A CO₂ table is a series of breath holds of the same length separated by rests that get shorter every round. Because carbon dioxide builds up faster than your body can clear it, each hold starts with more CO₂ in your blood than the one before — and that is exactly the stimulus that trains your tolerance to the urge to breathe. It is dry-land training, it takes about 20 minutes, and it is the single most reliable way to make a breath hold feel calmer.
What is CO₂ tolerance, really?
Most people assume that the burning need to inhale comes from running out of oxygen. It does not. Your brainstem and the chemoreceptors in your carotid arteries watch the partial pressure of carbon dioxide in your blood. When it climbs a few millimetres of mercury above your resting value — often just 3 to 5 mmHg — the drive to breathe switches on. Your oxygen saturation at that moment is still high enough to run a marathon.
That gap between “I want to breathe” and “I need to breathe” is where all breath-hold training happens. You are not training your lungs to hold more air. You are training your nervous system to stay calm while a chemical alarm is ringing.
This has one practical consequence that matters more than anything else on this page: never hyperventilate before a hold. Blowing off CO₂ silences the alarm without adding any meaningful oxygen. You feel great, you hold longer, and you may lose consciousness with no warning at all. Every safe protocol works with the CO₂ signal, not against it.
The two phases of a breath hold
Swedish physiologist Erika Schagatay’s terminology splits a hold into two parts, and it is the most useful mental model you can carry into training:
- The easy-going phase — from the start of the hold until the first involuntary contraction of your diaphragm. It feels like nothing much is happening.
- The struggle phase — from the first contraction until you break. Contractions get stronger and closer together. Nothing dangerous is happening yet; it is uncomfortable, not harmful.
Here is the striking part. When Oisín O’Croinin and colleagues trained 22 beginners for 13 days and combined the holds with deliberate psychological coaching, total apnea time rose from 44 ± 21 s to 75 ± 33 s — a 70 % improvement (Applied Physiology, Nutrition, and Metabolism, 2025, DOI 10.1139/apnm-2025-0033). But the easy-going phase barely moved: 26 ± 12 s to 30 ± 17 s, statistically indistinguishable (p = 0.329). The struggle phase went from 18 ± 18 s to 45 ± 34 s.
Almost the entire gain came from tolerating discomfort longer, not from a physiological change in the comfortable part of the hold. That is a training target you can practise deliberately — and CO₂ tables are the tool.
How a CO₂ table is built
The structure is deliberately boring:
- Pick a hold time you can complete comfortably in every round — roughly 50–60 % of your current maximum.
- Keep that hold constant for all 8 rounds.
- Start with a long rest (around 2:00) and shorten it every round, down to about 0:35–0:45.
Because the rest shrinks, you enter each hold with progressively less CO₂ cleared from the previous one. Round 1 feels trivial. Round 7 feels like a maximum attempt at half the duration. That is the point.
Enter your own maximum below and the generator builds the table for you:
Build your CO₂ table
Enter your current maximum breath hold. The table keeps the hold constant at 50–60 % of your max and shortens the rest each round — that is what trains CO₂ tolerance.
| Round | Rest | Hold |
|---|---|---|
| 1 | 2:00 | 0:35 |
| 2 | 1:50 | 0:35 |
| 3 | 1:35 | 0:35 |
| 4 | 1:25 | 0:35 |
| 5 | 1:10 | 0:35 |
| 6 | 1:00 | 0:35 |
| 7 | 0:50 | 0:35 |
| 8 | 0:35 | 0:35 |
Total session: 15:05
Dry, seated or lying down, no hyperventilation. Stop the table if you feel dizzy or get strong contractions early.
The full session lands somewhere around 15–20 minutes. Do it dry, sitting or lying down, once or twice a day. There is no need to do it in water, and every reason not to.
Three schools: classic, Wonka and no-contraction
Not everyone runs the same table, and the differences are worth understanding.
The classic table is what the generator above produces: fixed hold, shrinking rest. It is predictable, easy to log, and progresses cleanly — when the last round stops feeling hard, raise the hold by 5–10 seconds and start again.
The Wonka table keeps the rest fixed and lengthens the hold instead. Fans argue it trains a rising CO₂ load with a stable recovery, which feels more like real diving, where surface intervals are dictated by conditions rather than by a plan. It is harder to pace: the last rounds can turn into maximum attempts if you started too high.
