Shibu O-yu is a blended source — the certificate names it as a mix of the O-yu source and the Shibu Onsen combined source — and the sample was drawn at the bath spout rather than at the wellhead. Chemically it is a sodium-calcium sulfate-chloride water: sulfate gives it quiet body, chloride adds a salty, heat-retaining edge, and 1,257 mg/kg of dissolved solids puts it in the moderately mineralized band with noticeable substance. It is weakly acidic, faintly turbid pale yellow at the spout, with a metallic iron taste and a light saltiness on the lips. Metasilicic acid at 148 mg/kg leaves the kind of thin silica film that makes skin feel smoother afterwards.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺207mgP53 national52%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺109mgP75 national31%
- Aluminumshows up in acidic volcanic waterAl³⁺13.6mgP91 national9%
- Magnesiummild; a laxative if you drink itMg²⁺8.2mgP59 national4%
- Potassiumminor; matters for drinking limitsK⁺19.2mgP62 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺5.1mgP82 national1%
- LithiumLi⁺0.50mgP52 national0.4%
- ManganeseMn²⁺0.40mgP61 national0.1%
- StrontiumSr²⁺0.40mgP61 national0.1%
- AmmoniumNH₄⁺0.08mgP13 national0.0%
- ZincZn²⁺0.03mgP56 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻407mgP82 national50%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻285mgP64 national48%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻11.6mgP7 national1%
- FluorideF⁻1.3mgP52 national0.4%
- BromideBr⁻1.6mgP69 national0.1%
- IodideI⁻0.30mgP61 national0.0%
- HSO4HSO40.20mgP40 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃148mgP68 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂38.2mgP84 national
- Metaarsenious acidHAsO₂0.40mgP78 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.3mgP53 national
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.