An alkaline simple thermal spring with a notably high pH of 9.5 — strong enough to give the unmistakable slippery, skin-smoothing feel of the Japanese beauty-water family. Dissolved solids are light at 170 mg/kg, but a faint hydrogen sulfide note and a fluoride content above the therapeutic-spring threshold give the water more character than the simple-thermal label suggests.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺45.7mgP12 national96%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.4mgP5 national3%
- PotassiumK⁺0.30mgP3 national0.4%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- LithiumLi⁺NDmg
- Magnesiummild; a laxative if you drink itMg²⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- StrontiumSr²⁺NDmg
- ZincZn²⁺NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.8mgP77 national46%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻25.4mgP15 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.9mgP25 national14%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.3mgP8 national8%
- Fluoridetrace; drinking limits applyF⁻2.6mgP64 national7%
- HydroxideOH⁻0.60mgP78 national2%
- Hydrogen sulfideHS⁻0.10mgP28 national0.2%
- Bromidetrace, sea-originBr⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃44.1mgP24 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.90mgP12 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.