You keep sighing. A yawn doesn’t quite land, and you struggle to finish it. You take a deep breath, it stops short of full, so you take another. Your watch says your oxygen is 98%, which doesn’t help at all.
The instinct is to breathe more, take bigger breaths, and inhale more air. And if you’ve searched for CO2 tolerance, you’ve probably found a score to chase and a set of breath-hold drills to chase it with.
There’s a simpler account. The urge to breathe responds mainly to carbon dioxide, and your oxygen level has surprisingly little to do with it. In a body that’s anxious or has been breathing a little too much for a long time, that urge tends to arrive early and strongly. That’s why more air rarely satisfies and why slower, softer breathing, when practised often, usually helps.
What does CO2 tolerance mean?
CO2 tolerance is how much carbon dioxide your body lets build up before it pushes you to breathe. Low tolerance means the urge comes early and strongly. Researchers call this high CO2 sensitivity, or chemosensitivity. It’s a word from breathwork and coaching, and it isn’t a diagnosis.
Carbon dioxide is what your cells make as they use oxygen. Your blood carries it to your lungs and you breathe it out. Between breaths it builds up again, and when it climbs past a certain point, your body tells you to breathe. How early and how insistently that signal arrives is what people mean by CO2 tolerance.
You’ll meet three words for overlapping things:
| The word you’ll see | What it means | Where it’s used |
|---|---|---|
| CO2 tolerance | how much carbon dioxide you can sit with before the urge to breathe | breathwork, coaching, freediving |
| CO2 sensitivity (chemosensitivity) | how strongly your breathing responds as carbon dioxide rises | research |
| Air hunger | the uncomfortable urge to breathe itself | medicine and research |
Low tolerance and high sensitivity describe the same thing from two sides. This article uses “tolerance”.
Why does carbon dioxide drive the urge to breathe more than oxygen does?
Your body watches carbon dioxide closely because it changes fast. Sensors in your brainstem, with smaller ones in the arteries of your neck, track the carbon dioxide in your blood and the acidity it creates. They adjust your breathing breath by breath (opens in new tab).
Oxygen has its own sensors, but they act as a backup. In laboratory studies, low oxygen on its own was a weak trigger (opens in new tab): people given too little oxygen often lost consciousness before they felt any air hunger, while a modest rise in carbon dioxide made the urge to breathe climb steeply.
So the feeling of not having enough air is mostly a carbon dioxide signal. That’s how you can feel breathless with a reading of 98%. The reading is accurate. It just measures something the alarm doesn’t listen to much.
Many people know a larger version of this. You can understand perfectly well that your oxygen is fine and still feel there isn’t enough air. The understanding happens in your head. The feeling comes from your brainstem, which doesn’t take instructions from reasoning. If the feeling is going to change, the change has to come through the body, and the breath is the most direct way in.
What is the Bohr effect and how does it relate to CO2 tolerance?
Haemoglobin, the protein in red blood cells that carries oxygen, holds oxygen less tightly where carbon dioxide is higher or the blood is more acidic. That’s the Bohr effect (opens in new tab). It’s useful because working muscle makes more carbon dioxide, so oxygen is released where it’s needed most.
You’ll see it invoked often in breathing circles, sometimes accurately and sometimes not.
What holds up in research? When over-breathing lowers carbon dioxide, the effect runs the other way, and haemoglobin holds on to oxygen a little more tightly, at least for a while. A second effect is better documented and easier to feel: low carbon dioxide narrows the blood vessels in the brain (opens in new tab) and reduces blood flow there. That’s why over-breathing makes you light-headed and tingly. The breathwork safety article walks through what over-breathing does in the body, step by step.
What’s stretched? Some breathing books leap from there to the idea that everyday over-breathers are slowly starving their cells of oxygen. I haven’t found good evidence for that in healthy people. The short-term effects are real, but lasting oxygen starvation has never been shown.
You’ll also read that carbon dioxide helps your body “absorb” oxygen. It works the other way round. Carbon dioxide helps haemoglobin release oxygen to the tissues.
What are the signs of low CO2 tolerance?
Low CO2 tolerance usually shows up as a breathing habit. Some of the signs are frequent sighing and yawning, breaths that never feel full, and breathlessness that doesn’t match what you’re doing. It points to a sensitive alarm or to a habit of breathing more than you need. It isn’t a diagnosis.
Common signs:
- Frequent sighing and yawning
- Breaths that never feel full or satisfying
- Breathing through your mouth or high in your chest while resting
- Feeling breathless when you talk or after very little effort
- Light-headedness or tingling when you’re stressed
- A short, comfortable breath-hold (a low BOLT score)
Researchers describe a pattern called dysfunctional breathing (opens in new tab), with symptoms such as air hunger, dizziness, and palpitations. They note that it’s often missed or mistaken for asthma. An NHS hospital leaflet describes the symptoms of over-breathing as very unpleasant but not harmful (opens in new tab) and suggests slow, regular breathing at about 8 to 10 breaths a minute when they come on.
These signs overlap with medical conditions. If breathlessness is new, getting worse, or comes with chest pain, fainting, or a racing heart, see a doctor before you work on your breathing.
Are some people more sensitive to CO2?
Yes. People vary in how they react to rising carbon dioxide, and anxiety is a large part of it. People prone to panic tend to meet that rise with more alarm, and anxious breathing tends to keep carbon dioxide low, which can make an ordinary rise feel like an emergency.
In a well-known study, breathing air with 5% carbon dioxide brought on a panic attack in about half of people with panic disorder and in about 9% of people without it (opens in new tab). Their breathing reflex responded much like everyone else’s. What differed was the fear. And the people with panic disorder who panicked had lower carbon dioxide before the test began, a sign they were already breathing more than they needed.
