The CO₂ Rethink
From Principles to Practice: Rethinking CO₂ Training for Static Apnea
Tom Way, Singapore, 16th October 2025
Most freedivers think CO₂ training is about suffering.
Holding your breath until contractions feel unbearable. Counting down shorter and shorter breathing intervals. Fighting through discomfort. Toughening up.
But that’s not adaptation—that’s attrition.
Others believe that its about not suffering at all. No contractions no utb. Threatening or effortless. It’s black and white. But my belief is that most things in freediving are shades of grey, and CO₂ training is noe exception.
True progress in static apnea comes from understanding what CO₂ really does in your body and brain, how it interacts with O₂, and how to design training that teaches you to stay calm rather than endure pain.
This is the type of stuff I talk about with Pat ad nauseum. Well, ad nauseum for her at least. Over the last few years we’ve built some of our learnings in to her training and I use it in my own, and we’ve seen steady progress for us both.
Why CO₂? Well, it’s not the only thing to train, of course, but it’s the thing that gets the biggest results for the most amount of people.
And so this post takes you from the principles behind CO₂ training into how you might apply it practically (spoiler alert: you probably need a static coach! There’s some great ones out there!). We’ll give you ideas on how to structure sessions that actually make your dives feel better and last longer.
Each section aligns with a short reel from the GetEcstatic series, and together they form a mini-framework for how we think about CO₂ development, and we’ll be hyperlinking to relevant research or providing PMIDs where possible.
Now for a disclaimer: I’m not biologist, or physician, a clinical physiologist, physician assistant, cardiologist or neurologist; neither do I have a science or research role. I’m a teacher who knows a little about communicating knowledge and who is sceptical in his thinking—that makes me ask questions about common beliefs and rules-of-thumb. And on that note, I’m going to quote Florian Dagoury, who he said to me during my first week of coaching three or four years ago, “Pretty much all freediving science is psuedoscience”. What we do is weird. It combines well understood physiology in ways that aren’t at all understood. And our wonderful freediving scientists do fantastic work, but if you read the majority of their papers they lack the scale and funding that allow for true generalisations: small sample sizes and a lack of blinding and controls predominate; what’s worse, many of the studies are on non-freedivers and as a result don’t really represent elite or even intermediate level divers very well.
Okay, that’s enough of an intro. Let’s get started on section one.
Me, my limitations, my biases
Spoiler alert: the conclusion for this series of reels will be something along the lines of, "you should seriously consider getting a specialist static apnea coach if you want to make progress in this discipline".
Please don't contact me, I'm not a coach and don't have any paying students! I mean, I occasionally work with Pat on our own training programmes and she pays in as much as she has to listen to me yammer on about papers I've just read, and in return she says stuff that grounds me in the practical rather than the theoretical.
Her: "You just spent 25 minutes telling me I should do more breath holds, right?"
Me: "Yes".
Her:...
Her: "I think you could make that shorter."
My own thinking and interest in exploring CO₂ adaptation further has been influenced by my own coaches and mentors, foremost Florian Dagoury (@Mr10Minutes). Then there's the influence of Eric Fattah and his writings since the early 2000s and onwards, the coaching of Rami Bladlav, and more recently breath-scientists and communicators like Martin McPhilimey (who heavily influenced the direction of this post). If you're interested in the science of breathing, I'd recommend taking a course with him. And although there's still a long way to go, it's fantastic to see a growing number of women questioning the man-science because, quite frankly, most of the studies in this field and much of physiology are of men. Maybe reach out to Heike Shwertner, Paula Johnsson, or Iiris for suggestions on coaching specifically for women.
Okay, let's go. This is going to be the longest blog post by far but, for me at least, it's the foundation to understanding how to approach training (or understanding why a coach is giving you specific routines).
What CO₂ Does
Understanding Dual Adaptation to Carbon Dioxide in Freedivers.
CO₂ Isn’t the Enemy—and yet it also kinda is. Let's explore that for a second.
Rising CO₂ makes the blood more acidic. Why? Well CO₂ reacts with water (in your blood plasma, or in the cerebrospinal fluid) to form carbonic acid (H₂CO₃). Carbonic acid is unstable, however, and immediately disassociates into hydrogen ions (H⁺), an acid, and bicarbonate ions (HCO₃ ⁻), a base.
