Breathwork covers a wide range of practices, from slow diaphragmatic breathing to structured breath-holds intended to build what enthusiasts call "CO2 tolerance." The underlying idea is that the urge to breathe is driven largely by rising carbon dioxide rather than falling oxygen, and that gently exposing yourself to mild air hunger can make that urge feel less urgent over time. This is physiologically real, but the language around it often outpaces the evidence.

The physiology in plain terms

Your drive to breathe is controlled by chemoreceptors that respond primarily to CO2 and blood pH. When you breathe slowly, CO2 levels rise slightly and the nervous system often shifts toward a calmer, more parasympathetic state. Studies on paced breathing near six breaths per minute report changes in heart-rate variability and, in some participants, reduced self-reported stress. These effects are modest and vary a lot between individuals.

"CO2 tolerance" is not a clinical measurement. Popular tests such as timing a comfortable exhale hold give a rough, highly variable snapshot of how comfortable you are with mild air hunger. They are influenced by relaxation, practice, and expectation, so treat them as a personal trend line, not a diagnostic number.

What people are actually measuring

  • Resting respiratory rate over days or weeks, which some wearables estimate during sleep.
  • Perceived calm or focus before and after a session, logged simply.
  • Heart-rate variability trends, keeping in mind these are noisy day to day.
Comfort with slow breathing is trainable. Extreme performance in breath-holds is a separate, riskier pursuit and is not required for the calming benefits most people are after.

Where caution is essential

The single most important safety point: never practice breath-holds or hyperventilation-style breathing in or near water, in a bath, or while driving. Fainting during these practices has caused drownings and injuries. People with cardiovascular conditions, a history of fainting, or who are pregnant should speak with a clinician before starting intense protocols. Gentle slow breathing is low-risk for most people, but aggressive routines are not.

If you experiment, favor short, seated sessions, stop if you feel dizzy, and prioritize consistency over intensity. Measurement should serve comfort and calm, not become another source of pressure.

For research and educational purposes only. This is not medical advice.