You’re on a run, a hard hike, or in the middle of a workout. Your legs feel fine, your body is capable, but you’re gasping for air.

It’s a frustrating, familiar feeling. Nobody likes that sensation of shortness of breath during exercise, like you just can’t take a full, satisfying breath. You have a good base level of fitness, so why does this keep happening?

I know your immediate, logical conclusion might be that you’re “out of shape.” You think you need more cardio. You need to push harder. You need to “train your lungs to take in more oxygen“.

But what if the problem isn’t your cardio, your lungs, or your fitness level?

The feeling of being “out of breath” is one of the most misunderstood sensations in human performance. We’ve been taught it’s a sign of poor cardiovascular fitness. But for many active, disciplined people, it’s not.

It’s a sign of low CO₂ tolerance.

Curious of what that is? Let’s break it down together.

CO₂ tolerance is the hidden variable that separates those who can sustain a calm, steady pace from those who are constantly fighting for air. The good news? It’s not a permanent flaw. It’s a physiological response that is 100% trainable.

This guide isn’t about more effort. It’s about smarter effort. It’s time to stop blaming your lungs and start training your brain’s internal alarm system.

The Great Misconception: The “Gasping for Oxygen” Myth

Why do I feel like I can’t take a full breath?

When you feel that desperate urge to breathe , your first thought is, “I need more oxygen.” This is a deeply ingrained, intuitive assumption. And it’s almost entirely wrong (exception: when you’re at very high altitude).

I’m not calling you a liar – you are very smart, I’m sure. But your sensation of breathing isn’t driven by a lack of oxygen (O₂). It’s driven by the increasing presence of carbon dioxide (CO₂) .

Here’s how it works:

1. You Have These Sensors: Deep in your brainstem, you have specialized sensors called chemoreceptors. The ones I’m referring to here are your central chemoreceptors.
2. They Monitor the pH of your CSF (Cerebral Spinal Fluid): The pH of your CSF is directly correlated to your blood, which is impacted by the amount of CO₂ is in it.
   1. On the other hand, your peripheral chemoreceptors, the ones that monitor oxygen levels, are located in your carotid bodies. These really only “pay close attention when your O₂ levels become dangerously low, which is rare outside of very high altitude).
3. CO₂ is the Trigger: As you exercise , your muscles work harder than normal, create energy, and release CO₂ as a byproduct. This carbon dioxide enters your bloodstream. This is normal and necessary.
4. The Alarm Sounds: Your chemoreceptors detect the change in pH as a result of this rise in CO₂ and sound the alarm. This “alarm” is the urge to breathe. It’s your brain saying, “Time to exhale and clear out this CO₂.”
   1. Remember, your breathing is a constant back-and-forth of O₂ → in, CO₂ → out.

Hence, the feeling of “I am out of breath” is not your lungs failing to get oxygen in. It is your brain panicking about the rise of CO₂.

What is CO₂ Tolerance (And What Are Its Symptoms)?

CO₂ tolerance is, quite simply, the measure of your chemoreceptors’ sensitivity to carbon dioxide.

Think of your chemoreceptors as your home’s smoke detector.

• High CO₂ Tolerance: You have a calm, well-calibrated detector. When you’re just… cooking toast (a normal rise in CO₂ from exercise), it stays quiet. It understands the difference between a little smoke and a real fire. This allows you to breathe efficiently and calmly even as CO₂ levels rise. This is your ideal state.
• Low CO₂ Tolerance: You have a “panicky” detector. The slightest whiff of smoke—the lightest browning of your delicious sourdough toast—sets it off, blaring at full volume. Your brain thinks a normal, safe rise in CO₂ is a five-alarm fire. This is a poor state.

This “panicky alarm” is what hijacks your performance. Your brain overrides any rational thought and forces you into a chaotic, inefficient breathing pattern (gasping, upper-chest, shallow mouth breathing) to “flush” the CO₂ at all costs. And it actually does a poor job of it, only shuttling air back and forth in the “dead space” of your airways (space consisting of un-used air between your nose/mouth and trachea, before reaching your bronchi and, ultimately, alveoli).

