few normal breaths through the nose. Don't deep-breathe or hyperventilate — that lowers baseline CO₂ and inflates the test result.

Step 2 — Five warm-up breaths at 5.5 in / 5.5 out

Inhale through the nose for 5.5 seconds. Exhale through the nose for 5.5 seconds. Repeat five times. This standardises your starting CO₂ level so your test result is reproducible across retests. Without the warm-up, your "baseline" varies by mood, posture, and recent activity — making score trends noisy.

Step 3 — The test exhale

Take one final full inhale through the nose. Then begin exhaling — slowly, softly, continuously. Through the nose or pursed lips. Start a timer (or watch a clock with seconds) at the start of the exhale.

Sustain the exhale as long as you can without forcing. The exhale should remain soft — no squeezing, no straining. When you absolutely must inhale, stop the timer. That number, in seconds, is your CO₂ tolerance score.

Step 4 — Record and interpret

Note the number. Write it down or save it. The first time you take this test, your score is your baseline, not a verdict — it's the starting point you'll track.

What your score means

Score ranges and their corresponding tiers in the Auralize system:

Under 20 seconds — Wanderer. A common starting point. High CO₂ sensitivity; the urge to inhale arrives quickly. Most untrained adults score here on first attempt.

20–29 seconds — Seeker. Early adaptation. You can slow the exhale and tolerate some CO₂ buildup, but the drive still arrives relatively quickly.

30–59 seconds — Voyager. Solid tolerance. The chemoreflex is calmer; CO₂ pressure builds without panic. This range is consistent with regular practice.

60–79 seconds — Trailblazer. Elite range. Exceptional control and lower chemoreflex sensitivity. Uncommon without dedicated training.

80+ seconds — Pulmonaut. Rare. Free-diver-level tolerance. Profound physiological adaptation.

How to use the score

The score is most useful as a calibration for your daily breathing practice. Lower scores benefit most from gentle coherence breathing (5.5 inhale / 5.5 exhale) and short box-breathing intervals — the goal is to teach the chemoreflex that mildly elevated CO₂ is safe. Higher scores can handle longer box intervals and extended-exhale work without overwhelming the nervous system. ^{[1] [2]}

Auralize uses your score to prescribe a 14-day CO₂ Capacity Builder program with box-breathing intervals matched to your baseline. After the program, you retest, the score updates, and the next prescription updates with it. No guessing what interval is right for you.

How often should you retest?

Once every two to four weeks if you're actively training. More frequent testing introduces noise — daily variability in baseline can swing scores by several seconds even without any real change in tolerance. Weekly is the floor for meaningful trends; bi-weekly to monthly is the practical sweet spot. Read more on training cadence and the Bohr effect.

Safety notes

The CO₂ tolerance test is safe for most healthy adults because it doesn't involve a breath hold after an inhale — only a soft, controlled exhale you stop on your own terms. Even so, skip it (or consult a clinician first) if you have uncontrolled cardiovascular disease, severe respiratory conditions, a history of fainting with breath work, late-stage pregnancy, or are recovering from eye, sinus, or chest surgery. Never perform breath-related assessments while driving or in water.