CO2 tolerance and breath-hold tests: what they show

What “CO2 tolerance” means

Sensors in your brain and near large blood vessels track carbon dioxide and oxygen, and your brain adjusts each breath to match. The level of carbon dioxide in the blood is held within a few points of a set value. Small changes go unnoticed; a large, fast rise causes breathlessness, a strong urge to breathe and even panic. More in how breathing works.

“CO2 tolerance” is a breathwork term, not a medical one. Scientists talk about chemosensitivity: how strongly your breathing responds to carbon dioxide. It differs between people, and the popular idea is that you can measure it, and train it, with breath holds.

What happens during a breath hold

A breath hold goes through phases:

1. The easy phase. Nothing much happens; you just do not breathe.
2. The physiological break point. Small, involuntary movements of the chest and diaphragm begin. Your breathing rhythm has not stopped, it was only being held back. These movements depend mainly on carbon dioxide.
3. The struggle phase. The urge grows until you breathe.

How long a hold lasts varies a lot between people. It gets longer with more oxygen, less carbon dioxide or bigger lungs, and shorter with effort. It is not even purely physical: in one study, changing how 26 people perceived time changed how long they held their breath. Researchers still have no simple explanation for exactly when a breath hold breaks.

Breath-hold tests: BOLT and the control pause

The best-known test is BOLT, from the breathing coach Patrick McKeown, based on Konstantin Buteyko’s “control pause”: after a normal breath out, pinch your nose and time how long until the first clear urge to breathe. It is often said to show how sensitive you are to carbon dioxide.

The evidence does not back that up:

• BOLT has not been scientifically validated. In 49 highly trained speed skaters, it had no link at all with any measure of fitness.
• When researchers tested Buteyko’s claims in 83 adults, breath-hold time and carbon dioxide levels were related in the opposite direction to what he predicted.
• No study we found shows that BOLT or the control pause measures CO2 sensitivity, or how reliably it gives the same result twice.

So a breath-hold time is a mix of chemistry, lung size, effort and mindset. At most it can track your own practice over time; it is not a health score.

The Stanford breathing study (Balban 2023) used a different “CO2 tolerance test”: breathe in fully, then breathe out as slowly as you can, and time it. It was used only to choose the pace of box breathing (3-4 seconds per step for up to 20 seconds, 5-6 seconds for 25 to 45 seconds, 8-10 seconds for 50 seconds or more) and was not itself tested.

Buteyko and asthma

The Buteyko method was developed to reduce over-breathing in asthma. The UK asthma guideline (SIGN 158) says breathing exercise programmes, including physiotherapy-led breathing retraining and Buteyko, give adults modest improvements in symptoms and quality of life and reduce the need for reliever inhalers, with little effect on lung function. It recommends offering them alongside medicine, not instead of it. A 2020 Cochrane review of 22 studies found moderate-certainty evidence that breathing exercises improve quality of life in asthma.

CO2 and panic

People with panic disorder are, on average, more sensitive to carbon dioxide: breathing air with a high dose of CO2 reliably triggers panic symptoms in them, while healthy people react only at higher doses. In one study, a simple breath hold triggered a panic attack in 46% of people with panic disorder and in 4% of healthy volunteers. One influential theory describes this as a false suffocation alarm, but it is still debated.

If you have panic attacks, skip breath-hold tests and choose slow, gentle breathing instead. See breathing for anxiety and panic.

Safety

Never hyperventilate before swimming underwater, and never hold your breath underwater for long: you can pass out and drown without warning. Practise breath holds only sitting or lying down on dry land, and keep them comfortable. More on why in Wim Hof breathing.