People oxidize and rust, just like metals rust, including the skin as they age. Meanwhile, reduction, the opposite of oxidation, has an aging control effect that reduces or resets rusting. A common characteristic of hot springs is this reduction action that resets rusting. The effect of reducing oxidation or aging can be expected when we bathe in such a hot spring that gives great benefits to the skin.
The effect of the reduction action of hot springs decreases through a lapse of time after welling up from the source due to oxidation by the oxygen in the air, just like people,  This phenomenon is called the aging of hot springs. Because of this, freshness is important for hot springs. Extracted from “Ikiteiru Onsen Towa Nanika” (What is a living hot spring?) by Shoichi Okouchi