I moderate a few hearing health forums. From time to time I see people saying they got into this and damaged their hearing / caused tinnitus.

Yeah, there are several threads about that in the subreddit [0]. I did get the ringing in my ears when I first started. The solutions normally given are a) You are straining yourself. "Listen to your body" when to stop holding your breath (No issues there for me) and b) Breath in through your nose, out through your mouth.

Breathing in through my nose completely removed the ringing noise for me.

[0]: https://old.reddit.com/r/BecomingTheIceman/

I got ringing afterwards when I tried it, found the permanent tinnitus warnings and quit immediately. I wasn’t straining. I might try once more, being careful to follow your suggestion.

Yeah, I was pretty scared as well and instantly googled, saw the tinnitus warnings and became worried. Luckily, the nose breathing helped, because I quite enjoy the breathing exercises.

Tinnitus is mental

It is. Hyperventilation is the body's response to CO2 levels become too low.

Hyperventilation removes CO2 from the blood

...which causes hypocapnia, which can be dangerous if prolonged.

Are you sure? I would expect it to be the response to high co2 levels, since it removes co2.

It's why breathing into a bag works. You need to slow your breathing during hyperventilation to increase your CO2 back to baseline. Not speed it up.

Can CO2 be measured in a simple way? (Like an oximeter in a smartwatch measures oxygen)?

Yes there is some equipment to do transcutaneous pCO2 but that's mostly used in sleep studies AFAIK. Most of the time you use a blood test. I heard in some research they use IR on expired air not sure how common that is. If you find a "home made" way to get a (even just relative) pCO2 reading I would be highly interested.

The desire to breathe in a healthy individual is a really good proxy for CO2 levels based on a wonderful design with distributed sensors throughout the body.

It is not a good proxy for O2 levels in healthy individuals, but may be in people with COPD or other chronic breathing problems.

I want to offer you two responses each of which are absolutely genuine:

You must be fun at parties.

If you have never experienced your own body responding to your selective input (other than Mt. Dew) you are missing out... Ever hear of a "runner's high"? Dangerous indeed!

Why don't you try it out and see what happens, you have to breathe anyway

No thanks, I don't have tinnitus and want to keep it that way.

Uh oh, you have tinnitus

https://www.psychiatrictimes.com/view/psychopathology-tinnit...

Getting intoxicated isn't the end goal but to relax the body and mind to boost your immune system. I take you haven't even read the article.

edit: so if you think Wim Hof is just getting recreational high then you should continue taking whatever drug you are on such as lead.

The article doesnt talk about these benefits, only about how the author was able to re-achieve the high they were seeking. I take it you didn't read the article.