I am just glad another time when an academic point of view totally coincides with my own previous breath exercises. We have cold wind outside and the best way to breath in the streets is like this, described with the help of Catherine Despeux, one of the authors of Encyclopedia of Taoism:
“Tŭnà is an abbreviation of the phrase tŭgù nàxīn 吐故納新, “exhaling the old and inhaling the new (breath).” This term is first found in chapter 15 of the Zhuangzi, which states: “Breathing in and out [while emitting] the sounds chuī 吹 or xū 噓, exhaling the old and inhaling the new [breath], hanging like the bear and stretching like the bird, these are only methods for longevity” (see trans. Watson 1968, 167-68).”
“Tŭnà and tŭgù nàxīn are generic terms for breathing practices meant to expel the impure and pathogenic qi from the body. Liu Gen, a third-century fangshi, is attributed with this description:“Feeding the body with the living breath (shēngqì 生氣) and exhaling the dead breath (sĭqì 死氣) allows you to subsist for a long time. When you inhale through the nose , you actually inhale the life breath. When you exhale through the mouth, you exhale the death breath.””
That is a special art to be happy with earth and air, and it is a small but not the smallest lesson I could extract using my humble achievement studying Chinese.