It's the acid which triggers the central chemoreceptors in the brainstem that drive the impulse to breathe and which initiate the familiar increase in bradycardia and peripheral vasoconstriction; the CO₂ component of the mammalian dive response (PMID 30855833). Those reflexes are adaptive: they conserve oxygen and redistribute blood flow to where its needed.
Note: the carotid chemoreceptors—located in the branch of the carotid arteries—primarily detect oxygen but also detect CO₂ to an extent. I’ll discuss these in a separate post.
But CO₂ also influences how the brain feels the urge to breathe—turning a neutral interoceptive signal into anxiety or even panic depending on how the cortex interprets it.
So CO₂ conserves oxygen through the mammalian dive response (good), but it creates anxiety which can make us abort a hold (breaking point) or drive hormonal responses that increase oxygen consumption and lead to early hypoxia (bad).
As such, max-attempt performances in apnea often involve an attempt on how to balance the benefits with the negatives. That might mean finding strategies to reduce overall CO₂ load (for example through lifestyle and diet choice, or through hyperventilation—more on this to come in other posts) until such a time when your adaptation to CO₂ is sufficient to redress the balance and capitalise on early CO₂ and stronger MDR effects.
This might not be the message you've heard from instructors who love to cite the MDR and tell us that we should worship CO₂, full-stop; what they don't tell us is that the adaptation to loving CO₂ is long, badly understood, and will probably be the main focus of our training for the rest of our (freediving) lives. If not addressed with care, being slavish to CO₂ can also be demoralising, painful and put us off the apnea part of freediving for good.
Like, really, how many of your freediving buddies like static apnea?
With an understanding of the effects of elevated CO₂ in the body and brain, let's now move on to the two questions which I hope to address through this post:
i. What's the evidence for adaptation to CO₂?
ii. And, in the event it exists what exactly is being adapted: is it a physiological change or a psychological change?
Physiological adaptation: the body’s long‑term tuning
We often talk about adaptations that are physiologically based in freediving, and for sure there are some. When it comes to CO₂ however, the evidence is more challenging to interpret. Broadly speaking, there are two types of exposure to CO₂ that may create the pressure required for adaptation: chronic exposure (as in some disease models) and shorter but more intense exposure (as in freedive training).
Adaptations through chronic exposure
We know that chronic CO₂ exposure (elevated levels of co2 over long periods of weeks, months or years) leads to adaptations. For example, Chronic CO₂ retention in patients with diseases like COPD produces a clear, long‑lasting physiologic desensitization of the central chemoreceptors and other adaptations such as metabolic compensation via renal bicarbonate retention. The medical literature sometimes describes this as “submissive hypercapnia,” a state where CO₂ tolerance is genuinely physiological (PMID: 25891787)
Rehabilitation studies for COPD with Chronic CO₂ retention suggest that even after partial recovery, some patients retain elements of this tolerance—demonstrating that some long‑term physiological adaptation to CO₂ can persist when exposure is chronic (Neder et al., 2021).
However, the evidence from disease models of chronic CO₂ exposure is not one that naturally applies to freedivers in training. Even those of us training pretty extreme levels of volume, for example receiving an elevated dosage of co2 for 5 hours a week through training, or through commercial spearfishing for example, are nowhere near the durations of chronic exposure experienced by COPD patients.
In addition, we must be mindful that while the findings in the medical literature are often robust, with large sample populations, long study periods, controls and, where possible, designed with blinding to ensure stronger validity, studies in freediving are often the opposite: small sample populations, no or few controls, short study periods, and no blinding.
So with a healthy pinch of salt: the literature on voluntary breath-holding, controlled exposure to higher CO₂ levels through training *may* produce measurable physiological effects—though the changes are subtle rather than dramatic. For each freediver study I'll take the additional step of providing the population size as an indication of how generalisable the results are.
The evidence for CO₂ adaptations through breath-hold training
Blood buffering and acid-base balance
For example, studies of experienced freedivers show that they maintain impressively stable arterial pH even as CO₂ levels rise. This appears to reflect a slightly enhanced bicarbonate buffering system and renal regulation, similar to what we see in people who live with chronically elevated CO₂ (Rizzato et al., 2018; N=8). Note: This study was conducted by taking arterial blood samples before during and after depth dives to 40m, not static apnea.
