CO2 Tolerance Training and the BOLT Test: What Science Says

CO2 Tolerance Training: What the Evidence Actually Says

When 49 elite speedskaters at Poland’s Institute of Sport took the Body Oxygen Level Test (BOLT) — a breath-hold test marketed as a proxy for carbon dioxide tolerance and exercise capacity — researchers found no meaningful link between their scores and their performance on laboratory tests. The study, published in Frontiers in Physiology in 2024, adds a sobering data point to the conversation about CO2 tolerance training, a practice that has spread from freediving circles into fitness culture largely ahead of the science.

Elite Athletes: No Link Between BOLT Scores and Performance

Kowalski and colleagues at the Institute of Sport — National Research Institute in Warsaw and Wrocław Medical University recruited 49 competitive speedskaters and had each perform three assessments: the BOLT test, the Wingate Anaerobic Test, and a full cardiopulmonary exercise test on a cycle ergometer. The BOLT measures the time from a normal exhalation to the first definite urge to breathe, and its proponents claim the score reflects CO2 sensitivity and, by extension, exercise tolerance.

The data disagreed. Correlations between BOLT scores and peak power, total work, power drop, time to exhaustion, and maximum oxygen uptake ranged from r = −0.172 to 0.013, with p-values between 0.248 and 0.984 — statistically indistinguishable from zero. A regression model combining BOLT with age, body measurements, and training experience explained just 8% of the variance (R² = 0.08, RMSE = 9.78 seconds). The authors’ conclusion is direct: interpret BOLT scores in highly trained populations “with a great degree of caution.”

Why might the test fail here? Breath-hold time depends heavily on motivation and on how the test is coached, and elite endurance athletes may already have blunted chemoreflex sensitivity compared with untrained people — a ceiling effect that makes a single breathing test unable to rank performers within that group. A tool can be sensitive to differences between a sedentary person and an athlete while being blind to differences between two athletes.

What Apnea Training Actually Changes in the Body

Separate research from Ghent University, led by Declercq, Bourgois, and colleagues (European Journal of Applied Physiology, 2024), examined how apnea novices respond physiologically to maximal breath holds and to two structured training formats: O2 tables and CO2 tables. The distinction matters. O2 tables use fixed, generous recovery times with progressively longer holds, exposing the body to falling oxygen. CO2 tables keep holds relatively short but shrink the recovery between them, so carbon dioxide accumulates while oxygen stores stay comparatively intact.

Mechanistically, the urge to breathe is driven far more strongly by rising CO2 than by falling O2. CO2 diffuses across the blood-brain barrier and acidifies cerebrospinal fluid, where central chemoreceptors in the brainstem detect the pH shift and trigger air hunger. Peripheral chemoreceptors in the carotid bodies add a faster signal. Repeated exposure to hypercapnia — what CO2 tables deliberately produce — can recalibrate this reflex arc, reducing the intensity of breathlessness at any given CO2 level. Apnea training also recruits the mammalian dive response: bradycardia, peripheral vasoconstriction, and in trained freedivers, spleen contraction, which releases stored red blood cells into circulation.

Where CO2 Tolerance Training Genuinely Helps

Absence of a performance benefit in speedskaters does not mean the whole field is empty. Evidence in clinical settings is more encouraging. Research on patients with chronic lung disease suggests that improved CO2 tolerance reduces perceived breathlessness, including during strength training, which can make pulmonary rehabilitation feel less punishing — a topic covered in our article on CO₂ tolerance and breathlessness in strength training for lung disease. Reduced CO2 sensitivity also appears relevant to heart failure, where chemoreflex imbalance affects prognosis; see our piece on CO2 tolerance as a heart failure marker.

There is also a psychological dimension. People with panic disorder show heightened CO2 sensitivity, and heightened sensitivity to CO2 is a known feature of hyperventilation-related breathing pattern disorders, as discussed in our coverage of hyperventilation syndrome. Gradual CO2 tolerance work, alongside slow breathing practices, may lower the alarm threshold — though panic-prone individuals should progress slowly and ideally with professional guidance.

A Sensible, Safer Way to Practice

If you want to try CO2 tolerance work, structure matters more than intensity. A simple land-based protocol: after a normal exhale, hold your breath until you feel a clear urge to breathe, note the time, then recover for a period shorter than the hold — for example, hold 30 seconds, recover 20 seconds, repeat six to eight times. Over weeks, recoveries shrink as tolerance grows. Always train seated on land, never in water (shallow-water blackout kills experienced freedivers every year), never hyperventilate beforehand, and stop at the first signs of dizziness or tingling. If you have cardiovascular or respiratory disease, clear it with a clinician first.

For athletes, the honest summary: CO2 tolerance training is a specific skill with specific effects — better breath-hold duration, reduced breathlessness perception — and the Polish data suggest it will not show up in your VO2max or sprint power numbers. Treat it as a tool for breathing comfort, not a shortcut to fitness.

Frequently Asked Questions

Does a higher BOLT score mean I’m fitter?

Not necessarily. In 49 elite speedskaters, BOLT scores showed no significant correlation with aerobic or anaerobic performance, so a high score shouldn’t be read as proof of fitness — especially in trained populations.

What’s the difference between CO2 tables and O2 tables?

CO2 tables shorten recovery between breath holds so carbon dioxide builds up while oxygen stays relatively stable; O2 tables fix recovery time while lengthening holds to stress low-oxygen tolerance. They train different adaptations.

Is CO2 tolerance training safe?

Done seated on land without prior hyperventilation, gradual CO2 work is generally tolerable for healthy people. It carries blackout risk and should never be performed in water, and people with heart or lung conditions should consult a doctor first.

Can CO2 tolerance training help with breathlessness?

Yes — that’s where the supporting evidence is strongest. Reduced sensitivity to CO2 can lessen the intensity of air hunger, which is why it’s being studied in chronic lung disease, heart failure, and panic-related breathing disorders rather than athletic performance.

Sources:
https://pubmed.ncbi.nlm.nih.gov/39290618/
https://pubmed.ncbi.nlm.nih.gov/39044031/
https://pubmed.ncbi.nlm.nih.gov/36877666/
https://pubmed.ncbi.nlm.nih.gov/34559419/
https://pubmed.ncbi.nlm.nih.gov/34127052/