Carbon dioxide (CO2) has a bad rep. Classically we are taught that when you breathe, blood cells become oxygenated in the lungs, and then the oxygen is delivered to the organs and tissues, and carbon dioxide is then exhaled as a waste product. Although that is not wrong, it’s missing some key steps. So more oxygen (O2) is what we need right? Take a big deep breath, right? Well the issue is not the amount of oxygen in the blood, but delivering the oxygen to the tissues so it can be used. Turns out, carbon dioxide is a big factor, and our tolerance to carbon dioxide is crucial to athletic performance.

Oxygen levels (or saturation) in the blood doesn’t change much unless you have a breathing dysfunction like COPD. It might bounce between 95% and 98%, not a drastic change. So why do we feel the need to take deeper breaths during exercise? It may be due to a sensitivity to accumulation of CO2.

A quick lesson: Blood cells transport both O2 and CO2. When a blood cell is in an area where there is higher concentration of one of these molecules, it will pick up more of said molecule. In the lungs there is higher O2, so the blood picks up O2. In the muscle tissue, there is more CO2, so it picks up more CO2, and the O2 is offloaded to the muscles. As this happens, the CO2 concentration rises and there are receptors in the vessels that monitor this. If they are sensitive, they will stimulate the urge to breathe sooner and less O2 is offloaded to the muscle. So the key is to build your tolerance to CO2 accumulation so more O2 can be offloaded and used. This can have a dramatic effect on athletic performance.

There is a simple test you can perform to assess where you stand with CO2 tolerance; it’s called the BOLT test. Here is how it works:

Take a normal breath in through your nose and a normal exhale out through your nose.
Hold your nose to prevent air from entering your lungs.
Count the number of seconds until you feel the first definite desire to breathe, or the first stresses of your body urging you to breathe. This may include a need to swallow, or a constriction of the airways, abdomen, or throat.
Release your nose and breathe in through your nose. You should be able to resume normal, calm nasal breathing immediately. If you need to take a big mouth breath, you have held your breath for too long.

Average tends to be around 20 seconds. 25-40 seconds is ideal. If you are under that, you may be sensitive to CO2. The good news is you have room to grow. Lower scores are associated with anxiety and stress. This is a good litmus test and can be a snapshot of where you are currently. If work has been stressful, you can expect a lower score.

So how do we improve CO2 tolerance? Free divers have been exploring this for years and there is a dearth of knowledge out there with different techniques. But there are a few simple techniques. Nasal breathing is key, and you can perform light aerobic activity like walking with nose only breathing. Nasal breathing will increase nitrous oxide in your body which is a strong vasodilator. Nasal breathing also activates the diaphragm and slows down breathing. Faster breathing tends to lead to chest breathing.

You should also have a daily breathing routine. Take about 10 minutes a day and you can either work on cadence breathing, which means only 6 breaths per minute with longer exhales. Or you can put your hands on your chest and stomach, slow down the speed of air coming in your nose and out of your nose. When you feel the need to breath, take a slow breath in. This is a great exercise in the evening prior to bed.

You may feel warm after doing this. That is because CO2 is a vasodilator and allows oxygen to pass into the tissues easier. When we get short of breath and begin breathing too hard and get rid of too much CO2, the blood vessels constrict, and the bond between hemoglobin and oxygen is strengthened.

For more information, check out Patrick McKeown. He wrote, “The Oxygen Advantage”. If you aren’t a reader, you can check out this podcast:

SCP Podcast 168: Oxygen Advantage with Patrick Mckeown

CO2 Tolerance