A blended sodium chloride–sulfate spring drawing on the Ichihara No. 2 and No. 3 wells, served at a hot 51.7 °C. Weakly alkaline and soft on the skin, the chloride lead carries a quiet sulfate body — colourless and clear in the glass, with the faintest salty edge on the tongue. Sampled from the mixing tank in the bathhouse's mechanical room rather than at the wellhead, so what you bathe in is exactly what the cert measures.
Good for
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, nettō 熱の湯).
cold sensitivity
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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⁺584mgP78 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺63.3mgP62 national11%
- PotassiumK⁺3.5mgP29 national0.5%
- Aluminumshows up in acidic volcanic waterAl³⁺0.01mgP19 national
- FeTotalFe0.0030mgP10 national
- Manganesetrace; companion to iron springsMn²⁺0.0010mgP14 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻765mgP79 national72%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻336mgP77 national23%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻15.6mgP59 national2%
- Fluoridetrace; drinking limits applyF⁻7.9mgP87 national1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃37.6mgP20 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.3mgP44 national
- Metaarsenious acidHAsO₂0.80mgP89 national
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.