I remember the first time I saw a whitetail walk in front of my tree stand. I was 12-years-old and you could have sworn that I was running a jackhammer. The buck fever shakes took control of me. Hell, it wasn’t even a buck, and I didn’t even shoot. But that’s to be expected from a boy on one of his first solo sits. And if we’re real honest, we still get feverish when the right critter walks in front of us. There’s nothing wrong with that. Excitement is part of the fun. There is, however, a problem with excitement that turns into stress that’s not managed. I’m going to teach you two, simple C02 tolerance tests to help see if that’s an issue for you.

A good score on the breathing tests, and it’s likely that you manage hunting stress well — when paired with the right mental skills and perspective. A poor test, and, well, you might be easily overwhelmed by circumstances out of your control in the field. That’ll set you up for some poor decision making. It’ll also cause you to have a shitty time, and maybe even piss off your hunting partners.

I’m going to walk you through the tests, explain why they matter for controlling your stress response while hunting, and how to train to improve your score if you don’t max them out.

(Bonus: I’ll explain how to use them to test your training readiness.)

What is C02 Tolerance and What Does it Look Like Physically and Psychologically?

C02 tolerance is the ability to handle rising and falling blood carbon dioxide levels without a stress response. Let’s talk about that in regards to your nervous system and what it means in practical hunting situations.

You have a nervous system. You already knew that. What you might not know is that part of it is called autonomic, and it handles all of your involuntary functions. The autonomic nervous system has two parts — sympathetic and parasympathetic. 

The sympathetic part handles your fight, flight, or freeze response. It jacks you up when there’s a perceived threat or when you’re excited. It increases your heart rate, sends more blood to your muscles, and dilates your pupils and airways. 

Your parasympathetic nervous system does the opposite. It is the rest and digest aspect of your nervous system. It tones you down by slowing your heart, decreasing your blood pressure, and bringing you back down to baseline or below. It also helps you poop — it’s responsible for digestion. So, if you’re constipated, it might be because you’re too jacked up all the time. (You probably also need to drink some water and eat some fiber.)

Poor C02 tolerance is linked to an overactive sympathetic nervous system, and it happens at both ends of the carbon dioxide spectrum. Inability to handle low C02 levels, often brought on by shallow breathing, triggers a sympathetic response. You exhale too much C02, and that makes your brain think there’s trouble. It prepares your body for conflict. Inability to handle higher C02 levels can cause an anxiety spike because it sends the message that you need to breathe right fuckin’ now! So, as a hunter and a human, you need to handle both conditions. 

All this talk about poor C02 tolerance begs the question, what does good C02 tolerance look like in real life? 

Physical Indicators of Good C02 Tolerance

Mostly, it looks like calm, easy breathing. While you’re just hanging out, you can breathe calmly through your nose, and you only need to breathe about 6 to 10 times per minute.

Another indication of good C02 tolerance is being able to breathe through your nose while exertion increases. The longer you can keep breathing through your nose while doing aerobic exercise, likely the better your C02 tolerance.

It also looks like a calm breath hold. If you can hold your breath out for an extended period (we’ll talk about just how long) with a calm face and without throat tension, you likely have good C02 tolerance. 

Breathing recovery is also a solid indicator of good C02 tolerance. If you recover breathing within a handful of breaths after an extended exertion, say a steady uphill climb, then it’s likely that you have decent C02 tolerance.

Apart from these physical indicators, there are also psychological indicators of good C02 tolerance. 

Psychological Indicators of Good C02 Tolerance

The truth is, it’s tough to separate the psychological indicators of good C02 tolerance from developed mental skills. But that’s not necessarily a bad thing. They coincide. For example, developed mental skills might override poor C02 tolerance, but you’ll have a hard time developing mental skills under stress if your C02 tolerance is abysmal. You won’t be able to go long enough, and your anxiety spike will likely override your sense of control. You work to develop them together. Here’s what it looks like when you do that well.

Viktor Frankl, the Austrian psychiatrist and Auschwitz survivor, wrote an incredible line which sums up the first psychological indicator of good carbon dioxide tolerance:

“Between stimulus and response there is a space. In that space is our power to choose. In that response lies our growth and our freedom.” 

