Understanding CO2 Tolerance & How To Train It

CO2 Tolerance

Let's talk CO2 tolerance

Breathwork and CO2 tolerance have started to gain attention not only for improving physical performance but also for enhancing mental improvement. I started my deep dive into sports science and high performance in 1999, and at that time, the emphasis was on increasing an athlete’s VO2 max – the maximal amount of oxygen that an individual can use. However, it was only in 2016 that CO2 tolerance and the benefits of CO2 became apparent.

Table of Contents

How is CO2 created?

Breathing out removes carbon dioxide (CO2) from the body. CO2 is a waste bi-product from the biochemical processes in our body, more specifically from cells creating energy from glucose and oxygen. However, it’s impossible to rid the body entirely of carbon dioxide, so a tiny amount remains in the body.

Why is CO2 good for you?

Carbon dioxide is not all harmful; the amount of it is. Very much like lactate acid, which was seen as a negative but turned out to be positive. The way that the body detects the amount of CO2 is vital for the respiratory system to work. Levels of CO2 inform the brain that there is a need to breathe in more air. Therefore, by using different breathing techniques during training and breath-hold techniques, we can increase tolerance to CO2.

co2 tolerance training

Why Improving CO2 Tolerance Is Important?

As mentioned, there are not only physical benefits to increasing CO2 tolerance; there is also a close correlation to anxiety and our ability to manage stress. In addition, increased tolerance allows us to have better control over the rate and depth of breathing.

It doesn’t matter if you are an elite athlete or high performing senior leader. Managing our state and nervous system is vital for health, well-being, and performance. If you want to know more about getting in the right state head over to Cognitive Athlete where we talk more about flow states and performance.

From a physical performance standpoint, increasing CO2 tolerance will help:

1. Dilating blood vessels, allowing more significant movement of blood.

2. Increase the amount of oxygen that moves from the haemoglobin to the cells. Known as the Bohr effect; CO2 in the body lowers the blood pH and increases the body’s ability to absorb oxygen. The body is susceptible to CO2; reducing these sensitives can help reduce our maximum minute ventilation.

3.Reducing the performance indicator helps with giving us more time to absorb the oxygen.
CO2 tolerance is linked to breathing mechanics. Improvement in CO2 tolerance helps our breathing pattern.

How Do I Start to Improve CO2 Tolerance?

Like with everything, it is essential to understand your starting point. There are several field-based tests (meaning we do not need special equipment).

The one test that I have found to be the best is from the guys at shift adapt.

How To Implement The CO2 Tolerance Test

You will need to repeat the test to see improvement, so therefore before beginning the test, make sure that you are comfortable and relaxed. Ideally, with both my athletes and cognitive athletes, we complete the test at the start of the day as a monitoring tool for readiness to train or work.

The Test

Take four full breaths (this needs to be completed nose-to-nose); these should be controlled breaths.

At the top of the 4th inhale (fill your lungs as much as possible), start a timer. 

Then exhale as slowly as possible (from your nose). 

Stretch out the exhale for as long as possible. It helps to close your eyes so that you can more effectively stay relaxed.

Stop the timer when your air runs out or you feel the need to inhale

The Results

>80 seconds: Elite
Advanced pulmonary adaptation, excellent breathing control, excellent stress control

60-80 seconds: Advanced
A healthy pulmonary system, reasonable breathing control, relatively reasonable stress control

40-60 seconds: Intermediate
Generally improves quickly with a focus on CO2 tolerance training

20-40 seconds: Average
Moderate to high stress/anxiety state, breathing mechanics need improvement

<20 seconds: Poor
Very high anxiety and stress sensitivity, mechanical restriction possible, insufficient pulmonary capacity

How To Improve Your CO2 Tolerance

We need to understand that there are two main ways to improve our tolerance. Passive and Active.


Sit down in a controlled, quiet area. Make sure that you are comfortable.

Exercise 1

Breath Counting

Input your results into the shiftadapt guide; you will get an idea of the speed and rate of breathing from there. For example:

Set a timer for 10 minutes breath-count guidelines.
Beginner: exhale slightly more extended than inhaling. Start with inhaling for 8 seconds and exhaling for 10 seconds.
Intermediate: exhale longer than inhale, and utilize a breath-hold at the top of each inhale. Starting with inhaling for 8 seconds, hold your breath for 4 seconds at the top of the inhale, and exhale for 10 seconds.
Advanced: exhale much longer than inhale, and utilize a breath-hold at the bottom of the exhale. Start with inhaling for 8 seconds, exhale for 10 seconds, and hold your breath for 4 seconds at the bottom of the exhale.

Exercise 2

Box Breathing

Close your eyes. Breathe in through your nostrils while counting to four slowly.
Hold your breath while counting slowly to four. Do not clamp your mouth or hold your nose. Avoid inhaling or exhaling for 4 seconds.
Begin to exhale for 4 seconds slowly.
Repeat steps 1 to 3 at least three times. Ideally, repeat the three steps for 4 minutes. Note this practice can be done multiple times during the day.


This type of breathwork is carried out during training.

Exercise 1


Breathing control via your nose during training. This encourages the body to offload the carbon dioxide and increase the amount of CO2 in the body. The more you do it, you will find that your pace will increase.

Exercise 2


Hold your breath for 5-15 seconds while performing a maximal effort. Once finished, complete 9, 7, 5 breathing. Nine breaths mouth-to-mouth, seven breaths nose-to-mouth and then five nose-to-nose breaths. Once completed, breathe nose-to-nose until starting the breath-hold again. This is an advanced technique; if you have any concerns, please speak to a doctor before attempting this


CO2 tolerance and training to improve it can have a significant performance-enhancing effect both physically and mentally. However, we need to spend time testing and completing both passive and active training methods to improve. If you would like to know more about building this type of training in to a performance programme, get in touch here. 

Or head over to either the Instagram page or YouTube channel for some examples of exercises to increase CO2 tolerance.

We hope that you have enjoyed this resource about CO2 tolerance. We use CO2 tolerance in all our programmes which you can check out here coaching.

If your interested to learn more I strong recommend the following books

Breath By James Nestor

Oxygen Advantage By Patrick McKeown


Director and Founder


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