CO2 Tolerance Explained: What Changes During a Breath Hold

What CO2 tolerance means

During a breath hold, the body continues to use oxygen and produce carbon dioxide. Rising carbon dioxide contributes to the increasing urge to breathe. Falling oxygen, lung volume, respiratory-muscle activity, and other signals also affect when a person ends the hold.

“CO2 tolerance” is a practical training term for becoming more familiar with the discomfort associated with rising carbon dioxide. It is not proof that oxygen remains safe, and it is not a medical diagnosis.

This explanation follows a recent review of respiratory responses during maximal apnea.

Why the urge to breathe grows

Carbon dioxide dissolves in blood and affects acid-base balance. Chemoreceptors in and around the brainstem and major arteries respond to changes in carbon dioxide, oxygen, and pH. Their signals increase respiratory drive during a hold.

The common shortcut that “the urge is CO2, not oxygen” goes too far. Carbon dioxide explains much of the discomfort, but oxygen still falls during apnea. A person can reduce or delay the warning sensation without removing the risk of hypoxia.

Read the full breath-hold safety guide before practising.

What happens during training

A CO2 table normally keeps the hold duration steady while shortening the rest between rounds. This creates repeated exposure to breath-hold discomfort without requiring a maximum attempt every round.

People may become more familiar with the sensations and better at relaxing during them. Research does not support one universal improvement percentage, training frequency, or timetable for every user. Protocol, baseline ability, health, sleep, and training history all matter.

The CO2 table guide explains the session structure. The free CO2 table generator can calculate a starting table without requiring an in-water attempt.

CO2 tables are not O2 tables

CO2 tables usually shorten rest while keeping the hold steady. O2 tables usually keep rest steady while increasing the hold. The second pattern moves the later rounds closer to longer apnea and should not be treated as interchangeable with a CO2 table. See the full CO2 table vs O2 table comparison.

Both belong on dry land when you are training alone. Every in-water attempt requires a trained buddy.

What the research can and cannot say

Reviews of apnea training report possible physiological and sports-performance effects, but the protocols and findings vary. They do not justify promises that a particular table will add a fixed number of seconds, treat anxiety, improve sleep, or create a broad health benefit.

Useful starting points:

• Respiratory system responses to a maximal apnoea
• Effects of apnoea training on aerobic and anaerobic performance
• The application of breath-holding in sports