A sodium-calcium chloride spring — the Kawara-no-yu source of Shima Onsen in Nakanojo, Gunma — with sulfate close behind as a second anion, adding quiet body to the water. Neutral in pH and hypotonic, it feels soft rather than sharp, while roughly 2.0 g/kg of dissolved solids gives it genuine mineral weight and good heat retention. Metasilicic acid at 142 mg/kg places it squarely in Japan's beauty-water family, leaving the kind of silica film that makes skin feel noticeably smoother. At the wellhead the water is colorless and clear apart from a few faint brown flecks in suspension.
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, netto 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⁺467mgP75 national69%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺150mgP81 national25%
- Potassiumminor; matters for drinking limitsK⁺58.0mgP86 national5%
- MagnesiumMg²⁺1.3mgP27 national0.4%
- ManganeseMn²⁺0.71mgP72 national0.1%
- StrontiumSr²⁺0.46mgP61 national0.0%
- Iron (II)Fe²⁺0.32mgP43 national0.0%
- AluminumAl³⁺< —mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻695mgP78 national67%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻420mgP82 national30%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻46.7mgP27 national3%
- BromideBr⁻4.5mgP80 national0.2%
- FluorideF⁻0.70mgP39 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃142mgP67 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂49.8mgP88 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂30.8mgP61 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.6mgP97 national
- CopperCuNDmg
- 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.