“I can’t breathe… I have to stop.”
I still remember that humid Bali morning. I was hunched over, gasping for air, while the rest of my running group pulled ahead like it was no big deal. I honestly thought being out of breath was just part of the deal — something I’d always have to live with.
Fast forward a few years, and now I’m that guy cruising up steep jungle trails, barely winded. What changed? I started training my breath — specifically, my carbon dioxide (CO₂) tolerance. Fancy name, but trust me, the idea is simple. And powerful.
This isn’t hype. This is real science mixed with experience — both mine and the runners I coach. We’ll get into what CO₂ tolerance really means, why it matters more than you think, and how it can change your running for good. Plus, I’ll walk you through how to start training your breath today with some simple, real-life tips.
So take a deep breath (through your nose if you can!), and let’s get into it.
I Couldn’t Breathe… Until I Learned to Breathe Right
Back when I started running, I couldn’t make it one block without wheezing. Picture this: a sweaty twenty-something guy in Denpasar, Bali, trying to jog in the morning heat, heart pounding like a drum solo, and lungs screaming for air.
One memory stands out — I was running up Campuhan Ridge, one of the prettiest routes in Ubud. But I was too busy sucking wind to enjoy any of it. I remember thinking, “Maybe I’m just not built for this.”
Turns out, I wasn’t short on oxygen — I just didn’t know how to use it properly. I was mouth-breathing like crazy, flushing out CO₂ too fast, which left me more breathless. It was a frustrating loop: the more I gasped, the worse it got.
My wake-up moment? An ultra runner from our group passed me with his mouth shut, breathing calmly through his nose. Afterward, over post-run coconuts (yes, very Bali), he explained something that floored me:
“You’re not running out of oxygen — you’re just not handling carbon dioxide well.”
Wait, what?
Apparently, it’s not low oxygen that makes you feel breathless. It’s the rise in CO₂. And the crazy part? You actually need a bit of that CO₂ to get oxygen where it needs to go. That lit a fire in me. I started researching, experimenting, and slowly learning to breathe better. It completely changed the way I run.
What is Carbon Dioxide Tolerance (and Why Should You Care)?
CO₂ tolerance is your body’s ability to stay cool when carbon dioxide builds up — especially during hard efforts.
Here’s how it works:
When you move, your muscles create CO₂ as they burn fuel. This gas builds up in the blood. Your brain doesn’t panic because oxygen is low — it panics when CO₂ gets too high.
Most people freak out when that happens. They breathe faster to get rid of it. But here’s the catch:
CO₂ is actually what helps oxygen get delivered to your muscles.
Thanks to something called the Bohr effect, higher CO₂ levels make hemoglobin release oxygen more easily. So if you hyperventilate and blow off all your CO₂, your body holds onto oxygen — and your muscles get less of it.
Let that sink in…
The very thing that feels like the right move — breathing faster — can backfire and leave your muscles starved for oxygen.
That explained a lot about my old struggles.
The Real Benefits of Training Your CO₂ Tolerance
Building CO₂ tolerance isn’t about being a breath-holding wizard. It’s about improving how your body handles stress, effort, and fatigue.
Here’s what happens when your body learns to tolerate more CO₂:
🫁 More Oxygen Where You Need It
Higher CO₂ = better oxygen delivery (Bohr effect again). That means your legs get what they need during those tough miles — especially on hills or long runs.
❤️ Better Blood Flow
CO₂ helps your blood vessels relax and open up. That means more blood to your muscles, more nutrients delivered, and more waste carried away.
🧘 Lower Heart Rate, Slower Breathing
Once you stop panicking about the breath, you naturally breathe slower and deeper. That leads to a lower heart rate at the same pace. I now talk through runs that used to leave me gasping.
💪 More Endurance, Faster Recovery
When you use less energy on every breath, you’ve got more gas in the tank for your legs. Plus, your body bounces back faster post-run — breathing calms down, heart rate drops, and you’re ready for round two sooner.
🧠 Mental Toughness
Breath training teaches you to stay calm when your body says, “Panic!” That skill? It’s gold during mile 20 of a marathon or the final rep of a brutal workout. It also spills over into life — a lot of runners report feeling less anxious day-to-day.
