塩の湯Shio-no-yu Spring
Shio-no-yu is a free-flowing source at Shojukan in Nakanojo, Gunma. The water is a sodium-calcium chloride spring with sulfate adding quiet body, weakly alkaline and soft against the skin. At the wellhead it carries only the faintest salty tang, and with 89.3 mg/kg of metasilicic acid it belongs to Japan's beauty-water family — enough silica to leave skin feeling noticeably smoother. Moderately mineralized at 1,522 mg/kg of dissolved solids, it has real substance without heaviness.
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⁺311mgP63 national60%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺166mgP83 national37%
- Potassiumminor; matters for drinking limitsK⁺20.9mgP64 national2%
- Magnesiummild; a laxative if you drink itMg²⁺2.8mgP38 national1%
- AluminumAl³⁺0.10mgP54 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- AmmoniumNH₄⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻503mgP75 national65%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻297mgP72 national28%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻93.4mgP42 national7%
- BromideBr⁻3.2mgP78 national0.2%
- FluorideF⁻0.60mgP36 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃89.3mgP50 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂33.9mgP82 national
- Metaarsenious acidHAsO₂1.4mgP91 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.7mgP30 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —mg
- CopperCu< —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.
- 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.