Kamiyu Onsen is a sodium-bicarbonate spring with a notable iron(II) signature — the jūsō-sen softness of an Na-HCO3 base, overlaid with the rusty character of dissolved iron that oxidizes reddish-brown on contact with air. The water emerges at 53.6 °C, classified as a hot spring, and reaches the bath at the same temperature without cooling. Weakly alkaline at pH 7.7, it carries the soft, skin-smoothing feel of the bicarbonate family alongside a metallic edge from the iron content.
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.
- Aluminumshows up in acidic volcanic waterAl³⁺—mg
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺—mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺—mg
- Potassiumminor; matters for drinking limitsK⁺—mg
- LithiumLi⁺—mg
- Magnesiummild; a laxative if you drink itMg²⁺—mg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺—mg
- AmmoniumNH₄⁺—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻—mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻—mg
- Fluoridetrace; drinking limits applyF⁻—mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻—mg
- Hydrogen phosphateHPO₄²⁻—mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻—mg
- NitrateNO₃⁻—mg
- HydroxideOH⁻—mg
- Thiosulfateoxidized sulfurS₂O₃²⁻—mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃—mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂—mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂—mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S—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.