Folks with good C02 tolerance display executive control under stress. They can still choose even when their physical sensations are screaming at them.​​ They can think. They remain focused. They don’t panic when air gets low. This is because their higher brain centers stay online even while the ancient parts of the brain at the brain stem scream threat! 

That leads us to the next psychological characteristic — accepting discomfort instead of avoiding it. Rather than trying to mentally remove themselves from discomfort or letting it overwhelm them, they stay in the moment and objectively look at what’s happening. Instead of trying to put themselves mentally on some beach somewhere or letting physical sensations build to the point that they want to quit, people with good C02 tolerance tend to get curious and ask questions.

My legs and my lungs are burning, is any permanent damage happening? No. Do I need to slow down? No. Am I fine but uncomfortable? Yes.

Acceptance provides a degree of separation between the stimulus and the response. It’s the interplay between psychology and blood chemistry. Folks who can manage the psychological response to changes in carbon dioxide in their blood chemistry are better able to accept discomfort as part of the deal instead of trying to avoid it.

What about at baseline when there are no stressors? It looks like calmness and patience. 

Good C02 tolerance lends itself to parasympathetic dominance, keeping a person calm and relaxed. That calm and relaxed state promotes greater emotional bandwidth. These folks can handle a greater number of stressors without being reactive. They remain even keel. They don’t display histrionics or dramatize stress.

You have a clear physiological and psychological picture of C02 tolerance. Let’s move on to what it means for you as a hunter.

C02 Tolerance and Managing Hunting Stress

Do you recognize the name Rudyard Kipling? If you do, it’s probably because you know that he wrote The Jungle Book. He also wrote a famous poem called “If—”. Kipling wrote it for his son to give him a solid definition of what it means to be a man and a leader. The opening line fits nicely into our current conversation:

“If you can keep your head when all about you are losing theirs…”

Keeping your head regardless of circumstances is the main aim of developing C02 tolerance. There is a fitness component we’ll also discuss, but mostly it’s about managing stress in the field so that you better enjoy yourself while also increasing your chances at bagging a critter.

When you get better at managing carbon dioxide, you:

• Manage your breathing better during exertion — think uphill climbs under your pack or hustling to get into position for a shot. That means it’s easier for you to get back to baseline breathing after breathing hard.

• You don’t tighten up when the circumstances get tense. You remain loose and calm.

• You maintain your ability to make good decisions when the pressure mounts. Should you push on to the next ridge to try to get in front of those elk? Should you take the shot or not?

• You maintain a quiet confidence. You’re the guy your hunting partners can count on instead of being the liability who freaks out when shit gets sideways.

Those are just a few examples of how improving your C02 tolerance helps you as a hunter. I’m sure you can add to the list from your personal experience. Now, let’s talk about the tests you’ll do to learn about your C02 tolerance. After that, we’ll talk about how to improve it and how to use the tests to gauge your training readiness.

Two, Simple C02 Tolerance Tests

The best things about these tests are that they don’t require any special equipment and you can do them on your own, anywhere. The first is a long exhalation test. The second is the BOLT test, which has you hold your breath out after an exhale.

Long Exhalation Test

What You’ll Need

• A quiet environment

• A stopwatch

• A chair that promotes upright posture

Before the Test

• Avoid caffeine, hard exercise, or any kind of breathing training for at least two hours

• Relax and breathe normally in your quiet testing environment for two to three minutes

Step by Step Testing Protocol

1. Sit tall with relaxed shoulders and your mouth closed. You’ll breathe only through your nose.

2. Take Four ramp-up breaths

   1. Inhale for 3-5 seconds through your nose

   2. Exhale for 5-10 seconds through your nose

   3. Pause for 1 second at the end of each exhale

   4. Repeat 4 times

3. The Test

   1. After the fourth inhale, you’ll do the long exhale.

   2. Start your timer as soon as you begin the exhale. Exhale as slowly and smoothly as you can through your nose.

   3. Stop the clock whenever any of the following happen:

      1. You run out of air.

      2. You swallow.

      3. You feel the first urge to inhale.

      4. Your exhale becomes shaky or forced.

   4. Rest for 90 seconds, then repeat.

   5. Record your time for each test, and then average them.

Test Scoring

1. <20 seconds: Very low C02 tolerance, highly reactive to stress and/or under recovered

