Pavel warns that hyperventilation (or excessive O2) acts as a poison. But, the dose makes the poison, and I believe that the drastic effect of hyperventilation is supposedly what could promote some of the health benefits that are claimed to be caused by Wim Hof breathing. However, keep in mind that is is just one part of the Wim Hof method: For example, you are supposed to relax befor and after the hyperventilation, and you are also supposed to use cold and employ a positive mindset, among other things.
I think Wim Hof breathing is a serious stressor - but as with all stressors, one that might just trigger the right adaptations in the body. I guess for Pavel to be convinced of the Wim Hof method, we would need some more research on it. But I am not Pavel, so what the heck do I know
Really looking forward to watching the Second Wind Express vids