CO2 Table vs O2 Table: What's the Difference?

If you've spent any time in the freediving world, you've almost certainly come across the terms CO2 table and O2 table. Both are structured breath-hold training protocols used by freedivers worldwide — from pool beginners to competitive depth divers — to improve their ability to stay underwater longer. But despite being mentioned in the same breath (pun intended), these two methods target completely different physiological systems and serve distinct purposes in your training. Mixing them up — or worse, skipping one entirely — can significantly slow your progress.

In this article, we'll break down the science behind each type of table, walk through real examples, compare them side by side, and give you a clear framework for incorporating both into your weekly training.

The Physiology of Breath Holding

To understand why CO2 and O2 tables work so differently, you first need to understand what actually happens in your body during a breath hold.

The urge to breathe is not triggered by low oxygen — it's triggered by rising carbon dioxide (CO2). As your muscles consume oxygen during a dive, CO2 builds up in your blood. Your brain's chemoreceptors detect this rise in CO2 and send an increasingly urgent signal to your diaphragm: breathe now. This is what creates those uncomfortable contractions during a breath hold. This CO2-driven urge is a warning system — but it activates long before your oxygen levels are actually dangerous.

Your actual physiological limit, however, is set by oxygen depletion. When blood oxygen drops below a critical threshold — roughly 50% SpO2 for untrained individuals — loss of consciousness (shallow water blackout) can occur without any warning. This is why freediving without a buddy is always dangerous.

These two systems — CO2 tolerance and O2 efficiency — can be trained independently. That's precisely what CO2 and O2 tables do.

What Is a CO2 Table?

A CO2 table is designed to raise your tolerance to carbon dioxide accumulation. The protocol is simple: you perform a series of breath holds at a fixed duration, but the rest periods between holds get progressively shorter. Because each rest is shorter, your CO2 level never fully drops back to baseline before the next hold begins. Your body gradually adapts to functioning — and tolerating the urge to breathe — at higher CO2 levels.

The practical result? Your contractions start later, feel less overwhelming, and your mental resistance to the urge to breathe improves dramatically. For beginners, this is often the single most impactful training they can do. The fear and discomfort of contractions is frequently the biggest limiting factor — not actual oxygen depletion.

CO2 tables are typically done in a pool (static apnea) or on a training mat. The hold time is set at roughly 60–75% of your personal best, making the session demanding but repeatable.

Notice that the hold time is constant throughout. What changes is your recovery window. By round 8, you're heading into a full 1:30 hold with only 30 seconds of rest — a genuine challenge that forces your body to adapt.

What Is an O2 Table?

An O2 table flips the formula. Here, the rest period is fixed and the hold durations get progressively longer. The goal is no longer CO2 tolerance — it's training your body to extract more oxygen per breath and to push your actual physiological breath-hold limit further out.

With each successive hold, you begin from a slightly lower oxygen baseline. Your body learns to cope with lower arterial oxygen saturation, your heart rate response becomes more efficient (the mammalian diving reflex deepens), and your spleen — which acts as a natural oxygen reservoir in trained freedivers — learns to release more red blood cells on demand.

O2 tables are more physiologically demanding and carry a greater safety risk than CO2 tables. They should only be practiced with a trained buddy present, ideally in a pool environment where you can be immediately rescued in case of a hypoxic incident.

Each hold is a little longer than the last, while the rest stays constant. By the final rounds, you're approaching — or even slightly exceeding — holds you could typically perform fresh. This is what makes O2 tables powerful, and why they require caution and supervision.

Key Differences at a Glance

Which Should You Train First?

The answer is clear: start with CO2 tables. For most beginners, the limiting factor isn't a lack of oxygen — it's the overwhelming discomfort of CO2 build-up. The contractions, the panic, the mental resistance to staying still when every instinct screams "breathe!" — these are CO2-driven responses.

By building CO2 tolerance first, you develop the mental composure and physical familiarity with discomfort that make all subsequent training safer and more productive. You'll also discover that your personal best improves significantly just from learning to stay calm during contractions — without ever needing to push your actual oxygen limits.

O2 tables become valuable once you've built a solid CO2 base — typically after several months of consistent training. At this point, CO2 tolerance is no longer your main limiter, and you need to address the oxygen side of the equation to keep progressing.

How to Combine Both Tables in a Training Week

For intermediate freedivers training 3 days per week, a well-structured weekly plan might look like this:

• Monday: CO2 table (static apnea, pool or dry) — focus on relaxation during contractions
• Wednesday: O2 table (pool only, buddy required) — push hold duration progressively
• Thursday or Friday: CO2 table variation or dynamic apnea training

Never do O2 tables two days in a row. Your body needs recovery time between sessions that push oxygen limits. CO2 tables are less taxing and can be done more frequently, but even these benefit from at least a day of rest between sessions.

Avoid doing an O2 table the day after any training that left you physically or mentally exhausted. Fatigue dramatically increases the risk of a hypoxic event. When in doubt, do a CO2 table instead — it's always the safer choice.

Tracking Both with Anima Apnea

One of the most common questions from freedivers is: "How do I know what hold time and rest time to use for my level?" The answer depends entirely on your current personal best — and it changes as you improve.

Anima Apnea is a freediving training app that automatically generates both CO2 and O2 tables calibrated to your personal record. As your PR improves, the tables adapt in real time. You don't need to manually calculate percentages or wonder whether you're training at the right intensity — the app handles all of it.

Beyond table generation, Anima Apnea tracks your session history, visualizes your progress over time, and provides structured training plans that balance CO2 and O2 work across your week. Whether you're a beginner just starting out with your first CO2 tables or an experienced diver pushing 4-minute static apnea, the app scales to your level.

The combination of understanding the science, practicing consistently, and using intelligent tools to guide your training is what separates freedivers who plateau from those who keep progressing year after year. Start with CO2, layer in O2 when you're ready, and let the data guide you forward.