2. 20 - 30 seconds: Low C02 tolerance, limited C02 control, keep training load low volume/low intensity

3. 30 - 60 seconds: Functional C02 tolerance, balanced stress response, train as usual

4. 60 - 90 seconds: Strong autonomic control, great training readiness

5. 90 seconds+: Exceptional C02 tolerance, great training readiness

Bolt C02 Tolerance Test

What You’ll Need

• A quiet environment

• A stopwatch

• A chair that promotes upright posture

Before the Test

• Avoid caffeine, hard exercise, or any kind of breathing training for at least two hours

• Relax and breathe normally by inhaling and exhaling through your nose in your quiet testing environment for a minute.

Step by Step Testing Protocol

• After the minute of normal, relaxed breathing, exhale normally (no forcing), pinch your nose close, and start the timer

• Hold your breath out until you feel the first urge to breathe. You read that? The first urge to breathe, not when it becomes so uncomfortable you can’t take it anymore.

• Release your nose and resume nasal breathing

• Record your time in seconds

• Rest for 60 seconds and repeat two more times

• Average the three tests to get your score

Test Scoring

1. <10 seconds: Very low C02 tolerance, highly reactive to stress and/or under recovered

2. 10 - 19 seconds: Low C02 tolerance, limited C02 control, keep training load low volume/low intensity

3. 20 - 29 seconds: Functional C02 tolerance, balanced stress response, train as usual

4. 30 - 39 seconds: Strong autonomic control, great training readiness

5. 40 seconds+: Exceptional C02 tolerance, great training readiness

How to Improve Your C02 Tolerance

There’s good news even if your C02 tolerance currently sucks — you can train to improve it. A combination of breathing exercises and aerobic training, done consistently, will improve your ability to manage your blood chemistry. And, as a result, improve your ability to manage stress while hunting. Here are a few simple things you can do to help your blood and your brain along. 

(Side note: These practices are helpful even if you scored well on the test.)

We’ll start with tiered breathing drills that progress from beginner to advanced. Then we’ll talk about aerobic training.

Breathing Exercises to Improve Your C02 Tolerance

We’ll scale these up from a simple way to focus on your breathing throughout the day to advanced breathing exercises. 

Foundational Focus

It’s best to begin with this foundational work if you scored in the low or very low categories on the tests. However, these are great drills for everyone to do because each is great practice at managing your C02 levels and your autonomic nervous system via breathing.

Even Nasal Breathing

This is simple. You can do it anywhere, anytime. It helps, however, to do it after something stressful (like training) or to tone down before you go to sleep. I’d recommend doing it under both of those conditions, while also focusing on it intermittently throughout the day.

Inhale through your nose for 4 seconds, then exhale through your nose for 4 seconds. Do it for a focused 5 minutes.

Box Breathing

You’ve likely heard of, or seen, this drill before. 

Do the following by breathing through your nose the entire time:

Inhale 5 seconds > Hold in 5 seconds > Exhale 5 seconds > Hold out 5 seconds

You’ll do this for 5-10 minutes every day. You can do it twice for extra credit and to accelerate the process.

Extended Exhales

You’ll inhale for 4-5 seconds, then exhale for 8-10 seconds. This is all through the nose, mind you.

You’ll do this for about 5 minutes. It’s a great practice to do before bed because it tones you down to go to sleep.

Intermediate Drills

These drills purposefully increase the C02 levels in your blood so you learn to manage your breathing while staying calm.

Hypoventilation at Rest

“Under breathing” increases carbon dioxide levels in your blood. This creates something called “air hunger” or the feeling that you need to take a deeper breath. Here’s how you’ll use it to improve C02 tolerance:

Take short, light inhalations and exhalations through your nose for 2-3 minutes. If you’re doing it right, you’ll start to develop a mild air hunger. Then breathe normally for 1 minute. Then repeat the under breathing. Do the whole shebang for 2-3 cycles.

