by David Deppeler, PT, DSc


There is a rapidly growing recognition of the value of improving CO2 tolerance for health, physical wellbeing, and athletic performance [1-9]. The CO2TT has been used by Free divers to help determine daily training tables. The CO2TT can be used as a test and an activity. The benefits of the activity are derived from first engaging (stimulating) the nervous and biochemical systems, then allowing them to quiet to a blissful place.




The Science

The science on this test is scarce. A recent study, yet to be published by researchers at USC and California State University – Fullerton, showed an inverse relationship between the CO2TT score and anxiety levels[10]. The authors of this study caution against this measure being used for anything more than an “in-this-moment” assessment.


The Procedure – 4 Breaths

  • Take three easy, calm breaths in and out through the nose
  • Take one full nasal inhale, filling the abdomen, upper chest, and lungs
  • Start a timer as you exhale through the nose, as slow as possible
  • Stop the timer when you run out of air, swallow, or feel that you must take a breath in

How we use the CO2TT

As a test, it can be used to help determine the appropriate BYT phase in which to play.

 Note: Your gut wisdom will also be a valuable guide and may override the results of this test. If a repeated test yields improved scores, this is a valuable activity for you, in this moment. Keep playing.

As an activity, the CO2TT can be done as either a stealth version of Over breathing and Recovery or the BYT Deep Meditation practice. If you feel better after doing this activity once, we recommend a few rounds. Follow up the activity with meditation, quiet reading, light yoga, or QI Gong. This will allow your nervous and biochemical system to realize the greatest rewards.



More Interpretations

SH//FT, an athlete training system report:

>80 seconds –> Elite. Reflects an advanced pulmonary adaptation, excellent motor control, and low arousal. You get to play big.
60-80 seconds –> Advanced. Reflects a healthy pulmonary system, good motor control, and relatively low arousal.
40-60 seconds –> Intermediate. This range generally improves quickly with a focus on CO2 tolerance training.
20-40 seconds –> Average. Moderate to high arousal state. Breathing mechanics may need improvement.
<20 seconds –> Awesome candidate for breathwork. Very high arousal and stress sensitivity.

Option: See the SH//FT calculator or recommended apnea and cadence breathing schedules


  1. McKeown, P., The oxygen advantage : the simple, scientifically proven breathing techniques for a healthier, slimmer, faster, and fitter you. First edition. ed. 2015, New York, NY: William Morrow, an imprint of HarperCollinsPublishers. xiv, 352 pages.
  2. Camacho-Cardenosa, A., et al., Repeated sprint in hypoxia as a time-metabolic efficient strategy to improve physical fitness of obese women. Eur J Appl Physiol, 2020. 120(5): p. 1051-1061.
  3. Galvin, H.M., et al., Repeated sprint training in normobaric hypoxia. Br J Sports Med, 2013. 47 Suppl 1: p. i74-9.
  4. Haider, T., et al., Interval hypoxic training improves autonomic cardiovascular and respiratory control in patients with mild chronic obstructive pulmonary disease. J Hypertens, 2009. 27(8): p. 1648-54.
  5. Hamlin, M.J., et al., Hypoxic Repeat Sprint Training Improves Rugby Player’s Repeated Sprint but Not Endurance Performance. Front Physiol, 2017. 8: p. 24.
  6. Hinghofer, H., Intermittent hypoxic training: Risks versus benefits. European journal of applied physiology, 2009. 108: p. 417.
  7. Serebrovska, T.V., Z.O. Serebrovska, and E. Egorov, Fitness and therapeutic potential of intermittent hypoxia training: a matter of dose. Fiziol Zh, 2016. 62(3): p. 78-91.
  8. Nestor, J., Breath : the new science of a lost art. 2020, New York: Riverhead Books. pages cm.
  9. Nestor, J., Deep : freediving, renegade science, and what the ocean tells us about ourselves. 2014, Boston: Houghton Mifflin Harcourt. 266 pages, 8 unnumbered pages of plates.
  10. Bentley, T. and B. Mackenzie CO2 Tolerance and Anxiety Study. 2020.