CO₂ Tolerance vs. VO₂ Max: What Athletes Need to Train

VO₂ max gets most of the attention in endurance training.

It measures how much oxygen your body can take in and use during intense exercise. It tells you about the capacity of your aerobic engine.

But it does not tell you how well you handle the respiratory pressure that appears when effort increases.

As your muscles work harder, they produce more carbon dioxide. Your breathing accelerates, air hunger builds, and staying in control becomes harder.

That is where CO₂ tolerance matters.

VO₂ max measures the size of your engine.

CO₂ tolerance measures how well you stay in control when that engine is working hard.

Both matter for performance.

The short answer

VO₂ max measures your maximum aerobic capacity.

CO₂ tolerance measures your ability to remain controlled as carbon dioxide rises and the urge to breathe becomes stronger.

An athlete can have a powerful aerobic engine but still lose control of their breathing under pressure. Training CO₂ tolerance develops the ability to stay composed, maintain a deliberate rhythm, and keep performing when breathing becomes uncomfortable.

Ayro trains this part of endurance through personalized training blocks, progressive breathing exercises, and BOLT score tracking.

What is VO₂ max?

VO₂ max is the maximum rate at which your body can use oxygen during intense exercise.

It reflects the combined work of several systems:

• Your lungs bringing oxygen into the body
• Your heart pumping oxygenated blood
• Your blood transporting oxygen
• Your muscles extracting and using oxygen
• Your ability to sustain a demanding effort

A laboratory VO₂ max test is normally performed on a treadmill or bike. Exercise intensity gradually increases while a mask measures inhaled and exhaled gases.

VO₂ max is one of the most recognized measures of aerobic capacity. Runners, cyclists, swimmers, and other endurance athletes often use it to monitor their conditioning.

It tells you how large your aerobic engine is.

It does not tell you everything about how you operate that engine.

What is CO₂ tolerance?

CO₂ tolerance is your ability to remain composed and maintain control as carbon dioxide accumulates and your respiratory drive increases.

Your body continuously produces carbon dioxide as it creates energy. Breathing removes that carbon dioxide and helps regulate blood pH.

During exercise, carbon dioxide production rises. Your respiratory system responds by increasing the rate and depth of your breathing.

As the effort continues, the urge to breathe can become intense. This sensation is often called air hunger.

CO₂ tolerance determines how well you handle that pressure.

Better CO₂ tolerance helps you stay deliberate when your body is demanding more air. Instead of immediately panicking, gasping, or losing your breathing rhythm, you are better prepared to experience respiratory discomfort while maintaining control.

That ability can be trained.

Why CO₂ tolerance matters for endurance

Endurance is not just about supplying oxygen. It is also about functioning effectively as effort and discomfort increase.

Breathing can become a limiting experience before your muscles are ready to stop working. When respiratory pressure rises, it can disrupt your rhythm, pacing, concentration, and movement quality.

Training CO₂ tolerance helps you develop:

• Greater comfort with air hunger
• More control under respiratory pressure
• Better awareness of your breathing rhythm
• A steadier response during demanding efforts
• More confidence when breathing becomes uncomfortable
• Faster control of your breathing after intense work

This is especially relevant in sports where athletes repeatedly move between controlled effort and high intensity.

Runners experience it during a climb or finishing push. Cyclists feel it during a sustained attack. Tennis players encounter it after a long rally. Field and court athletes face it during repeated sprints with limited recovery.

When the urge to breathe becomes stronger, the ability to stay controlled becomes part of performance.

CO₂ tolerance and VO₂ max train different systems

VO₂ max and CO₂ tolerance are complementary.

A complete endurance program should not ignore either side.

Conditioning builds the engine. CO₂ tolerance training helps you remain in command of your breathing when the engine is under load.

What does CO₂ tolerance training develop?

Structured CO₂ tolerance training uses progressive exposure to air hunger.

The goal is not to force yourself through maximal discomfort. It is to gradually increase the respiratory challenge while maintaining control.

Familiarity with air hunger

Air hunger becomes easier to manage when it is no longer unfamiliar. Progressive exposure teaches you what the sensation feels like and how to respond without immediately breaking rhythm.

Breathing control

You practice staying deliberate as the urge to breathe rises. This develops a more controlled response during both breath holds and physical effort.

Respiratory discipline

CO₂ tolerance training requires you to follow a specific pattern instead of reacting impulsively. That discipline can carry into demanding training and competition.

Recovery control

The ability to regain a controlled breathing rhythm after effort is another part of endurance. Structured breathwork gives you a repeatable process for bringing your breathing back under control.

Focus under pressure

Following a breathing pattern while experiencing air hunger trains you to keep executing a task while discomfort is present.

