Wim Hof explains how to breathe during ice baths: set an intention, then use long, deliberate exhalations while fully immersed. Avoid rapid, anxious breathing. About a minute of prolonged out-breaths lets your body adapt quickly to the cold.
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How should I breathe when I do cold exposure? Could be a plunge, could be immersion, or whatever the name. Icy water is having an effect directly on the lungs, by which it happens. Before you go in, say to yourself, "I'm gonna do long out breaths when I go in." And when I'm fully immersed, I keep on doing the long out-breath until I feel the body is completely adapted. The body knows what the body is capable of. Only if you are anxiously breathing, you are blocking its mechanisms. So, then it Doesn't happen in the body. But if you stay intentionally in long out-breath— And it's hard enough, the exercise. For 1 minute, then suddenly the body has completely adapted to the icy water. Thus, it has become strong Inside. Long out-breaths.