A CO₂ table keeps the hold constant and shrinks the rest. An O₂ table keeps the rest constant and grows the hold. That one structural swap changes everything: what accumulates in your blood, what the discomfort means, and how much can go wrong. CO₂ tables train your tolerance to the urge to breathe and are the safer, do-first tool. O₂ tables push you towards genuinely low oxygen — useful, but only with strict rules attached.
Key takeaways
- A CO₂ table keeps the hold constant at roughly 50–60 % of your maximum and shrinks the rest. An O₂ table keeps the rest constant at about 2:00 and grows the hold.
- CO₂ tables train tolerance to the urge to breathe and stay submaximal throughout. O₂ tables walk you toward real hypoxia with the alarm deliberately quiet, which is why their rules are stricter.
- Oxygen economy responds fast: in Engan et al. (2013), two weeks of training raised the lowest saturation at the same hold duration from 84 % to 89 %, and the diving bradycardia arrived about three seconds earlier.
- No protocol wins outright. The Massini meta-analysis found a large pooled effect on static apnea time (Hedges g = 1.30) but could not name a single best method, so progression matters more than table type.
Two tables, one goal, opposite levers
Both protocols exist for the same reason: a breath hold ends long before your body actually runs out of options, and structured repetition moves that endpoint. The pooled evidence backs that up — across the protocols Massini and colleagues collected, static apnea time improved with a large effect (Hedges g = 1.30; Journal of Sports Medicine and Physical Fitness, 2022; PROSPERO CRD42021230322). But the two tables pull on opposite levers.
A CO₂ table is a series of fixed, submaximal breath holds separated by rests that shrink round by round. Because carbon dioxide builds up faster than short pauses can clear it, every round starts with more CO₂ on board than the last. The holds themselves stay comfortably submaximal — the challenge is that the alarm rings earlier and louder each round. You are training your response to the urge to breathe, which, as the CO₂ tables guide explains in detail, is a carbon-dioxide signal rather than an oxygen one.
An O₂ table is the mirror image: the rest stays fixed and each hold gets longer. The rest is long enough to clear most of the CO₂ between rounds, so the alarm stays relatively quiet. Instead the holds stretch further and further, walking you step by step towards lower oxygen saturation. You are training your body’s economy at the hypoxic end of a hold, the part a CO₂ table barely touches.
For example, take a diver with a 3:00 maximum. Their CO₂ table is eight holds of about 1:30 each with the rest going 2:00 → 0:40. Their O₂ table is eight holds growing from roughly 0:45 to 1:34, with the rest parked at 2:00 the whole way. Same spreadsheet aesthetic, opposite physiology.
The mechanics, side by side
Here is the whole difference in one table:
| CO₂ table | O₂ table | |
|---|---|---|
| What stays constant | The hold (≈ 50–60 % of your max) | The rest (typically ~2:00) |
| What changes each round | Rest shrinks, e.g. 2:00 → 0:40 | Hold grows, e.g. 45 s → 1:34 |
| What accumulates | CO₂ — the urge to breathe arrives earlier every round | Progressively lower O₂ — late rounds approach your maximum |
| What it feels like | Uncomfortable early, never near your limit | Easy early, the last rounds feel like max attempts |
| Primary training target | Tolerance to the CO₂ alarm | Efficiency at low oxygen |
| Risk profile | Miserable but submaximal | Pushes towards real hypoxia — blackout territory |
Drag the slider through the eight rounds below, or press play, and watch the numbers move: on the CO₂ side (gold) the hold stays put while the rest shrinks; on the O₂ side (teal) the rest stays put while the hold grows.
CO₂ table vs O₂ table
Drag the slider through all eight rounds — or press play. Watch the numbers: on a CO₂ table the hold stays put while the rest shrinks; on an O₂ table the rest stays put while the hold grows.
| Round | CO₂ Rest | O₂ Hold |
|---|---|---|
| 1 | 2:00 | 0:45 |
| 2 | 1:50 | 0:50 |
| 3 | 1:35 | 1:00 |
| 4 | 1:25 | 1:05 |
| 5 | 1:10 | 1:10 |
| 6 | 1:00 | 1:15 |
| 7 | 0:50 | 1:25 |
| 8 | 0:35 | 1:30 |
The visual is the honest summary. A CO₂ table squeezes the recovery; an O₂ table stretches the work.
Why O₂ tables are the riskier tool
The urge to breathe is a CO₂ alarm — and an O₂ table is deliberately built to keep that alarm quiet while oxygen drops. That is the whole point of the protocol, and also the whole problem: the sensation that normally forces you to stop is the one signal an O₂ table suppresses by design. As the holds grow towards your maximum, you can approach genuinely low oxygen saturation with less subjective warning than the numbers deserve. Hypoxia is a state in which the oxygen reaching your blood and tissues falls below what they need to work properly, and in a breath hold it is what precedes a blackout. A blackout is a sudden loss of consciousness from that oxygen shortfall, and it announces itself with a few seconds of tunnel vision — or with nothing at all. For scale: the untrained participants in Engan et al. (2013) were already dipping to a lowest saturation of 84 % on holds that were not maximal (PMID 23802288).
So the rules for O₂ tables are stricter, and none of them are negotiable:
- Dry first, dry mostly. Sitting or lying on land. There is no training benefit that requires water.