What are the symptoms of low CO₂ tolerance?

This isn’t just a problem that rears its ugly head during workouts. Chronically low CO₂ tolerance can affect your entire day.

Symptoms at Rest:

• Frequent Sighing or Yawning: A sigh is your body’s “reset button” for a dysfunctional breathing pattern. If you’re doing it often, it’s a sign your CO₂ alarm is miscalibrated.
• Brain Fog & Fatigue: Can low CO₂ cause fatigue? Absolutely. A chronic, low-grade “panicky” breathing pattern is metabolically expensive. You’re using your accessory breathing muscles (neck, shoulders) all day instead of your diaphragm. It’s exhausting.
• Feeling “On Edge” or Anxious: This is a major symptom. Shallow, rapid breathing is the physical language of anxiety. A low CO₂ tolerance can trap you in a feedback loop where your physiology tells your brain to be anxious, even when there’s no external reason. It’s a huge source of chronic stress.
• Disturbed Sleep: You might wake up feeling breathless or find it hard to settle at night.
  • Also, if you have a dry mouth in the morning, it’s a sign you’re mouth-breathing. I recommend using mouth tape. It’s not weird, I promise.

Symptoms During Exercise:

• Disproportionate Breathlessness: Your shortness of breath feels way out of sync with your actual effort. Your heart rate is reasonable, comfortably in Zone 2, for example. Your legs could also do more. However, your lungs feel like the limiting factor.
• Inability to Nasal Breathe: You find it impossible to breathe through your nose during even moderate exercise. You have to “gasp” through your mouth because you feel your nose is too small of an airway to “get enough air.”
• Air Hunger: That persistent, frustrating feeling that you “can’t get a full breath ” or “can’t get enough air in.”
• Rapid Fatigue: Your performance hits a wall, and it feels like your endurance has fallen off a cliff.

The Vicious Cycle: Why “Training Harder” Makes It Worse

This brings us to the core of the “train smarter” concept.

You feel out of breath climbing the stairs at your house or office, and you decide you need to get in better shape. So, you go for a run. Your CO₂ level inevitably rises. Your panicky alarm (your central chemoreceptors) goes off. You resort to quick, shallow breaths through your mouth as you gasp for air. You are, in effect, practicing and reinforcing a dysfunctional breathing pattern. You’re teaching your body that gasping through your mouth is the correct response to effort.

But it gets worse. This “gasping” pattern actually sabotages your body’s ability to use the oxygen you’re breathing in.

Enter The Bohr Effect: The “Gasping” Trap

There is a fundamental law of physiology called the Bohr Effect. It’s the single most important concept you’re not using.

• The Fact: Oxygen is carried through your blood by hemoglobin (a protein in your red blood cells).
• The Lock: Hemoglobin is “locked” to oxygen.
• The Key: Carbon dioxide (well, really the subsequent drop in pH) is the key that unlocks oxygen from hemoglobin and delivers it to your muscles, brain, and other vital tissues in your body.

When you have a low CO₂ tolerance, you panic and over-breathe (hyperventilate), often through your mouth. This “flushes” too much CO₂ from your system.

Your blood CO₂ levels drop. Your blood pH rises. And suddenly, you don’t have the key to unlock oxygen from your hemoglobin.

This is the ultimate performance paradox. You are gasping for more oxygen , but the very act of gasping is preventing the oxygen you already have in your bloodstream from ever reaching your working muscles.

You aren’t “out of shape.” You are, quite literally, breathing yourself into a state of fatigue.

This also answers the question, “Can exercise cause low CO₂ levels?” Yes, improperly performed exercise with a dysfunctional breathing pattern absolutely can. You’re training your body to be exceptionally good at “flushing” CO₂—the one molecule you desperately need to unlock oxygen.

