Hyakurakusen is an alkaline simple thermal water (pH 8.52) sitting in Japan's beauty-water family — soft on the skin from the high pH alone, even without strong mineralization. Sodium dominates the cations and chloride and bicarbonate sit at nearly equal mval shares on the anion side. Metasilicic acid at 61.5 mg/kg adds a thin silica film for a noticeably smoother finish. The source emerges at 31.8 °C, a cold-mineral by the temperature axis (though classified as a low-temperature spring on the cert), and is reheated for bathing.
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⁺127mgP36 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.8mgP19 national5%
- Potassiumminor; matters for drinking limitsK⁺5.9mgP36 national2%
- Magnesiummild; a laxative if you drink itMg²⁺0.80mgP24 national1%
- LithiumLi⁺0.09mgP19 national
- ZincZn²⁺0.07mgP68 national
- AmmoniumNH₄⁺0.05mgP13 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.03mgP25 national
- CopperCu²⁺0.03mgP81 national
- Iron (III)Fe³⁺0.03mgP52 national
- Manganesetrace; companion to iron springsMn²⁺0.03mgP19 national
- StrontiumSr²⁺0.02mgP11 national
- BariumBa²⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻135mgP47 national61%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻134mgP48 national35%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻2.5mgP24 national1%
- FluorideF⁻1.1mgP47 national1.0%
- SulfateSO₄²⁻2.5mgP11 national0.8%
- BromideBr⁻0.40mgP41 national0.2%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃61.5mgP37 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.1mgP19 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.70mgP15 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- 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.