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 How to Raise your CO2 levels

The Importance of Controling Your Breathing and How Improve CO2 Tolerance

breathing exercises and drills running pace training Oct 31, 2023

In the running world, we talk a lot about markers of physical and cardiovascular fitness, like lactate threshold and VO2 max. We purposefully train at specific paces to help move the needle here However,  there is one thing many of us are forgetting. 

Training CO2 Tolerance!

That urge to breathe and increase respiratory rate is not caused by a lack of oxygen. It’s actually a stress response to carbon dioxide (CO2). BUT carbon dioxide gets a bad rap, we actually need it around for a wide variety of important physiological regulations! For Example:

* Vital metabolic processes

* Brain synapse

* Immune system (antibody affinity)

* Glucose uptake

* Oxygen distribution

* Respiratory regulation


We need carbon dioxide to effectively use oxygen!  We can’t talk about improving VO2 max until we Raise your CO2 levels! AND training CO2 tolerance helps train your stress resilience. Most of us can benefit from (re)training this system.

Deconditioning combined with chronic stress (compounding from training and life) makes us hyper-sensitive to carbon dioxide, increasing our respiration rate while actually decreasing the ability to use oxygen for energy.


CO2 Tolerance Test:

So how do we know what our CO2 tolerance is?

Start with 2-3 second breaths through the nose only

  • Inhale 2-3 seconds
  • Exhale 2-3 seconds
  • Repeat 3 times

On the 4th breath cycle, take a big inhale through the nose

Exhale long, slow, and continuous through the nose. 

Time this exhale. Stop the timer when you:

  • Pause 
  • Panic
  • Swallow
  • Run out of air 

The long slow exhale is also a skill, so feel free to try a few times but don’t push past those stop signs!


< 25 seconds definitely could benefit from improvement in this area. Stress and difficulty recovering from said stress may be playing a factor in your performance/pain.


30-60 seconds is considered average. Improvements here could still make a significant impact on your performance and stress resilience. 


>60 seconds is considered elite level.


How to Improve CO2 Tolerance:


I’ve laid them out in steps but you can work all steps 1 through 3 concurrently.

Step 1:

Bring awareness to your breathing. Close your mouth and breathe through your nose as often as possible. Let your tongue rest gently on the roof of your mouth. Unclench your jaw.

Step 2: 

Work on rhythmic nasal-only breathing with two 5-minute sessions a day. 

Level 1: Breathe evenly in for 4 seconds and out for 4 seconds through your nose.

Level 2: Start adding a 4-second pause at the end of the inhale. It will become inhale for 4, pause for 4, exhale for 4 and repeat - all through the nose.

Level 3: In addition to the pause at the end of the inhale, add a 4-second pause at the end of the exhale. It will become inhale for 4, pause for 4, exhale for 4, pause for 4, and repeat - all through the nose.

Level 4: Start stretching the time to 5 seconds, then 6.

You can find some other variations of this idea here

Step 3:

Start with walking using relaxed, easy breathing through your nose only. It should be a slow rhythmic breath. From here you can work up to being able to run easily with this same relaxed nasal breathing. Staying truly relaxed nasal breathing only with this will keep you humble in your pace!

Once you can do this with at least walking for 30 minutes, you can move on to step 4.

*A bonus here combines a little bit of step 2 with step 3:
For 5 to 10 minutes of an easy walk use the following strategy. Take 5 nasal-only easy, rhythmic breaths. On the 5th breath let the exhale out as long and slow as you can. Count your steps with the exhale. Repeat with every 5 breaths. Tracking your steps per exhale is a great way to see your CO2 tolerance improve over time.*


Step 3b:

You might need to get good at the old snot rocket (farmer blow, bush hankie) here, especially as the days start to cool off!

Step 4:

Then start pushing the boundary of your nasal breathing using run: walk (or run: jog) intervals as follows:

On the run interval, breathe through your nose only but start pushing the pace to where the rhythm quickens and it becomes more forceful (and more uncomfortable).  Push to that boundary of where you are uncomfortable but still able to breathe through your nose only for the whole run interval. As you begin the walk interval, take 3-5 deep breaths, in through the nose and then out through the mouth to help clear out some of that built-up CO2. Then return to easy nasal-only breathing with the rest of the walk interval.

Start with a 1-minute run: 1 1-minute walk. In the beginning, you can increase the walk interval if needed to fully return to easy nasal breathing. Stay at this 1 minute run until you need 1 minute (or less) to recover.

Then start increasing the run interval until you can complete the whole 30-45 minute run at RPE 5/6 nasal breathing only. This is a serious challenge for some! It will likely take me a year of consistency to build up this tolerance!


I’ve been working on steps 1-3 for years but just started working on that little bonus piece from step 3 and the run: walk intervals from step 4 after my colleague Laci Weeks presented on this topic as part of my continuing education.

I’ve been loving the challenge of it, knowing I’m tapping into my physiology. I’m using run: walk intervals already because it’s part of my graded return while rehabbing my Achilles. Because of the potential with this strategy, I plan to continue building up my interval duration this way once per week even when my Achilles is back to tolerating longer straight running.


Join me in tapping into our physiology to unlock our full potential! Speaking of tapping into our physiology, I’ve got a little something for you! My 2-hour masterclass, called The Runner's Lab, where physics and biomechanics meet physiology to unlock your full potential.  It’s part PowerPoint-style education, part move and feel in your body 

Click here to learn more.


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