This may be all too obvious… but I’ll say it anyway. One of the most common struggles I hear from climbers is their battle with breathlessness while climbing at high altitude.

If you’ve climbed at altitude, you know the sensation: legs seem fine, pack’s not too heavy, food/fuel plan is working, but your breathing is labored—and it’s getting worse with each ascending step. Many (and I mean MANY) assume this is because of a lack of oxygen, but that’s only part of the story.

Surprisingly, a significant reason you feel out of breath has to do with Carbon Dioxide (CO₂) Tolerance. And the good news? You can train your breathing system to handle it better, so

Why CO₂ Tolerance Matters at Altitude

As elevation levels increase, oxygen levels decrease—and that certainly creates risk for hypoxia. But the more under-the-radar element is that your body may be highly sensitive to build-up of CO₂. As you exert yourself climbing, your muscles create CO₂ as a byproduct of energy production (your Biology 101 class takeaway today: cellular respiration → ATP → CO₂ as a byproduct in your blood). Your chemoreceptors closely monitor the pH of your blood, and CO₂ alters that pH (causing it to go down). And if your CO₂ tolerance is low, these receptors speak loudly, triggering that panicky, breathless feeling—even when oxygen is available.

Before we can talk about training your CO₂ tolerance, though, let’s first help you get a gauge for your own chemoreceptors’ sensitivity to CO₂. The answer is simple: do the BOLT and CO2TT (CO₂ tolerance test) assessment scores. A BOLT of <20, or CO2TT <60 is a sign you may want to train your CO₂ tolerance.

Simply put: It’s not just about getting more oxygen. It’s about training your body to tolerate CO₂ better, so you don’t feel like you’re suffocating with every step.

So, how do you actually train this?

4 Ways to Train Your CO₂ Tolerance for High-Altitude Climbing

1. Warm-Up with Breath Holds

This is simple but effective. Before you start any training, or your actual climb, do this breathing exercise to prime your system for optimal oxygenation:

• Take a normal exhale and pinch your nose closed.
• Hold your breath for 10-15 seconds while nodding your head up and down.
• Walk a few steps while holding your breath.
• Repeat 5 times to warm up your respiratory system.

By increasing nitric oxide in the lungs, as well as CO₂ levels, this exercise stimulates vasodilation (widening of blood vessels) and helps your body use oxygen, even if limited in the air around you, more efficiently.

2. Breathe Through Your Nose During Training – so that you can Eventually do it During Your Climb

Given that feeling of breathlessness, as the intensity level rises, along with your heart rate, your instinct may be to breathe through your mouth—but resist that urge as much as possible. Nasal breathing helps you:

• Engage your diaphragm and pull air DEEP into your lungs
• Keep your heart rate under control (and therefore also panic level)
• Keep your respiratory rate slow, which retains CO₂ for better oxygen delivery (See: the Bohr effect)
• Keep your airways warm, humidified, and free from the chance of bronchoconstriction

At first, this will feel uncomfortable. But over time (weeks to months of training), your CO₂ tolerance will increase, making nasal breathing easier and more effective.

• Personally, it took about 8-10 months for me to adjust my breathing, but I’ve never looked back. So much so that I ran a marathon with my mouth taped shut…. And if I did it, you can too. Trust me. 

3. Breathe Light Training

This exercise teaches your body to tolerate more CO₂, and it’s one of my favorite ways to torture myself. But in all seriousness – this is an efficient way to build mental resilience and prepare for your high altitude climb.

• Sit in a relaxed position.
• Slow your breathing until you feel mild air hunger
• Maintain this for 3-5 minutes

Over time, this retrains your chemoreceptors, so your body stops overreacting to CO₂ buildup—helping you breathe more easily at altitude.

4. High Altitude Simulation (Walk → Jog)

Here we go. It’s time to talk about high altitude simulation; this is an advanced exercise (and the most advanced I feel comfortable publishing publicly on the internet) for building CO₂ tolerance and mental resilience. You should not do this if you are pregnant or suffer from a panic/anxiety disorder.

this is an efficient way to build mental resilience and prepare for your high altitude climb.

• After a normal, passive exhale, pinch your nose and start walking.
• When you reach a medium level of air hunger, begin to jog.
• Go as far as possible before the urge to breathe becomes too strong.
• When you stop, try to take only 6 minimal nasal breaths before breathing normally again. It’s like you simply shuttling a tiny bit of air in and out of your nose, not deep into your lungs.
  • You may feel your diaphragm contract during those minimal breaths – and that’s a good thing.
• Do 5 rounds in a row with 12-18 recovery breaths in between round.

The Bottom Line

If you want to feel less out of breath at high altitude, don’t sleep on breathwork training. Train your breath the same way you train your legs: with targeted exercises full of purpose, intention, and enthusiasm.

The better your CO₂ tolerance, the more you can maintain control of your breathing, and ultimately the more oxygen your body can put to use amidst low levels climbing at high altitude.

 ~ You have a goal to climb a mountain. Our goal is to get you there. ~

Happy training!

Anthony and the Recal Team