If you want a quick, surprisingly revealing snapshot of your nervous system, you don't need a wearable.
You can learn a lot just by watching how long you can comfortably exhale after one deep breath.
That's the essence of the CO2 tolerance test—a simple, back-of-the-envelope measure of how well your body and brain handle carbon dioxide.
Why CO2 Tolerance Matters
Your urge to breathe is driven not by a lack of oxygen, but by a rise in carbon dioxide (CO2) in your blood.
- High CO2 = your brain says "Breathe now."
- Better CO2 tolerance = you can stay calmer and clearer for longer in the face of rising CO2.
Low CO2 tolerance is often associated with:
- Higher baseline anxiety
- Overbreathing (too many shallow breaths per minute)
- Reduced endurance
- Having a harder time staying calm when stressed
Improving CO2 tolerance is like increasing your buffer against stress—physical and emotional.
How to Do the CO2 Tolerance Test
⚠️ Don't do this while driving, in water, or operating machinery.
Step-by-Step
-
Sit comfortably
- Upright, relaxed shoulders
-
Breathe normally for ~30 seconds
-
Prepare a timer
- A stopwatch or phone timer works great
-
Take a full inhale through your nose
- Fill your lungs, using your diaphragm (let your belly gently expand)
-
Start the timer
- As soon as you begin your exhale
-
Exhale slowly through your nose
- Controlled, continuous, no pausing
- Your goal is to make the exhale as long and smooth as possible without straining
-
Stop the timer
- When your lungs feel empty and you naturally need to inhale again
Important: This is not a breath-hold test. You're timing the length of a single, controlled exhale, not how long you can sit with empty lungs.
How to Interpret Your Result
Everyone's physiology is different, but here's a simple framework:
≤ 20 seconds
- Likely low CO2 tolerance
- Nervous system may be more reactive
- You might be overbreathing during daily life
~25–40 seconds
- Moderate CO2 tolerance
- Good foundation, room to improve
≥ 50 seconds
- High CO2 tolerance
- Often associated with better endurance and calmer baseline
You can write your category as a simple number:
- Low: 3
- Medium: 5–6
- High: 8–10
We'll use these numbers in the training protocol below.
How to Train Your CO2 Tolerance (Box Breathing)
Once you know your category, you can use box breathing to gently train your system:
Inhale → Hold → Exhale → Hold
Each phase is the same length, forming a "box."
1. Choose Your Basic Count
Using your CO2 tolerance result:
- Low (≤ 20s): Use 3-second box
- Medium (25–40s): Use 5–6-second box
- High (≥ 50s): Use 8–10-second box
2. The Protocol
For 2–3 minutes:
- Inhale through your nose (e.g., 3 / 5 / 8 seconds)
- Hold (same count)
- Exhale through your nose (same count)
- Hold (same count)
- Repeat
Keep everything smooth and comfortable—no gasping or straining.
Over time, as this becomes easier:
- Retest your CO2 tolerance
- Increase your box count if appropriate
What's Actually Changing?
With regular practice, you're training:
Neuromechanical control of the diaphragm
- Better command over your breathing muscles
Your brain's comfort with CO2
- You learn that slightly elevated CO2 is safe, not an emergency
Baseline breathing patterns
- People often naturally shift to slower, deeper breathing at rest
The payoff:
- Feeling less jittery or "on edge"
- Greater focus under stress
- More stable energy during the day
- Better sleep quality (for many people)
How Often Should You Do It?
A realistic, sustainable plan:
- 2–3 times per week
- 2–5 minutes per session
You don't need to turn this into a 30-minute ritual. Short, consistent sessions are what reshape your baseline.
A Simple Weekly Plan
Week 1–2
- Day 1: CO2 tolerance test + 2 minutes of box breathing
- Day 3: 3 minutes of box breathing
- Day 5: 2–3 minutes of box breathing
Week 3–4
- Retest your CO2 tolerance
- If your exhale is now clearly longer, increase your box count by 1–2 seconds
- Keep the same 2–3x/week rhythm
What You Might Notice
As CO2 tolerance improves, many people report:
- Less "background anxiety"
- Easier time staying with difficult tasks
- Reduced tendency to overreact to stressors
- Improved physical endurance (running, cycling, etc.)
The beauty: you're not forcing yourself to "be calm." You're changing the underlying physiology that determines how reactive your system is in the first place.
About the Author: John Doe explores simple physiological tests and practices that give you more control over stress, focus, and performance—all grounded in neuroscience and respiratory physiology.
