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Exactly what is CO2 tolerance?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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J Campbell

Well-Known Member
Sep 17, 2001
Can someone explain in detail exactly what CO2 tolerance is?
What does it mean to be CO2 tolerant?
Conversely, what does it mean to be CO2 intolerant?
What does it FEEL like to be CO2 tolerant/intolerant?
How do I know if I am CO2 tolerant?
Phsiologically, how have I changed when I become CO2 tolerant?
Or is this just a psycological phenomenon?

Mark J.
CO2 Tolerance

The urge to breathe is mainly triggered by elevated levels of CO2 in your blood stream. CO2 tolerance is something that freedivers like to build up so that they can delay and reduce the onset of the urge to breathe. The more your CO2 tolerance, the less urge to breathe at the same CO2 level.

CO2 tolerance can be built up by doing repeated breath-hold activity. Underwater hockey is an excellent way to build up CO2 tolerance because of the short recovery times. CO2 levels are already high at the start of every dive.

Kirk Krack has also developed some CO2 tolerance tables which may help. These are a set of breath-hold schedules that you go through, outlining how long to breathe and how long not to breathe. The general idea is that the breath-holds remain constant while the breathing interval gets progressively shorter. There is one of these schedules that you go through per week and the breath-hold time per week gets progressively longer.

So is CO2 tolerance a physiological or psychological thing? I'm pretty sure it's a bit of both. Physiologically, the urge to breathe comes later and psychologically, the urge to breathe becomes less worrisome.

Of course, you don't want to become too CO2 tolerant. After all, the mechanism is there to save your butt! However, the process of building up CO2 tolerance is an excellent way to learn the signals your body is telling you and where your limits really are.

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Are there any other ways to build up this thing? Other than repeated breathhold?

Interval Hypoxia Training (IHT) is a weird process of breathing reduced amounts of O2, it is likely that you could do the same thing with CO2. This is a note Eric Fattah wrote to Freedive Canada a while back about it.

IHT is applied via a machine which feeds you low o2 to cycle your SaO2 to
70-85 and back to normal, about 5-6 times in an hour. Treatment is 1-2 hours
per day, for a few weeks, then rest. Research shows that IHT increases EPO,
natural antioxidants, and increase breathing stimulus from low O2. The
machines cost about $15000. I talked to Oleg Takarov, a pioneer in the
field, and he confirmed my suspicion that the same effects can be had by just
holding the breath, because hypercapnia does interfere with the effects. Of
course, to get to 85%, you'll need to do at least 3'30-4'30 static, and to
get to 70%, you may need as much as 6'00--> 5 to 6 times per hour. Oleg's
sales pitch was that IHT machines make the process more 'pleasant.'

Eric Fattah

Strange but interesting,

IHT vs. ?

Quote from 'Profile - Eric Fattah' (in DB Freedive section):
Q: Where do you mainly train - i.e.; pool, open water, etc?

A: - - Interestingly, I don't do any apnea training unless I am training for static. The only times I actually hold my breath are during my pre-dive warmups, and during my actual dives.

How this goes hand-in-hand with Interval Hypoxia Training? Or is IHT valid just for static apnea? I'm not criticizing anything, just think I'm missing some point? How to develop CO2 & contraction tolerance if not doing IHT?

Juha I.:hmm
Hi Juha,

I tried an exercise to have contractions for a very long time
Perhaps it can help u to get used to contractions :

Without breathe up, and after small inhalation or mild exhalation hold yr breathe :
you'll get contractions very soon, then (e.g. 10 contr.) inhale that little more that contraction just don't go away.
When it gets hard, inhale a little more etc... With full lungs, hold on to what u can.

After 2 weeks of doing this 2 or 3 times a day I hold for 3'30" after contractions begin(80 to 100 contractions) within my limits.
In this 2 week my static has gone up from 4 to 5 minutes (dry).

Get used to the cramps and this way learn to love them. If you see them as the beginning of the end of your apnea, your negative attitude will affect your times.

Works for me, hope for u2
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