The reaction isn’t limited to panic disorder. Extra carbon dioxide reliably raises anxiety and heart rate in healthy volunteers (opens in new tab), which is why researchers use it to study anxiety in the lab.
So can anxiety lower your carbon dioxide? It can. Anxiety tends to speed up and deepen breathing, which lowers carbon dioxide. A body used to running low can then treat a normal rise as a threat, and the urge to breathe arrives with a jolt of alarm. That loop fits the research well, though it hasn’t been proven as a cause.
At the other end of the range, trained breath-hold divers tend to show a lower breathing response to carbon dioxide (opens in new tab). It isn’t clear how much of that comes from training and how much from who takes up the sport.
How do you test your CO2 tolerance?
Two home tests are popular: the BOLT score, a short, comfortable breath-hold after a normal exhale, and the CO2 tolerance test, a slow exhale after a full breath in. Both are rough personal benchmarks. Neither has been validated as a medical measure.
| BOLT score | CO2 tolerance test | |
|---|---|---|
| What you do | After a normal breath out, pinch your nose and time how long until the first definite urge to breathe | After a full breath in, breathe out as slowly as you can and time it |
| Breath-hold? | A short, comfortable one | None |
| Where it comes from | Buteyko practice, popularised by Patrick McKeown | Breathing coach Brian Mackenzie |
| What it’s said to reflect | How sensitive you are to carbon dioxide | CO2 tolerance and breath control |
| The catch | The end point is a judgement call, and charts disagree on what’s “good” | The score bands copied across the internet have no published source |
| Try it | BOLT score test | CO2 tolerance test |
The research on these tests is thin. A 2024 study of 49 elite speedskaters (opens in new tab) found their BOLT scores weren’t linked to their performance, and its authors note that the test hasn’t been validated. The bands you’ll see for the exhale test, from “poor” under 20 seconds to “elite” over 80, trace back to one coach. Treat any chart as one practitioner’s rough guide.
Scores also move with sleep, stress, food, and the time of day, so your trend over weeks says more than any single number. And keep either test gentle. Holding on past the first real urge turns a check into breath-hold training.
How to increase your CO2 tolerance
You need to take this up slowly but consistently. Breathe through your nose, slow down, lengthen the exhale, and let your breaths get a little smaller, for a few minutes every day. Slow breathing has the best evidence behind it. The rest is sensible practice with thinner research.
- Breathe through your nose, at rest and on easy walks. In one study, nose breathing during exercise kept carbon dioxide higher and breathing slower (opens in new tab) than mouth breathing.
- Slow down to around six breaths a minute, for five to ten minutes a day. Slow breathing is linked to calmer states and less anxiety (opens in new tab). You can try this free online coherent breathing pacer to set the rhythm for you.
- Lengthen the exhale so it runs a little longer than the inhale. A longer out-breath is one of the simplest ways to shift the nervous system toward its calmer side.
- Breathe smaller. Let each breath become soft and small, until there’s a slight, comfortable hunger for air. This is the idea behind Buteyko’s “breathe light”. Stay with it for a minute or two. If it tips into panic, gasping, or strain, stop and breathe normally.
- Catch the sighs. When you notice a big sigh or a gulp of air, follow it with a few slow, small breaths.
- Track the trend. Take either test once a week at the same time of day and watch the direction over a month or two.
Breathing retraining for over-breathing is widely used, but the research on it is still thin (opens in new tab): a Cochrane review found only one small trial. The calming effect of slow breathing is the part with the most support.
How long does it take?
There’s no reliable timeline. What the research does suggest is that comfort comes before chemistry. In one study, two weeks of daily breath-holds lengthened people’s holds by almost half (opens in new tab), while their sensitivity to carbon dioxide didn’t change. They got better at meeting the urge. Changes in sensitivity itself, if they come, seem to take much longer. Expect the feeling to shift before the number does.
Are breath-holds and CO2 tables safe?
Breath-hold training carries risks that slow breathing doesn’t. The main one is blackout without warning, especially after breathing fast or deeply first, and it has killed people in and near water. For everyday CO2 tolerance, you don’t need it.
Freedivers build tolerance with CO2 tables: repeated breath-holds with shrinking rests in between. The danger comes from how the alarm works. Breathing hard before a hold empties out carbon dioxide, so the urge to breathe is delayed while oxygen keeps falling (opens in new tab), and people black out before they feel any need to come up.
If you do practise breath-holds
- Never in or near water
- Never after fast or deep breathing
- Sitting or lying down
- Never alone if you're pushing for longer holds
- Stop at the first strong urge to breathe
- Skip them if you're pregnant or have a heart or lung condition
The safety article covers breath-holding and water in more detail.
What happens to CO2 in a breathwork session?
In Conscious Connected Breathwork, carbon dioxide moves both ways. Some phases lower it (opens in new tab); others let it rise again. Part of my job as a facilitator is watching how each person meets those sensations, air hunger included, so they can stay with them and come back out the other side.
Therapists treating panic use a similar principle on purpose. Meeting uncomfortable body sensations in a safe setting (opens in new tab) is an established way to make them less frightening. You can know a sensation is safe. The body has to learn it.
Sessions are screened, done lying down, and paced by a trained facilitator. Between sessions, the slow breathing above is what builds a steadier baseline, with the BOLT test or the CO2 tolerance test as a reference for your trend.
Breathing less, more often
The urge to breathe is old and fast, and it doesn’t take instructions from your reasoning. It can learn, though, the way the body learns most things: through repetition, in small doses, when it feels safe enough. Five slow minutes a day is a reasonable place to start.
If you’d like a rhythm to follow, the coherent breathing pacer sets six breaths a minute with no holds.