CO2 table vs O2 table: what each one trains and which to choose

Ask ten freedivers what the difference between a CO2 table and an O2 table is and most will answer correctly in one sentence: one shortens the rest, the other lengthens the hold. Ask them why that matters, what each one does to the body and which one they should be doing this week, and the answers get much vaguer.

That vagueness has a cost. People who only do CO2 tables often plateau because they never practise staying relaxed near their maximum. People who jump straight to O2 tables spend their sessions fighting contractions they have never learned to handle, and push holds that sit uncomfortably close to their limit. This guide explains the mechanism behind each table, compares them on the same record, and gives you a simple way to combine them.

Two tables, two different stressors

During any breath-hold, two things happen at the same time. Your body keeps producing carbon dioxide, which builds up in the blood because you are no longer breathing it out. And it keeps using oxygen, so your oxygen saturation slowly declines. These two processes run in parallel, but they do not feel the same and they do not carry the same risk.

Rising CO₂ is what you feel. It is detected by chemoreceptors in the brainstem and in the carotid arteries, and it produces the urge to breathe and, later, the involuntary diaphragm contractions¹. Falling oxygen, on the other hand, is mostly silent. You do not get a reliable warning when saturation drops too low: the end point of severe hypoxia is a loss of consciousness, often without any sensation beforehand. That asymmetry is the key to understanding both tables.

Static apnea performance is usually described as depending on three things: how much oxygen you can store, how slowly you use it, and how well you tolerate the combination of high CO₂ and low O₂ near the end of the hold². The tables do not change your lung volume. They mostly work on the last two: relaxation and efficiency, and tolerance.

What a CO2 table trains

A CO2 table is a series of holds of identical length, typically around half of your personal best, separated by recoveries that get 15 seconds shorter every round. Because each recovery is too short to clear the CO₂ from the previous hold, every new hold starts with a slightly higher CO₂ level. The urge to breathe arrives earlier, contractions start sooner and the last rounds feel genuinely hard, even though the hold itself never gets longer.

The key point is that the hold stays far from your limit. Your oxygen saturation drops a little during each hold but never gets near critical levels. What you are training is your response to discomfort: staying loose while the urge builds, letting contractions come and go without bracing against them. Research on elite apneists shows a blunted ventilatory response to CO₂ compared with non-divers³. How much of that is trained and how much is innate is not fully settled, but anyone who has done a few weeks of CO2 tables knows the subjective change is real: the same hold simply feels calmer.

For the full construction rules and a calculator, see the complete CO2 table guide.

What an O2 table trains

An O2 table flips the structure. The recovery stays the same every round, often 2:00, which is long enough to clear most of the CO₂ between holds. The holds get longer each round, starting short and ending at a fraction of your record. Each hold therefore starts relatively fresh, but goes further into oxygen depletion than the one before.

The aim is to become comfortable in the last part of a long hold: the phase where contractions are strong, oxygen is lower and the temptation to stop is high. An O2 table lets you visit that territory several times in a controlled way, rather than only during record attempts. It is also commonly described as training hypoxic tolerance and the diving response, the reflex that slows the heart and constricts peripheral blood vessels to save oxygen⁴. There is, however, little controlled research comparing O2 tables with CO2 tables directly, so treat any precise claim about one being "better" with caution.

Side by side

Put the two tables on the same record and the contrast is clear. They are similar in total time, but the effort is distributed very differently: the CO2 table is hard because of density, the O2 table because of length.

Which one to choose

The honest answer is: both, in the right order and the right proportion. But your level and your goal decide where the weight goes.

You are starting out

If your record is recent, still moving fast, or you have never paid attention to your contractions, start with CO2 tables only. For most beginners the limit is not oxygen at all. It is the moment the urge to breathe turns into an alarm and the hold ends. CO2 tables teach exactly that skill, at an intensity that keeps you far from your maximum.

Your CO2 sessions feel controlled

Once you finish eight rounds of a CO2 table with contractions that are uncomfortable but manageable, add one O2 table a week. This is usually the point where people start to plateau, because they have learned to tolerate CO₂ but have never practised the end of a long hold. The O2 table fills that gap.

You are preparing a record attempt

In the weeks before an attempt, the O2 table becomes more relevant, because it rehearses the final phase you will meet during the attempt. Keep at least one CO2 session a week so you do not lose the base, and drop the intensity in the last days before the attempt. The guide on breaking 3 minutes in static describes a full attempt protocol.

The CO2 table teaches you to live with the urge to breathe. The O2 table teaches you to stay calm when the hold gets long. You need both, but not at the same time.

Combining them in a week

Adaptation happens during recovery, not during the session. Two hard tables on consecutive days usually means the second one is done tired, with worse relaxation and more risk of pushing through warning signs. A simple rule works well: never two hard table days in a row, and the O2 table on a day when you are rested.

On the light days, a few minutes of slow diaphragmatic breathing is useful but optional. Slow breathing is associated with lower arousal and better emotional regulation⁵, which is precisely the state you want to reach quickly before a hold. The breathing techniques guide covers the patterns that work best.

Different risk profiles

Both tables are low-risk when done dry and lying down, but they are not equally risky, and the difference follows directly from the physiology.

In a CO2 table, the holds are short. The discomfort is high, but your oxygen reserve is never seriously depleted. The main risk is psychological: pushing through every round at all costs, or shortening the recovery with fast, deep breathing.

In an O2 table, the last holds sit much closer to your limit, and low oxygen does not announce itself clearly. Fatigue across the session also matters: by round seven or eight you are more tired than during a fresh record attempt. Losing motor control or even consciousness is possible at the end of a long hold, which is why O2 tables call for extra caution and should never be pushed beyond the ceiling.

• Never hyperventilate before a hold. Fast, deep breathing lowers CO₂ and delays the urge to breathe without adding meaningful oxygen, which removes your warning signal⁶.
• Stop the session if you feel dizziness, tingling in the hands or lips, visual changes or anything unusual.
• Never in the bath, while driving, standing up or anywhere a loss of consciousness would be dangerous.
• See a doctor first if you have a heart or lung condition, epilepsy, or are pregnant.

Common mistakes

• Turning a CO2 table into an O2 table. Setting the fixed hold at 65 or 70 % of your record makes every round a struggle and mixes two stressors. Stay around 50 %.
• Using an O2 table as a record attempt. If the last hold is above 85 % of your record, the session no longer trains anything safely. Lower the ceiling.
• Starting with O2 tables. Without a CO2 base, the long holds are mostly spent fighting contractions rather than learning to relax through them.
• Training both every day. More sessions do not mean faster progress. Two CO2 tables and one O2 table a week are plenty for most people.
• Never updating the record. Both tables are built on your personal best. Retest it every few weeks and recalculate, otherwise the tables drift too easy or too hard.

Frequently asked questions