Every freediver knows the moment. The first stretch of a hold is easy, almost pleasant. Then your throat tightens, a small voice starts asking for air, and your diaphragm begins to twitch on its own. Most beginners stop right there, convinced they are running out of oxygen. In reality, their saturation is still high.
That alarm comes from carbon dioxide, and it is precisely what a CO2 table teaches you to handle: staying calm and loose while CO₂ climbs. This guide explains the mechanism, gives you a clear way to build your table along with a calculator, then walks through a typical session and a progression over several weeks.
Why CO₂ is what makes you stop
During a breath-hold, your body keeps consuming oxygen and producing CO₂. With no breathing to clear it, that CO₂ builds up in the blood, where it forms carbonic acid and lowers the pH. Specialised sensors called chemoreceptors, in the brainstem and in the carotid arteries, pick up this acidification and drive the command to breathe.
Research describes a static breath-hold in two distinct phases1. First comes an easy phase with no real discomfort. Then, from the physiological breaking point, meaning the first involuntary contraction of the diaphragm, a struggle phase begins, in which contractions repeat and grow stronger. In trained freedivers, this second phase can make up a large share of the total hold2.
Several studies show that elite breath-hold divers have a blunted ventilatory response to CO₂ compared with non-divers2. Part of that tolerance is physiological, and part of it is clearly learned: knowing that a contraction is not a danger signal, and staying relaxed while it passes. Contractions even do something useful, temporarily improving blood flow and oxygen delivery to the brain3.
A CO2 table does not give you more oxygen. It teaches you not to panic when CO₂ comes knocking.
What exactly is a CO2 table?
A CO2 table is a series of breath-holds in which the hold time stays the same while the recovery between holds gets shorter each round. Because you have less and less time to clear the CO₂ you have accumulated, each hold starts with slightly more CO₂ on board than the last. Contractions arrive earlier, and that is exactly what creates the training stimulus.
An O2 table works the other way round: recovery stays constant and the holds get longer, which trains hypoxia tolerance and takes you closer to your maximum. The two complement each other, but the CO2 table is the better place to start, because the hold stays well away from your limit. Our side-by-side comparison of the two tables will help you choose.
Build your table with four rules
Everything starts from your dry static personal best, measured recently and in good conditions. If you don't know it, measure it first: lying down, after a few minutes of calm breathing.
- Fixed hold: about 50% of your personal bestWith a 3:00 best, every hold is 1:30. Round to the nearest 15 seconds.
- Starting recovery: about two thirds of your personal bestWith a 3:00 best, start with 2:00 of recovery. Never go below 2:00 for the first round.
- Reduction: 15 seconds per roundThis is the standard step. It raises CO₂ gradually without turning the end of the session into a survival test.
- 8 roundsA session then takes 20 to 25 minutes. Start with 6 rounds if the last ones are too hard.
Same rules as the adaptive tables in the Anima Apnea app: hold at 50% of your best and starting recovery at two thirds but never under 2:00, rounded to 15 s, then −15 s per round (15 s minimum).
| Round | Recovery | Hold | What to expect |
|---|---|---|---|
| 1 | 2:00 | 1:30 | Comfortable |
| 2 | 1:45 | 1:30 | Comfortable |
| 3 | 1:30 | 1:30 | First sensations |
| 4 | 1:15 | 1:30 | Urge to breathe near the end |
| 5 | 1:00 | 1:30 | First contractions |
| 6 | 0:45 | 1:30 | Contractions come earlier |
| 7 | 0:30 | 1:30 | Clear struggle phase |
| 8 | 0:15 | 1:30 | Demanding, stay loose |
| Total time | 21 min | ||
If you get through all 8 rounds without a single contraction, add 15 seconds to the hold. If you have to cut a hold short before round 5, take 15 seconds off. The right setting gives you a hard but controlled finish.
How to run a good session
The table itself is only half the job. How you breathe and how you let go of tension, both between and during the holds, changes the outcome completely.
- Lie down, dryOn a bed or sofa, in a quiet room. Phone in airplane mode, apart from your timer.