C02 Tolerance Ladders

You’ll use your long exhale test score as a baseline for this drill.

Here’s how you’ll do the breathing ladders:

1. 4-5 deep ramp breaths (3-5 second inhalation / 5-10 second exhalation)

2. One long exhalation that lasts 60% to 80% as long as your test breath. You’ll do this after your last ramp inhalation.

3. Do 3 normal recovery breaths

4. Repeat 5-10x

Exhaled Breath Holds

This one builds off of the BOLT process.

Here’s how to do it:

1. Take 3, normal nasal breaths

2. Exhale normally after the third breath and hold your air out until you feel mild air hunger

3. Do 3 recovery breaths that are a little deeper than normal

4. Start over and repeat 5-10x

Advanced C02 Tolerance Training

I’ll offer only two techniques in this category. One you’ll do during training. The other you’ll do to recover during training. Only do these drills if you’re at the functional level or higher on each test.

Nasal Zone 2 Training

This is as simple as it sounds. You’ll do some kind of cyclical aerobic training (jogging, rucking, biking, etc.) while breathing only through your nose. If you’ve never done this before, start with 30 minutes and progress up to 60 minutes. You’ll likely have to go slower than usual, and you’ll note progress by going faster while nasal breathing gets easier and your heart rate stays lower.

Active Recover Nasal Breathing

You’ll do this during hard interval training sessions, such as lactate threshold training or a similar interval done at about an 8 out of 10 intensity. 

Once you finish a hard interval, switch immediately to nasal breathing. You’ll inhale for 3-4 seconds, and exhale for 6-8 seconds. You’ll do this for at least the first 1 to 2 minutes of the recovery interval, if the interval is longer than two minutes. After that, you can resume breathing normally if you want, but you likely won’t have to. 

With breathing drills covered, let’s talk about how aerobic training improves your C02 tolerance.

Aerobic Training and Improved C02 Tolerance

Solid volumes of aerobic training do some very nice things to improve C02 tolerance. I’ll list them for you.

1. Zone 1 and Zone 2 training, along with V02max training improves your aerobic efficiency. You get better at utilizing oxygen. That means there’s less metabolic waste. As a result, you accumulate less C02 and get better at managing the C02 that’s in your blood.

2. Sustained aerobic training strengthens your respiratory muscles. Stronger respiratory muscles mean better exhalations and an improved ability to offload C02.

3. You get better at buffering C02. That means your blood turns less acidic during exercise, which in turn dampens your stress or panic response.

4. And here’s a real big one: It improves your parasympathetic tone. There’s a direct link between your cardiorespiratory system and the vagus nerve. The better aerobic shape you’re in, the better your control over vagal tone. That means rising blood C02 levels have less of an effect.

The big takeaway from all of these points is that aerobic training better helps manage your blood chemistry while also improving the function of your autonomic nervous system. These combine to improve your stress response and, in effect, your C02 tolerance.

How to Use C02 Tolerance Tests to Gauge Training Readiness

This one ain’t rocket surgery, kids. You’ll choose one of the tolerance tests and do it consistently before training. Then you’ll gauge your training volume and intensity for the day based on the outcome of the test.

It’s especially useful if you’re feeling a bit behind the recovery curve and want confirmation. If you feel like 10 pounds of shit stuffed in a 5 pound bag, and you have a poor result on the tolerance test, it’s best that you back off training that day. If you feel like hell, but your tolerance test says you’re alright, start warming up and play it by ear. Sometimes getting moving is all you need to feel better. However, if you get moving and you still feel like hell, even if your tolerance test was good, then you probably ought to tone it back.

What you’re looking for are deviations from your normal baseline. If you’re a step or two below where you normally test, you need to pay attention and adjust accordingly.

Test Your Tolerance, Manage Your Stress and Your Training

Buck fever is one thing. Getting overwhelmed in the field by stress you can’t manage is another. Managing your blood chemistry and your nervous system by improving your C02 tolerance goes a long way towards improving your focus and mental game on the mountain. Test yourself. Use the drills to improve your tolerance. And come back to the tests if you need to double check your training readiness.