Natural hot spring waters spouting from wellheads are essentially reductive in terms of oxidation-reduction potential (ORP); however, it was found that these ORP values increase over time due to oxidation after surfacing. Furthermore, it was revealed that, because its exposure upon bathing, the skin was the most susceptible to be affected. Therefore, it has been proposed that habitual bathing in fresh reductive hot spring water can suppress skin oxidation and contribute to prevent the aging process thereupon. In this study, as a facile evaluation method of the reductive ability of natural hot spring waters, instead of conducting ORP-pH measurements, the reductive ability of hot spring water was evaluated by redox titration with iodine contained in a well-known and commercially available gargling agent (Isodine®; povidone iodine). For didactic purposes, experiments evaluating the effects of fresh hot spring waters on preventing melanin formation were carried out by visualizing their effects on freshly peeled apples and potatoes that normally develop a brownish discoloration upon exposure to air, which is indicative of melanin formation. The second part of this study aims at a practical implementation of a simpler means for producing reductive hydrogen-rich bathwater. The effects of hydrogen produced by magnesium hydride (MgH2), a bath salt, on the skin were examined. Results from the present study revealed that home-use bathwater can be changed from an oxidative state to a reductive state by the production of reductive hydrogen from the reaction oMgH2 with water. Furthermore, it was found that natural hot spring water (alkaline, untreated hot spring water) in large-sized bathtubs of the spa facilities of Ashino Onsen can also be made more reductive by periodically adding MgH2thereto. The elasticity coefficients of the skin (flexural side of forearm) of volunteers in the habitual bathing experiments showed that a statistically significant improvement was observed in the group of in dividuals subjected to bathing in hydrogenated Ashino hot spring water. Finally, the present work proposes that the organoleptic evaluation of hot spring waters, such as the development of a smooth or soapy sensation on the skin, can be quantitatively estimated by a balance of concentrations between the alkaline components Ca and Mg
