What Is CO₂ Tolerance? What Your Breath-Hold Time Really Means
There's a moment during a breath hold when everything changes.
At first, the absence of breathing feels almost uneventful. Then comes the pressure. The diaphragm begins to stir, your attention narrows, and taking a breath feels more like a demand. It would be reasonable to assume you're running out of oxygen, though that's not quite what's happening.
The growing urge to breathe is driven in large part by carbon dioxide accumulating in the blood. As CO₂ rises, the chemistry of the blood changes, and specialized receptors alert the brain that ventilation needs to increase. Oxygen is declining, too, but under ordinary breath-hold conditions, the discomfort that arrives first is often closely tied to rising carbon dioxide.
This distinction has helped turn “CO₂ tolerance” into a popular concept in breathwork and performance circles. The idea is straightforward: The better you can tolerate increasing levels of carbon dioxide, the longer you may remain comfortable without taking another breath. The physiology, however, is more complicated than the phrase suggests.
Carbon Dioxide Has Been Misunderstood
Oxygen gets most of the attention when we talk about breathing. We inhale it because our cells need it to produce energy. Carbon dioxide is often described as the exhaust; something the body simply needs to get rid of. Yet CO₂ plays an important role in regulating breathing itself.
The brain continuously monitors changes in carbon dioxide, oxygen and blood acidity, adjusting ventilation accordingly. Carbon dioxide is particularly influential and even relatively small increases can stimulate a stronger drive to breathe. This is why holding your breath can become deeply uncomfortable even while there is still substantial oxygen circulating in your blood. The sensation is real. Its interpretation is where things get interesting.
What Are You Actually “Tolerating”?
CO₂ tolerance is not a clinical measurement in the way blood pressure or VO₂ max is. It's better understood as shorthand for how an individual responds to the respiratory discomfort associated with rising carbon dioxide, and it's not purely physiological.
A breath hold depends on how much air was in your lungs when you started, your oxygen and carbon dioxide levels at the time, your metabolic rate and your body's sensitivity to changes in blood gases. It also depends on something decidedly harder to quantify: how willing you are to sit with discomfort. This means two people can stop a breath hold at very different times without one necessarily possessing a “better” respiratory system.
It also complicates one of breathwork's favorite metrics.
A Longer Breath Hold Isn't Necessarily a Better One
Tests such as the Body Oxygen Level Test, commonly called BOLT, typically ask someone to breathe normally, exhale and then measure the number of seconds before the first distinct desire to breathe. The resulting number is often presented as a window into breathing efficiency or CO₂ tolerance.
It is an appealing proposition. It is also easy to over-interpret.
In a 2024 study involving highly trained speed skaters, researchers found no significant relationship between BOLT scores and aerobic or anaerobic performance. The researchers also noted the limited scientific validation behind using the test to predict exercise performance.
Other research examining breath-hold measurements has similarly raised questions about whether a simple breath-hold time can reliably tell us what is happening with carbon dioxide in the body.
A person who can comfortably wait 40 seconds before breathing again is therefore not automatically healthier, fitter or better at breathing than someone who reaches that sensation after 20. The number may be less interesting than the response.
The Value Lives in Noticing What Happens Next
Spend enough time deliberately working with your breath and you begin noticing sensations that ordinarily pass beneath conscious awareness. The first tightening of the diaphragm. The impulse to gulp air. The temptation to tense the shoulders. The moment discomfort becomes urgency. Breathwork gives you an unusual opportunity to observe those responses without immediately obeying them.
This doesn't mean overriding the body's warning signals or treating increasingly long breath holds as an achievement, but rather developing familiarity with sensations that can otherwise feel alarming simply because they are unfamiliar.
Perhaps that is a more useful way to think about CO₂ tolerance. Not as another score to optimize, another metric to put on a dashboard or another test separating “good” breathers from bad ones. Instead, consider it a conversation with a regulatory system that has been quietly keeping you alive since the moment you were born.
The point isn't necessarily to hold your breath longer, it's to understand what your body is telling you when you finally want it back.