How to Fix It: A Smarter Approach

The solution isn’t to push harder through the panic. The solution is to re-calibrate your alarm. You must teach your brain that a normal rise in CO₂ is safe and necessary for performance.

You do this by training your CO₂ tolerance.

Step 1: Get a Baseline. Stop Guessing.

You can’t improve what you don’t measure. You don’t guess your 5k time; you use a timer. You shouldn’t guess your breathing.

This is why we built the CO2TT Assessment. It’s a simple, data-driven diagnostic test that gives you an objective score for your CO₂ tolerance. It cuts through the guesswork and tells you exactly where your “alarm system” is currently set. It’s the starting line.

Step 2: What breathing exercises increase CO₂ tolerance?

The answer is specific, targeted drills. Not all “breathwork” is created equal.

The goal isn’t to “flush CO₂ out of your body” —you’re already too good at that. The goal is to practice tolerating its presence. This is done through controlled, deliberate breath-holding.

Think of it like this:

• Running is to your cardio system…
• …what CO₂ Tolerance Tables are to your chemoreceptors.

There are structured drills—like Breathe Light (video below) that are designed to train your CO₂ tolerance. This technique involves slowing down your breathing to reach a medium and tolerable level of “air hunger.” Hold this level of breathing for at least 3 minutes. By doing this on a consistent basis, your chemoreceptors will begin to get used to a rise in CO₂ and won’t hit the alarm as quickly. 

→ for a full list of exercises, join our FREE Challenge: Feel Less Out-of-Breath in 10 Days. Click here to join.

Step 3: Rebuild Your Foundation

Alongside targeted drills, you must fix your “at rest” breathing pattern.

• Nasal Breathing: Your nose is your body’s built-in regulator. It warms, humidifies, and filters air. Most importantly, it adds natural resistance, forcing you to use your diaphragm and slowing your breath. This is the best way to breathe. Your goal should be 100% nasal breathing at rest, during sleep, and during all “easy” to “moderate” exercise. If you have to open your mouth, you’ve crossed a threshold.
• Diaphragmatic Breathing: Stop breathing with your chest and shoulders. Re-learn to use your primary breathing muscle —the diaphragm. This low, slow pattern is more efficient, calms the nervous system , and is the foundation of good breathing.

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Common Questions & Breathing Myths

Your newfound knowledge of CO₂ tolerance helps you cut through the noise of popular “breathing hacks.”

“How to train your lungs to take in more oxygen?”

This is the wrong question. Your lungs are likely not the problem. In most healthy people, the lungs are more than capable of saturating the blood with oxygen. The problem, as we’ve seen, is delivery (the Bohr Effect).

Stop trying to “take in more.” Start training to become more efficient with what you have. Improve your CO₂ tolerance , and you will automatically improve your body’s ability to use the oxygen you breathe.

“Why Free-divers are the undisputed masters of CO₂ tolerance?”

Their entire sport is a battle against the “panicky alarm.” They have, through years of training, desensitized their chemoreceptors to a degree that is almost superhuman. They prove that this “alarm” is not a fixed limit. It is 100% trainable.

You don’t need to hold your breath for 5 minutes. But the science they use is the same science you can use to stop gasping on a staircase. You’re just starting at a different point on the same-path.

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Stop Gassing Out. Start Training Smarter.

The feeling of “shortness of breath during exercise” is a signal. But it’s not a signal that you’re “out of shape.” It’s a signal that your respiratory physiology is inefficient.

You can continue to “train harder,” grinding out junk miles, reinforcing a dysfunctional breathing pattern, and wondering why you don’t feel like you’re getting any better.

Or, you can start to train smarter.

You can accept that the most powerful gains are no longer in your legs, but in your respiratory physiology. You can stop guessing and start measuring. This is the best way to improve your endurance and overall performance , whether you’re preparing for a boardroom meeting or a high-altitude mountain.

It’s time to find your baseline.

Let’s get to work.

For more information on our FREE Challenge: Feel Less Out-of-Breath in 10 Days, click this link.