The CO2 Baseline: Measuring When the Urge to Breathe Starts
How to record the point where the urge to breathe first appears, why it is a more useful training marker than a maximum attempt, and what it cannot tell you.
Short answer
Your CO2 baseline is the moment during a hold when you first notice the urge to breathe. It is a useful training marker because it tracks comfort rather than willpower, and because measuring it does not require going near your limit. It is a subjective marker, not a measurement of blood gases, and it says nothing about how much oxygen you have left.
Measure on dry land, seated or lying down, away from water. Never measure a breath hold alone in water, and never hyperventilate to push the first signal later. Suppressing the warning is the opposite of what this measurement is for.
Why the first signal is worth tracking
A maximum attempt tells you how far you will push through discomfort on a given day, which is heavily influenced by mood, motivation, and how competitive you feel. The first signal is closer to a physiological threshold and does not require a hard effort to record.
Practically, two divers with the same maximum can have very different sessions. The one whose discomfort begins later spends less of the hold managing urgency. That difference is what people usually mean by feeling comfortable in the water.
What is happening
During a hold, carbon dioxide accumulates and oxygen falls. Chemoreceptors in and around the brainstem and major arteries respond to changes in carbon dioxide, oxygen, and pH, and respiratory drive rises. A review of respiratory responses to a maximal apnoea describes this as multi-factorial rather than a single carbon-dioxide threshold.
The sensations typically progress from a faint awareness, to tightness, to involuntary breathing movements, to a strong demand to breathe. Your baseline captures the first step.
A later first signal is not proof of a safer hold. The published evidence is explicit that reduced discomfort does not indicate that oxygen remains at a safe level.
How to measure it
Set up on dry land
Sit or lie somewhere stable, away from water. Same room, same position, same time of day if you can manage it. Consistency is what makes the numbers comparable.
Settle your breathing
A couple of minutes of calm, normal breathing. Do not use rapid or repeated deep breaths; they lower carbon dioxide and artificially delay the very signal you are trying to measure.
Take one comfortable full breath
Fill low to high in a single smooth inhale. Full but not strained.
Hold and observe
Start the timer, stay still, and put your attention on the sensations rather than the clock. Do not anticipate the signal; wait for it.
Mark the first signal and release
Record the instant you first notice tightness or urge, then finish the hold shortly after. There is no reason to continue toward a maximum on this test.
The temptation is to wait until the signal is unmistakable so the number looks better. Resist it. A consistently early, honest mark is more useful than an impressive one, because you are comparing it only to your own previous readings.
What training does to it
A CO2 table repeats a fixed, sub-maximal hold with progressively shorter recovery, so later rounds begin with less complete recovery. The CO2 table guide explains how to run one, and the generator will build a conservative session.
What changes with practice is mainly familiarity: the same sensations feel less alarming and are easier to stay still through. Beyond that, be careful. Reviews of apnoea training and of breath-holding in sports describe varied protocols and mixed outcomes, and do not support a staged account of week-by-week physiological adaptation.
You will find tables of expected weekly gains elsewhere online. Treat them sceptically: no source we could verify supports those numbers for a general population.
What to expect instead
Track your own trend over weeks under consistent conditions. Some sessions will be worse than the last for reasons that have nothing to do with training. A single reading means very little; a direction over a month or two means something.
If you also record a maximum occasionally, use the two together: the baseline for how comfortable the hold feels, the maximum for scaling tables. See how to do a max breath-hold test.
What moves the number day to day
Stress, poor sleep, illness, caffeine, recent meals, recent exercise, and time of day all plausibly shift a subjective comfort threshold, and practitioners report exactly that. We do not attach a number of seconds to any of them, because these are self-reported influences rather than quantified findings.
The practical rule: do not change your training on the basis of one reading.
Sources
- Respiratory system responses to a maximal apnoea
- Hypoxia in Breath-Hold Diving, Divers Alert Network
- Freediving Safety, Divers Alert Network
- Involuntary breathing movements during breath holding (1981)
- Effects of apnoea training on aerobic and anaerobic performance
- The application of breath-holding in sports
Ready to start training?
Build CO2 and O2 tables, follow guided sessions, and keep your training history in one place.
Related Resources
CO2 Tolerance Explained: What Changes During a Breath Hold
Learn how carbon dioxide, oxygen, lung volume, and respiratory signals contribute to breath-hold discomfort and the urge to breathe.
GuideHow to Use a CO2 Table for Dry-Land Breath-Hold Training
Learn how a CO2 table works, choose a conservative starting session, avoid hyperventilation, and adjust the table without chasing a promised result.
GuideHow to Do a Max Breath-Hold Test Safely
A controlled dry-land protocol for recording a repeatable breath-hold baseline, what the number does and does not tell you, and the safety rules that do not bend.