Reality Check: It’s Not Magic, But It’s a Game-Changer
Let’s be real: training your CO₂ tolerance won’t turn you into Kipchoge overnight. It’s one piece of the puzzle. But it’s a powerful one most runners ignore.
We obsess over VO₂ max and oxygen intake — but forget the oxygen has to be delivered to matter. CO₂ is the delivery key.
By learning to sit with that breathless feeling — not panic, not over-breathe — you teach your body to run smarter, not just harder.
For me, it was like upgrading my engine’s software. Same legs. Same lungs. But a totally different level of control and calm under pressure.
How CO2 Tolerance Training Can Boost Your Running Without Fancy Gear
So, how does all this breath-holding stuff actually help when you’re out grinding miles? Let’s break it down runner-to-runner:
1. You’ll Breathe Easier at the Same Pace
After just a few weeks of CO2 tolerance drills, I noticed something weird (in a good way): I wasn’t gasping for air as much. Those routes that used to leave me huffing? Suddenly manageable. Not because my legs magically got stronger—but because my breathing got more efficient.
That’s the point. Better CO2 tolerance = more oxygen delivered to your muscles without hitting the panic button too soon. It’s like lifting the roof off your endurance—you’ve got more airspace to work with.
📚 According to one study, CO2 training boosts oxygen availability by improving your body’s ability to use what’s already there.
Try this: Next time you’re on an easy run, pay attention. Are you gasping at mile two, or staying steady? That change says more than your watch ever could.
2. Lower Heart Rate, Better Endurance
You know that panicky, “oh crap I’m dying” feeling when your breathing spikes and heart rate jumps through the roof? Been there.
Once I started slowing down my breathing and focusing on nasal inhales, my heart rate dropped—same pace, less effort. It made long runs feel smoother, even during marathon prep. I wasn’t burning out early because my body wasn’t yelling at me to breathe.
📚 Again, study backs this: Training yourself to tolerate CO2 shifts your ventilatory threshold—you can run faster or longer before that heavy breathing kicks in.
Coach’s Note: This is clutch in longer races. The calmer your breathing, the longer you can stay in the zone.
3. You’ll Recover Faster Between Reps or Hills
We’ve all topped a steep hill or hammered through a rep feeling like we just swallowed fire. But here’s the thing—after working on CO2 tolerance, I could actually catch my breath faster. No more gasping like a fish.
This happens because your chemoreceptors (the little sensors in your brain that scream when CO2 builds up) stop overreacting. You train your body to chill—even when things get spicy.
📚 Cyclists talk about this a lot too—more CO2 tolerance = slower, deeper breathing = faster bounce-back.
Runner Reality: When you crest that climb, you’re back in the game quicker. That’s a real edge on race day.
4. More Efficient Oxygen Use = Better Running Economy
Running economy isn’t just strong legs—it’s how little oxygen you burn for the same pace. I switched to mostly nasal breathing on my easy runs and felt like I traded in my old clunker for a hybrid.
📚 Some studies even suggest nasal breathing can help runners maintain VO₂ max while reducing how much air they need.
The crazy part? I could breathe through my nose at tempo pace. That used to sound impossible. Now it’s just training.
5. Stronger Mind, Sharper Focus
This might be my favorite part: mental toughness.
Those breath-hold drills where your brain is screaming “BREATHE NOW”—yeah, they’re uncomfortable. But that’s the whole point. You learn to stay calm when everything says “panic.”
In races, when your body’s freaking out, you fall back on that breath control. I’ve seen runners shave minutes off their times just by not panicking mid-race.
I tell my crew all the time: “When it gets hard, lock in on your breathing. Anchor yourself.”
📚 Breath control has legit mental benefits too. It trains your nervous system to stay in control under pressure.
⚠️ But Don’t Throw Out Your Mileage Just Yet…
Let’s not get carried away. Some scientists are skeptical. If you’re already in great shape, your oxygen saturation during workouts is likely around 98–100%—so boosting CO2 might not drastically change that.