The no-contraction approach flips the goal entirely. Rather than pushing deep into the struggle phase, you end every hold at the first contraction. Sessions are short, recovery is fast, and there is no accumulation of stress. It is the safest way to train frequently, and a sensible default if you train alone, are new to apnea, or are training in the days before a dive trip.
There is no single winning protocol, and that is not hand-waving — it is what the evidence shows. The meta-analysis by Massini and colleagues pooled 10 protocols from 8 studies with 138 participants and found a large effect on static apnea time (Hedges g = 1.30, 95 % CI 0.85–1.76, p < 0.01) across pure breath-hold work, physical training and cross-training alike, without being able to single out one ideal method (Journal of Sports Medicine and Physical Fitness, 2022; PROSPERO CRD42021230322). The practical reading: consistency and progression matter far more than which table you pick.
What the science says about how fast this works
Three data points are worth memorising:
| Study | Protocol | Result |
|---|---|---|
| O’Croinin et al. 2025 | 13 days, beginners | Total apnea time +70 %; struggle phase 18 → 45 s; diving bradycardia deepened from −10 to −20 bpm |
| Bourdas & Geladas 2024 | 14 days, dry apnea | Struggle phase +59.7 % on average (PMID 37797907) |
| Engan et al. 2013 | 14 days, untrained | Diving bradycardia arrived ~3 s earlier; desaturation slower — nadir SpO₂ 84 % vs 89 % at matched hold duration |
Two weeks. That is the timescale. Notice also what did not change in the Engan study: haematocrit and haemoglobin were unmoved after two weeks (Scandinavian Journal of Medicine & Science in Sports, 2013, 23:340–348). If someone tells you that a fortnight of breath holds gives you an EPO-like blood boost, the measurement says otherwise. The early gains are neural, autonomic and psychological.
There is a bonus finding in the O’Croinin data that has nothing to do with diving. The participants’ heart-rate response to a Stroop test — an unrelated cognitive stressor — was blunted after training, from 10 ± 7 to 6 ± 5 bpm (p = 0.009). Training your response to the urge to breathe appears to generalise to how you handle stress in general.
Safety rules that are not optional
- Never hyperventilate. More than 2–3 slow, relaxed breaths before a hold is already too many.
- Train dry. Sitting or lying on a soft surface, on land. Every table on this page is a dry protocol.
- Never in or near water alone. A hypoxic blackout is silent, gives no warning, and a face-down swimmer drowns in minutes.
- Skip the session if you are ill, dehydrated, sleep-deprived or hungover. Your numbers will be worse and your judgement will be too.
- Contractions are normal. Dizziness, tingling lips and tunnel vision are not. Stop and breathe.
How to progress over weeks
Keep it simple:
- Re-test your maximum every two weeks, not more often. Max attempts are stressful and tell you little day to day.
- Raise the hold by 5–10 seconds when the final round of the table stops feeling like a challenge.
- Alternate. CO₂ tables 3–4 days a week are plenty; add apnea walks for a movement-based CO₂ stimulus, and keep one day fully off.
- Log everything. The trend across weeks is the only number that matters.
If you do not know your starting point yet, run the free interactive baseline test first — the table above is only as good as the maximum you feed it.
Sources
- O’Croinin, O. et al. (2025). Applied Physiology, Nutrition, and Metabolism. DOI 10.1139/apnm-2025-0033
- Bourdas, D. I. & Geladas, N. D. (2024). Respiratory Physiology & Neurobiology 319:104168. PMID 37797907
- Engan, H. et al. (2013). Scandinavian Journal of Medicine & Science in Sports 23:340–348.
- Massini, D. A. et al. (2022). Journal of Sports Medicine and Physical Fitness 62. PROSPERO CRD42021230322.
Apnea Trainer is a breath-hold training app for iPhone and Apple Watch. It guides you through timed breathing cycles, builds progressive tables around your personal bests, and runs entirely on your wrist — live heart rate and haptic cues for every phase.
Keep reading
Find Your Breath-Hold Baseline: A Free Interactive Test (No Water Needed)
Before you train, measure. A guided dry test — paced breathing, one relaxed max hold — and an honest reading of what your result means.
What Science Says About Breath-Hold Training: 4 Studies in Plain English
Four peer-reviewed studies, no hype: what breath-hold training actually changes, how fast, and which popular claims the measurements refuse to back.