The mechanism for buffering is the reversible reaction between carbon dioxide, water, and carbonic acid:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
As CO₂ accumulates, blood pH drops; the kidneys respond by retaining bicarbonate (HCO₃⁻) and excreting hydrogen ions (H⁺), while intracellular buffers such as proteins and phosphate mop up additional acidity. In trained freedivers, repeated exposure to mild, transient acidosis may up-regulate this system—both through improved renal handling of bicarbonate and a faster mobilization of intracellular buffers—helping maintain pH stability even when CO₂ levels climb sharply during long apneas. And so, adaptations that lead to a more efficient buffering system would assist the body to keeping hydrogen ions and CO₂ lower in the blood, and reduce the CO₂ load crossing the blood brain barrier to the cerebrospinal fluid and the central chemoreceptors.
On the muscular level, biopsies from elite divers reveal increased mitochondrial density and myoglobin content—adaptations that enhance aerobic metabolism and reduce lactic acid buildup (PMID: 33510292; N=8). These changes indirectly stabilize CO₂ and pH during long breath-holds by improving how efficiently oxygen and lactate is used (despite the small population, this is a cool study btw, definitely worth a read!)
Chemoreflex desensitization
Several studies report that apnea-trained athletes show a reduced ventilatory response to rising CO₂ compared with untrained individuals, however none of these studies present, in my reading at least, a convincing case for actual structural chemoreceptor adaptations.
The suggested mechanisms include more efficient acid-base buffering (in the CSF as well as the blood), a general increase in hemoglobin in the blood, and enhanced oxygen transport (PMID: 33791844) - a lit review with most studies N<14; PMID: 3573399 - a lit review and hypthesis; PMID: 36237527- a meta-analysis N=126 combined participants from seven studies).
It's important also to note that the degree of desensitization varies a lot between divers, which suggests at most a partial physiological tuning rather than a fundamental rewiring of the human biology. Whether any adaptive effects are directly produced by the chemoreceptors or brain-stem regions like the medulla which are known to be directly responsible for ventilatory reponse are yet to be demonstrated.
Neural and affective adaptation: re‑training the interoceptive brain
Beyond the reflex arc, how we feel rising CO₂ is less about the chemistry itself and more about how the brain interprets that chemistry. The sense of discomfort, urgency, or “air hunger” emerges not from the medulla—which simply reports acidity—but from higher forebrain regions such as the insula , anterior cingulate cortex (ACC), and amygdala (PMID: 23439117). These structures form part of the interoceptive network - (PMID: 12154366) - the system that constructs conscious awareness of what’s happening inside the body and assigns meaning to those sensations.
The insula, in particular, plays a central role (PMID: 19096369; PMID: 40345467). It doesn’t generate “anxiety” directly; rather, it integrates sensory signals from the lungs, blood, and diaphragm into a unified perception of internal state — the urge to breathe. The posterior insula registers the raw physical intensity of this signal—something is happening inside—while the anterior insula and ACC transform it into conscious awareness and direct our attention toward it. Meanwhile, the amygdala provides emotional colouring: if past experiences associated this sensation with panic or loss of control, it’s tagged as threat; if it’s been re-experienced calmly, it becomes merely strong information — something to observe rather than fear.
Functional MRI studies have shown that even when brainstem drive is artificially held constant, these cortical regions still generate the subjective sensation of needing to breathe (PMID: 35965030). Many of us who practice breath hold diving will be familiar of days where our anticipation of urge to breathe seems to bring it on earlier and more intensely; this too, is evidenced in imaging studies of the brain - so cool! (PMID: 27648309; PMID: 31130876) In other words, the feeling of CO₂ discomfort isn’t a simple reflex — it’s a perception, shaped by expectation, emotion, and experience.
Repeated CO₂ exposure seems to recalibrate those circuits: freedivers show reduced anxiety and altered insular activation when anticipating breathlessness, paralleling studies in dyspnea habituation (PMID: 26082746). Over time, the interoceptive system learns to interpret rising acidity not as danger, but as signal. The diver’s conscious mind reframes—This isn’t panic. This is information. (PMID: 40014537; PMID: 28915367)
This process — interoceptive learning (PMID: 29884281) — likely explains much of the improved tolerance seen in short-term or weekly apnea training, even when the sensitivity of the chemoreceptors themselves remains unchanged. In essence, the freediver trains not only the lungs and blood, but the meaning-making machinery of the brain.
Looking ahead…
So we now we have a pretty thorough understanding of what can be adapted and some of the why the adaptations can occur. So the big questions is how can we leverage this to improve our CO₂ training?
Post 4 and 5 will deal specifically with this question.