📚 One study showed nasal breathing didn’t improve VO₂ max in trained runners. Another found that better BOLT scores didn’t always lead to faster race times over a few weeks.
That said, don’t ditch your long runs or intervals. Breath training isn’t a magic bullet—it’s more like adding polish to the engine. It’s about feeling smoother, not suddenly gaining superpowers.
In My Experience: What changes most is comfort, control, and your ability to stay composed. And that often does lead to faster times because you’re not tapping out early.
🧪 Testing Your CO2 Baseline: The BOLT Score
Want to know where your breathing stands? Try the BOLT test. It’s super simple and surprisingly telling.
I call it the “control pause.” It’s basically a snapshot of how chill your breathing system is.
Here’s how to test it:
- Sit and relax. Ideally first thing in the morning or after 10 minutes of rest.
- Take a normal inhale, then exhale normally through your nose.
- Pinch your nose and hold. Start timing.
- Stop when you feel the first strong urge to breathe. Don’t go full hero mode—no gasping allowed.
- Check your time. That’s your BOLT score. Resume nasal breathing calmly.
📚 Most folks score around 20 seconds.
If you’re under 10 seconds, your system’s stressed—maybe poor sleep, fatigue, or just shallow breathing. Around 20? Not bad, but room to grow. Hit 30+ and you’re breathing like a pro. Over 40? That’s elite territory.
Patrick McKeown—the guy behind The Oxygen Advantage—says 40 seconds is the gold standard.
Final Thoughts: Is It Worth It?
If you’re serious about running—not just surviving runs but feeling good out there—CO2 training is worth a shot. No gear. No subscriptions. Just your breath, your body, and a bit of discomfort.
I’ve used it. I coach it. And it works—not in flashy PRs out of nowhere, but in the quiet moments where you realize, “Hey, I’m not dying at mile five anymore.”
👟 Your Turn:
What’s your BOLT score?
Have you tried nasal breathing or breath-hold drills?
Drop a comment and let’s talk about how it’s helped—or frustrated—you. This stuff is simple, free, and can make a difference.
Let’s breathe smarter, not just harder.
CO2 Tolerance Training for Runners: A Real-World Breathing Upgrade
Let me be honest with you—my first BOLT score was 16 seconds, and I felt like I was suffocating.
That tight, panicky feeling? It was my body freaking out over CO2. But like any muscle, your tolerance can be trained. These days, I hover around 40 seconds, and I swear—my running feels like I unlocked a secret level. Easier breathing, smoother pace, and way less gasping. Keep your BOLT score handy—we’ll check in again after some training and you’ll see the change for yourself.
Coach tip: Treat the BOLT test like checking your battery. A big drop one morning? That could be your body telling you it’s stressed out, under-recovered, or riding the edge of burnout. CO2 tolerance tends to dip when you’re anxious or fatigued. On those days, back off a bit or focus on recovery instead.
Call to Action: Try This Mini Experiment
Tomorrow morning, measure your BOLT score. Write it down. Then commit to two weeks of simple breath work:
- Nasal breathing during runs
- Breath holds or box breathing before bed
After 14 days, test your BOLT again. Even a few seconds of improvement is proof that something’s shifting. But more importantly—check in with how your runs feel. Are you less winded on those easy miles? Recovering faster after speed work? That’s the real win.
Embrace the Discomfort: Turning Breath Training into Running Gains
Ultimately, improving your breathing is about improving your running experience. Running will feel less like a fight for air and more like a flow. You’ll likely run faster or farther before fatigue sets in, and even if you don’t become Mo Farah overnight, you’ll enjoy running a heck of a lot more when you’re not constantly gasping.
For me, that meant the difference between quitting running out of frustration and becoming a lifelong runner and coach. That’s the power of learning to breathe better.
So, take a deep (nasal) breath, and take the plunge into CO2 tolerance training. Your lungs, legs, and even mind will thank you. The next time you find yourself in that tough part of a run where your chest is burning, you’ll smile knowing you’ve trained for this very moment. And as you exhale, you’ll push onward, stronger and calmer than before.
Happy breathing and happy running!