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

Search "how to test your CO₂ tolerance" and you'll find two protocols described almost interchangeably: the BOLT test (Body Oxygen Level Test) and the CO₂ tolerance test (also known as the slow-exhale or CO₂ tolerance breath assessment). They measure the same underlying physiology — your body's tolerance to rising carbon dioxide — but they measure it through different protocols, and they reward different things. Picking the right one matters more than most guides admit.

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.

A typical untrained adult scores between 10 and 20 seconds. McKeown considers 25 seconds a working baseline, 40+ as comfortably healthy, and 60+ as athletic-elite. ^[1]

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.

The protocol-level differences

Breath hold vs. sustained exhale

BOLT scores a breath hold after a normal exhale. CO₂ tolerance scores an active, sustained exhale. The mechanical difference matters: a breath hold lets you sit in static air, recruiting willpower and stillness to delay the urge. A sustained exhale keeps the diaphragm and respiratory muscles working the entire time, making it harder to "white-knuckle" the score with sheer determination. The CO₂ tolerance score correlates more directly with practical breath control.

What ends the score

BOLT ends at the first urge to breathe — a subjective signal, easy to override or misjudge. CO₂ tolerance ends when you must inhale to continue — a less ambiguous endpoint because by then your respiratory muscles are physically saying "now." Both rely on honest self-reporting, but the CO₂ tolerance endpoint is harder to fudge.

How long the test takes

BOLT is fast: under a minute including the few seconds of setup. CO₂ tolerance takes about a minute total once you include the five warm-up coherence breaths. The warm-up is part of the protocol — it ensures your starting CO₂ level is consistent across retests, which makes the score trend reliable.

Reproducibility across retests

Because the CO₂ tolerance test includes a guided warm-up, the starting condition is more controlled. BOLT's "normal breath" varies by mood, posture, recent activity, and even time of day. Two BOLT scores taken thirty minutes apart can differ by several seconds based on nothing other than baseline drift. CO₂ tolerance scores are more stable across retests, which makes them better for tracking progress.

Which score is "better"?

Neither is universally better — they answer slightly different questions. Use BOLT when you want a fast, no-equipment proxy for chronic over-breathing and CO₂ sensitivity, especially as part of a clinical or coaching conversation. Use the CO₂ tolerance test when you want a trainable baseline you'll retest over a four-to-eight-week training block, because the guided warm-up makes the trend cleaner.

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]

Translating scores between the two

There isn't a precise conversion formula — the protocols measure subtly different aspects of CO₂ tolerance — but in practice, scores roughly correlate. A BOLT of 10–20 seconds tends to map to a CO₂ tolerance score in the 15–25 second range (Wanderer / early Seeker territory in Auralize's tier system). A BOLT of 25–40 generally corresponds to CO₂ tolerance in the 30–60s range (Voyager). BOLT scores above 40 frequently align with CO₂ tolerance scores above 60 (Trailblazer or higher). Treat these as soft heuristics, not lookup tables.

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 CO₂ tolerance score to prescribe a 14-day CO₂ Capacity Builder program with box-breathing intervals matched to your baseline. After the block, you retest, the score updates, and the prescription updates with it. The same training will tend to raise your BOLT score in parallel; you can verify by taking a BOLT measurement before and after the program if you want both data points.

The bottom line

The BOLT test and the CO₂ tolerance test both estimate the same thing — your body's relationship with rising carbon dioxide — and both respond to training. The CO₂ tolerance test is the more controlled, more reproducible, more training-friendly protocol. The BOLT test is the older, more widely-recognised one. If you want a number to track over weeks, use the CO₂ tolerance test. If you've already been tracking BOLT scores, keep doing so — and consider adding a monthly CO₂ tolerance test for a cleaner training signal.