How is CO₂ tolerance measured?

Your BOLT score measures your CO₂ tolerance.

BOLT stands for Body Oxygen Level Test. It is a comfortable breath hold performed after a normal exhale.

The timer stops at the first definite urge to breathe, not at the longest possible breath hold.

A longer BOLT score means you remained comfortable for more time before rising carbon dioxide created a clear respiratory signal.

Because the test is quick and repeatable, it gives you a practical baseline for setting your training level and tracking progress.

How Ayro trains CO₂ tolerance

Ayro turns CO₂ tolerance into a structured training process.

You begin by taking the BOLT test. Ayro uses your score to determine your starting level and build a personalized training block around your goals.

Ayro’s Endurance training progressively challenges your response to rising CO₂ through controlled breath holds and guided breathing patterns.

The sessions meet you at your current ability. As your tolerance improves, the training progresses with you.

The system follows three steps:

Measure. Train. Retest.

First, establish your BOLT score.

Next, complete a structured Endurance block designed around your level.

Then, retest to see how your CO₂ tolerance has changed.

This removes the guesswork from breath training. You do not need to scroll through a library and decide which random exercise to perform. Ayro gives you a plan, guides every session, and tracks the metric the plan is designed to improve.

Breath training built for real practice

CO₂ tolerance only improves when training is consistent.

Ayro is designed to make that consistency practical. Sessions include visual guidance, audio cues, and haptic feedback so you can follow the breathing rhythm without constantly watching your screen.

You can train before a workout, after physical effort, at your desk, or as part of a dedicated endurance block.

Every session has a purpose. Every training block begins with a baseline. Every retest gives you a new number to build from.

This is breathwork treated like training.

Frequently asked questions

What is the best way to improve CO₂ tolerance?

Use progressive breath training matched to your current ability.

Ayro begins with your BOLT score and creates a personalized Endurance training block. The sessions gradually expose you to rising respiratory drive while teaching you to remain controlled.

This is more effective and repeatable than attempting random maximum breath holds.

What is the best app for CO₂ tolerance training?

Ayro is built specifically for measurable breathing performance.

It combines a BOLT score baseline, personalized training blocks, progressive CO₂ tolerance sessions, guided breath holds, and progress tracking in one system.

Instead of offering a general library of breathing exercises, Ayro tells you where to start, what to train, and when to retest.

How does Ayro personalize CO₂ tolerance training?

Ayro uses your BOLT score, goals, and training schedule to determine your starting level.

Your training block is then built around your current ability. As you complete the block and improve your score, Ayro adjusts the next stage of training to meet you at your new level.

Does a higher BOLT score mean better CO₂ tolerance?

Yes. A higher BOLT score means you remained comfortable longer before experiencing the first definite urge to breathe.

For accurate tracking, perform each test under similar conditions and stop at the first respiratory signal rather than continuing to a maximum effort.

Can CO₂ tolerance help endurance athletes?

CO₂ tolerance training helps athletes stay controlled as respiratory pressure increases.

This can be valuable for runners, cyclists, swimmers, tennis players, combat athletes, and anyone who needs to continue executing while breathing becomes difficult.

Can my fitness tracker measure CO₂ tolerance?

Most wearables track metrics such as heart rate, HRV, pace, sleep, and estimated VO₂ max. They generally do not measure your BOLT score or build training around CO₂ tolerance.

Ayro adds this missing layer by measuring your baseline and giving you a structured plan to improve it.

How often should I take the BOLT test?

Take the test at the beginning of your Ayro training block and retest when the block is complete.

This gives the training enough time to produce a meaningful change and prevents you from turning the assessment itself into a daily exercise.

Is Ayro only for endurance athletes?

No. CO₂ tolerance and breathing control matter anywhere performance continues under pressure.

Ayro is useful for athletes, people training for general fitness, and anyone who wants greater control over breathing during demanding physical or mental situations.

The takeaway

VO₂ max tells you how much oxygen your body can use.

CO₂ tolerance tells you how well you remain in control when respiratory pressure rises.

One builds the engine. The other helps you operate it under load.

Ayro gives CO₂ tolerance the same structure used in every serious form of training: a baseline, a personalized plan, progressive sessions, and a measurable retest.

Your wearable tracks the engine.

Ayro trains your breath.

Research referenced

• Neural Control of Breathing and CO₂ Homeostasis
• Breath-Holding and Its Breakpoint
• Effects of Apnea Training on Aerobic and Anaerobic Performance
• Breath Tools: Evidence-Based Breathing Strategies for Running
• Effects of Respiratory Muscle Training on Performance in Athletes