The CO2 Tolerance Test
Time one slow exhale to read someone's aerobic efficiency and stress reactivity
- Difficulty
- Starter
- Time to result
- ~days to results
- Steps
- 5
- Confidence
- 82%
The CO2 tolerance test is a three-part timed exhale that reads how well a person tolerates carbon dioxide, which Mackenzie treats as the master variable behind aerobic efficiency and stress reactivity. The subject first settles with a couple of minutes of calm, comfortable breathing, ramps through a short four-breath pattern with a slightly extended exhale, then on the fourth breath inhales to the top, starts a timer, and exhales as slowly and controllably as possible. The elapsed seconds report three things at once: mechanical control of the diaphragm, physiological response to CO2 (how aerobically efficient you are at rest), and cognitive arousal state (how quickly you hit the panic switch). Scores under twenty seconds flag a volatile, reactive system; trained operators under a minute are considered reactive. The number is trainable and improves quickly once the person backs off and does breath work.
Origin
Developed by Brian Mackenzie through his Power Speed Endurance and Art of Breath work, after moving from endurance coaching and yoga into measuring breath with portable metabolic carts alongside colleague Rob Wilson in the special-warfare community.
Core principles
- 01Carbon dioxide, not oxygen, is the body's metabolic stress messenger
- 02How well you tolerate CO2 governs how efficiently you use oxygen
- 03A short tolerance score signals a reactive, overtrained, sympathetic-dominant system
- 04The breath is the first responder to rising CO2 — before heart rate reacts
- 05The test measures mechanics, physiology, and cognitive state in one number
How to run it
- 1
Settle with calm breathing
Sit or lie down for a couple of minutes doing slow, controlled breathing you are comfortable with. Keep it unforced.
Pro tip Comfort is the point here — this phase sets a clean baseline, not a challenge.
Watch out Do not breathe fast or hyperventilate; that inflates the score and invalidates the test.
- 2
Ramp through a four-breath pattern
Breathe up for three to four seconds, then relax and let the exhale go for roughly five seconds. Keep the exhale slightly longer than the inhale across the breaths.
Pro tip Let the exhale relax out rather than pushing it — you are cueing the diaphragm, not forcing air.
- 3
Start the timer at the top of the fourth breath
On the fourth breath, pull the air in fully. The moment it hits the top of the inhale, start the timer and begin exhaling.
- 4
Exhale as long and slowly as you can control
Release the breath as slowly and continuously as possible under full control. Stop the timer when you can no longer control the exhale.
Pro tip Control is the metric — a ragged, rushed exhale is not a longer score, it is a failed one.
- 5
Read the number against the thresholds
Under twenty seconds signals a very reactive, volatile system that needs cleaning up. A trained operator under sixty seconds is reactive with suppressed tissue. Higher scores indicate greater aerobic efficiency and calm.
Pro tip Retest after a few weeks of backing off and doing breath protocols; the gap opens up fast in most people.
Watch out Do not treat the score as a personality verdict; it reflects current training load and stress, and it moves.
In the wild
Mackenzie's colleague Rob Wilson was called in to assess a special-warfare operator who could not recover, carried nagging injuries, and had a host of problems. Before running any diagnostics, Mackenzie guessed the man's CO2 tolerance was under twenty seconds. Rob reported seventeen seconds. The number matched the whole clinical picture — a reactive nervous system, tight and painful tissue, and a body stuck in high sympathetic activity despite being an elite performer.
→ The single test confirmed the operator was overtrained and reactive, pointing to breath protocols and backed-off training rather than more work.
Mackenzie notes that a lifter who squats 1,000 pounds but has never done breath work will almost certainly test with a poor CO2 tolerance. All that lean muscle raises oxygen demand, and without aerobic-efficiency work the system defaults to anaerobic pathways within a few breaths of exertion. The test surfaces this even in someone who looks supremely fit.
→ A low score reframes an impressive strength athlete as aerobically inefficient, justifying a nasal-breathing reset.
Common mistakes
Hyperventilating before the timed breath
Breathing fast during the settle phase artificially lowers CO2 and inflates the exhale time, producing a score that misrepresents your actual tolerance.
Chasing a long time instead of control
The exhale must stay slow and controlled; a rushed or ragged release is not a higher score, because the test measures diaphragm control, not lung capacity.
Treating the number as fixed identity
The score reflects current training load, recovery, and stress, and improves quickly with breath work — reading it as a permanent trait misses the point.
Is it for you?
Best for
Coaches and self-coached athletes wanting a quick baseline of stress reactivity and aerobic efficiency before prescribing training or breathwork.
Not ideal for
People who mistake the number for a fixed identity rather than a trainable, day-to-day indicator.
From the transcript
“It's a three-fold test... you just do some slow controlled breathing... then a four breath pattern... Fourth breath, you pull it in, start the timer…”
“Anybody below 20 seconds and we have a very reactive, volatile environment that we need to actually start working to clean up.”
“carbon dioxide is the metabolic stress messenger of the body.”
From the episode
How To Breathe Properly - Brian Mackenzie - #121
Brian Mackenzie