What are CO2 Tables?

CO2 tables are a training method used by freedivers to increase their breathhold time.

There are many different kinds of tables that aim at training different aspects of the breathhold, such as O2 tables for oxygen training.

CO2 tables specifically target increasing tolerance to carbon dioxide, CO2, which is among other things responsible for triggering the urge to breathe. Carbon dioxide is commonly understood as a gas that is the byproduct of cellular respiration. We will talk more about how carbon dioxide is created in the body a little later, after we understand what a CO2 Table is.

Defining CO2 Tables

When we use the term CO2 Table what we are talking about is one full training session. A session consists of a number of rounds, and each round has two phases - a Rest Phase where you focus on relaxing your body into a calm and cyclical breathing cycle, and a Hold Phase where you hold your breath.

We call the first phase a rest phase because you are typically recovering from your previous breathhold. There are normally 8 rounds in a table, and if you were to write them down on paper your progress would look like a spreadsheet or table, with each line being a 'round'. Hence the word table.

How they Work

CO2 Tables are designed to gradually increase the level of carbon dioxide in your body. This happens by decreasing the length of the rest phase between for each round, usually by 15 seconds. In a standard table, your rest interval will start at 120 seconds, or 2 minutes, and decrease by 15 seconds each round. In this example, after 8 rounds you will have a 15 second rest phase between your last two holds. This decreasing of the rest interval gives your body less time to 'purge' the excess carbon dioxide which has accumulated as a result of your previous breathholds, thus increasing the level of carbon dioxide that you will start the next hold phase with.

Big Fat Disclaimer

Risks Involved

Holding your breath for an extended time can be dangerous. It can lead to lightheadedness, and faintness, as well to states of hypoxia, or low oxygen, or blackout. Blackouts can occur suddenly and without warning. They can be serious, especially if you are alone and without anyone to help you. You should always do any breathing exercises with a friend who also understands the associated risks.

You should never attempt any breathing exercises if you have any underlying medical condition. Before attempting any breathing exercises you should contact your primary medical care provider for their advice.

These breathing techniques are presented as educational, for those people seeking to understand these training methodologies. They are not intended nor recommended as any kind of treatment or therapy.

Breathing exercises should never be attempted in water. The risk of loss of motor control, blackout, and drowning are much greater and more severe in water.

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The Science of CO2

Cellular respiration is the process by which a cell takes oxygen and combines it with glucose in a chemical reaction that produces energy and byproducts. This energy produced is adenosine triphosphate, also called ATP, and the byproducts are carbon dioxide, CO2, and water. Carbon dioxide is then removed by the body through the lungs by exhalation.

Your body can be trained to tolerate greater levels of carbon dioxide through exposure. CO2 sensitivity is controlled by specialized neurons in the brain called the central chemoreceptors. The more time you spend with elevated levels of carbon dioxide in your body, the less sensitive the receptors will become, thus allowing a longer and more comfortable breathhold by delaying the onset of the urge to breathe.

Hemoglobin is a mocule that is responsible for carrying oxygen in the blood. It can carry up to four molecules of oxygen. The Khan Academy has done a really nice video explaining how hemoglobin moves O2 and CO2. The presence of carbon dioxide in the blood causes a chemical reaction with water, H2O, which produces bicarbonate, HCO3, and hydrogen ions, H+. These hydrogen ions have the effect of decreasing the pH, making the blood more acidic. During an extended breathhold as carbon dioxide levels increase in the blood and pH falls, hemoglobin's affinity for oxgen decreases. This causes oxygen to disassociate from their hemoglobin-carrier molecules. The result is that more oxygen becomes available as the level of CO2 in the blood increases.

CO2 and Freediving

In an extended underwater breathhold CO2 levels rise throughout the duration due to the fact that you cannot exhale. If you were to exhale to remove the carbon dioxide you would also exhale your remaining oxygen. These elevated levels of CO2 that develop throughout the breathhold are responsible for the urge to breathe, as well as the diaphragmatic contractions that occur in the later stages. As we have also illustrated, these elevated levels of CO2 cause more oxygen molecules to become available. This is known as the Bohr Effect.

Conversely, a decrease in CO2 levels causes the pH of the blood to increase. This causes oxygen molecules to bind more readily to hemoglobin, making less oxygen available. This explains why we never hyperventilate when freediving - hyperventilation purges the body of CO2, thus locking up oxygen and making less available for our breathhold.

Training Your CO2 Tolerance

Freedivers employ CO2 tables both dry and wet, but the purpose of this app is for dry training only. We will not discuss wet training and we strongly discourage you from attempting this unless with a trained and certified freediving instructor who can explain all aspects of the practice as well as monitor your progress. Do not ever use this app for the purposes of wet training.

Dry training is much safer and is recommended to always be done while laying in bed face upwards, or on a soft and comfortable surface. As with any breathwork exercises, there is always a risk of being faint or lightheaded afterwards, so take your time to return gently and rise slowly. There is always a risk of a blackout, which can occur without signs or warnings, which is why it is recommended to perform these training exercise laying down face up - if you do have a blackout, you will fall asleep and in a few moments return naturally.

A blackout occurs when the body's oxygen levels dip too low. In an attempt to conserve oxygen, the body reduces oxygen flow to the brain. Think of it as a low power mode. The body is in an energy conservation state. As soon as oxygen is available it will breathe and wake back up.

Why Train Your CO2 Tolerance

There are many reasons why you might want to train your body for a higher CO2 tolerance. With a greater tolerance to CO2 you delay the onset of the urge to breathe, and soften it when it does come. By regularly and safetly practicing tables you will learn to relax into the sensations that come with elevated CO2 levels. This will acclimizate you to many of the sensations and experiences that you may have while freediving, but with the benefit of not needing to be underwater to experience them.

Additionally, because carbon dioxide is a waste product of respiration, the more you exert your body underwater the more energy you will burn. This leads to a higher level of CO2 but without necessarily decreasing the amount of oxygen that you have used. In this way, CO2 training will make you more relaxed and comfortable in those elevated CO2 states where you also have plenty of available oxygen.

How To Use CO2 Tables

The recommended training schedule for CO2 Tables is no more than 3-4 times per week, with a rest day in between each session. It is best to do them on an empty stomach if possible, or at least 2-3 three hours after eating.

Start gently at first, with a hold interval of no more than half of your maximum breathhold. If in doubt, try a 60 second hold and see how a full session feels. You can gradually increase your hold times as the table becomes easier to complete.

The important part about CO2 Tables is that you should be able to finish them. It is not necessarily about a table duration being at the limit of what you are capable of, but more about them being challenging enough while still able to be completed. Remember, this is about increasing your CO2 tolerance gently over time.

Training with Tables

It is recommended to not combine training CO2 tables with O2 tables on the same day. Ideally you train them on different days. CO2 tables can be done every other day, usually with a rest day in between. They are generally quicker to complete than O2 tables, which can be long as well as exhausting.

And as always, Never Freedive Alone.