- Two to three minutes of calm breathingSlow belly breathing, with the exhale longer than the inhale. No big repeated breaths: you are not trying to "charge up" anything.
- One full but unforced inhaleFill the belly, then the chest, to roughly 85–90% of your capacity. A forced inhale creates tension, and tension costs oxygen.
- During the hold: active relaxationScan your body from jaw to feet. When contractions arrive, let them roll through without fighting them. They are waves, not danger signals.
- Afterwards: recovery breathsExhale only slightly, then inhale quickly and deeply, hold for a second or two with gentle pressure, and repeat two or three times. Stay lying down for the whole recovery.
The timing of your first contraction is one of the best markers of progress in CO₂ tolerance. If, on the same table, it shows up later from one week to the next, you are improving, even if your personal best hasn't moved yet.
Frequency and progression over several weeks
Adaptation happens during recovery, not during the session. Doing tables every day piles on fatigue without speeding anything up. Here is a typical week that leaves at least 48 hours between two CO2 sessions.
| Weeks | Goal | Adjustment |
|---|---|---|
| 1–2 | Learn the session | Base table, 6 to 8 rounds |
| 3–4 | Build density | +15 s on the hold once all 8 rounds feel controlled |
| 5 | Deload | A single CO2 session, with longer recoveries |
| 6 | Measure | New personal best test, then recalculate the table |
Five mistakes that undermine your tables
- Hyperventilating before the hold. Fast, heavy breathing artificially lowers CO₂: the urge to breathe arrives later, but your oxygen barely rises. You switch off your alarm without adding any reserve, which is the classic recipe for blackout4.
- Aiming too high. A hold at 70% of your best turns a CO2 table into an endurance ordeal. Stay around 50%.
- Tensing up during contractions. Clenching your jaw or fists burns oxygen and amplifies the discomfort.
- Neglecting recovery. The last recoveries are short, so every breath counts. Use recovery breathing.
- Training every day. More is not better. Two or three well-run sessions beat seven sloppy ones.
The non-negotiable safety rules
A hypoxic blackout can happen with no warning at all. In the water, it can be fatal within moments. Solo tables are done dry and lying down, full stop. In the water, only with a trained buddy who watches you the entire time.
- Never hyperventilate before a hold.
- Stop the session if you feel dizzy, notice tingling in your hands or lips, get visual disturbances or any unusual sensation.
- Cutting a hold short is not a failure. Coming up at 1:20 instead of 1:30 is part of training intelligently.
- Never while driving, in the bath, standing up, or anywhere a loss of consciousness would be dangerous.
- Get medical advice if you have a heart or lung condition, epilepsy, or are pregnant.
Frequently asked questions
How many CO2 tables should I do per week?
Two or three sessions a week, with at least 48 hours between them. Beyond that, fatigue builds up without any extra benefit.
Should I do CO2 tables dry or in the water?
Dry, lying on a bed or sofa, whenever you train on your own. In the water, only with a trained buddy who watches you the whole time.
What if I can't finish one of the holds?
End the hold, breathe, and finish the table if you feel fine. If it happens in the first few rounds, cut the hold time by 15 seconds next session.
How long before I see progress?
Most people notice a difference in comfort within two to four weeks of regular practice: contractions start later and feel easier to handle. Gains in maximum breath-hold vary a lot from person to person.
Anima Apnea builds your tables from your personal best, guides you round by round with sound and haptic cues, and lets you mark each contraction with a single tap so you can track your progress.
Sources
- Lindholm P, Lundgren CEG. The physiology and pathophysiology of human breath-hold diving. Journal of Applied Physiology, 2009.
- Bain AR, Drvis I, Dujic Z, MacLeod DB, Ainslie PN. Physiology of static breath holding in elite apneists. Experimental Physiology, 2018.
- Dujic Z et al. Involuntary breathing movements improve cerebral oxygenation during apnea struggle phase in elite divers. Journal of Applied Physiology, 2009.
- Craig AB Jr. Causes of loss of consciousness during underwater swimming. Journal of Applied Physiology, 1961.