- In water, only ever with a trained buddy watching you at arm’s length, one-up-one-down. Never in water alone — not once, not “just a short table”. A blackout in water is silent and a face-down swimmer drowns in minutes.
- No hyperventilation before any round. It silences the one alarm you have left.
- End the session early at dizziness, tingling, tunnel vision, or if a round felt closer to your limit than planned.
A CO₂ table is genuinely unpleasant — that is its job — but the holds sit at half your maximum, far from blackout territory. The discomfort is the training stimulus, not a danger sign. An O₂ table inverts that: it feels smoother while carrying more real risk. Do not let the comfort fool you.
What actually adapts — and an honest caveat
Does the low-oxygen end of a hold even respond to training? It does, and quickly. Engan and colleagues had untrained participants train for two weeks and found their oxygen use had become measurably more efficient. At the same hold duration, the lowest oxygen saturation improved from 84 % to 89 %. The heart-rate drop of the diving response also arrived roughly three seconds earlier (Scandinavian Journal of Medicine & Science in Sports, 2013, 23:340–348, PMID 23802288).
The methodology is the part worth reading twice. Participants were untrained, the programme ran two weeks, and saturation was compared at a fixed hold duration rather than at each person’s new maximum. That is why the 84 % → 89 % shift counts as oxygen economy rather than simply holding longer.
The honest caveat: that study shows apnea training improves oxygen economy — it does not show that O₂ tables specifically, or maximal holds, are required to get there. The adaptation showed up from a general two-week protocol. Which means a beginner loses very little by postponing O₂ tables, and gains a large margin of safety.
When to use which
The evidence gives you no excuse for dogma in either direction. The meta-analysis by Massini and colleagues pooled the available protocols and found a large overall effect on static apnea time (Hedges g = 1.30). What it could not do was crown a single best method (Journal of Sports Medicine and Physical Fitness, 2022; PROSPERO CRD42021230322). What works is structured, progressive training scaled to your level — not loyalty to one table type.
A sensible sequence looks like this:
- Measure first. Run the free baseline test — both tables are built from percentages of your max, and guessing defeats the purpose.
- CO₂ tables are the default, 3–4 sessions a week. They attack the phase where beginners actually break — the urge to breathe — and they do it safely.
- Add O₂ tables sparingly and late: once CO₂ tables feel routine, at most once or twice a week, dry, and never on the same day as a CO₂ session or a max attempt.
- Let the plan adapt. Re-test every two weeks and rebuild both tables from the new number, exactly as the CO₂ tables guide describes.
- Integrate Pranayama and Apnea modes into your training sessions.
For instance, if a fortnight of work moves your baseline from 2:30 to 3:00, your CO₂-table hold goes from about 1:15 to 1:30 and every rest in the ladder recalculates from there. The O₂ table rescales the same way: the fixed 2:00 rest stays, and the top round moves from roughly 1:15 to 1:30.
Sources
- Engan, H. et al. (2013). Scandinavian Journal of Medicine & Science in Sports 23:340–348. PMID 23802288
- 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
- Massini, D. A. et al. (2022). Journal of Sports Medicine and Physical Fitness 62. PROSPERO CRD42021230322.
Every source above links to its abstract, and each link was opened and re-read against this text during the last editorial pass. Where a study measured trained athletes rather than beginners, for example, the article says so instead of generalising.
Written by Jan Luther — keen amateur freediver, and the developer behind Apnea Trainer since 2010. About this blog explains how sources are chosen; the imprint has the contact details for corrections. All articles
Frequently asked
What is the difference between an O₂ table and a CO₂ table?
A CO₂ table keeps the hold constant, at roughly 50–60 % of your maximum, and shrinks the rest each round, so carbon dioxide accumulates and the urge to breathe arrives earlier every time. An O₂ table does the opposite. The rest stays fixed at around two minutes while the holds grow longer, walking you toward genuinely low oxygen. CO₂ tables train tolerance to the urge to breathe; O₂ tables train efficiency at low oxygen.
Are O₂ tables dangerous?
They carry more real risk than CO₂ tables. The long rests keep the CO₂ alarm quiet while oxygen falls, so the sensation that normally forces you to stop is the one signal the protocol suppresses by design. Do them dry, never in water without a trained buddy at arm's length, never after hyperventilating, and end the session at any dizziness or tunnel vision.
Should beginners do O₂ tables?
No, start with CO₂ tables. Engan et al. (2013) found that untrained participants improved their oxygen economy from a general two-week apnea protocol, with the lowest saturation at the same hold duration rising from 84 % to 89 %. That adaptation did not require maximal holds, so a beginner loses very little by postponing O₂ tables and gains a large margin of safety.
How often should you do CO₂ and O₂ tables?
CO₂ tables three to four times a week are the default. Add O₂ tables only once CO₂ tables feel routine, at most once or twice a week, dry, and never on the same day as a CO₂ session or a maximum attempt. Re-test your baseline every two weeks and rebuild both tables from the new number.
Apnea Trainer is a breath-hold training app for iPhone, Apple Watch and Android. It guides you through timed breathing cycles and builds progressive tables around your personal bests. On Apple Watch the whole session runs on your wrist — live heart rate and haptic cues for every phase.
Keep reading
CO₂ Tables Explained: The Science of Training Your Urge to Breathe
The urge to breathe is a CO₂ alarm, not an oxygen alarm. Here is how CO₂ tables train that alarm — and how to build one from your current max.
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.