Wim Hof breathing is unsafe in water, during pregnancy, and without a doctor's clearance for cardiovascular, seizure, or severe panic conditions. The mechanism behind each rule, with sources.
The Wim Hof Method is thirty to forty deep breaths in and out, then a full exhale and breath retention until the urge to breathe returns, then a recovery inhale held for about fifteen seconds. Three or four rounds make a session. The published evidence base is small but real. Here is what the protocol is, what the research supports, and who should skip it.