Gentle simple thermal water drawn from the Tsuru-no-yu source at Hirayu, in the Okuhida hot-spring district of Takayama. It is near-neutral and lightly mineralized — 622 mg/kg of dissolved solids — so it asks nothing of the skin, yet it carries 99 mg/kg of metasilicic acid, enough silica to leave the smooth, slightly softened feel that puts it in Japan's beauty-water family. The water is colorless, transparent and tasteless, but a faint hydrogen sulfide note rises at the spout. The certificate was sampled at the bath inlet rather than at the wellhead, so these figures describe the water as it actually reaches the tub.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺136mgP37 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.4mgP29 national9%
- Potassiumminor; matters for drinking limitsK⁺17.1mgP57 national6%
- Magnesiummild; a laxative if you drink itMg²⁺2.5mgP37 national3%
- LithiumLi⁺0.70mgP63 national1%
- StrontiumSr²⁺0.10mgP24 national0.0%
- 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
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻146mgP49 national54%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻145mgP51 national31%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻48.1mgP40 national13%
- FluorideF⁻1.0mgP46 national0.7%
- NitrateNO₃⁻0.60mgP69 national0.1%
- BromideBr⁻0.70mgP52 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃99.0mgP54 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.1mgP65 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂54.0mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- ArsenicAs0.0040mgP12 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.