A sodium chloride spring carrying dissolved sulfur — classed as a sulfur-bearing sodium-chloride water. At pH 9.1 it is alkaline with the slippery beauty-water feel on the skin, and the dissolved hydrogen sulfide gives it a sulfurous taste and the characteristic rotten-egg note. A hot source (47.8 °C) of moderate mineralization (about 3,900 mg/kg dissolved solids) with noticeable body.
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
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺1,367mgP89 national96%
- Magnesiummild; a laxative if you drink itMg²⁺16.5mgP68 national1%
- LithiumLi⁺6.3mgP96 national1%
- AmmoniumNH₄⁺18.1mgP97 national0.7%
- StrontiumSr²⁺2.9mgP87 national0.2%
- BariumBa²⁺1.4mgP92 national0.0%
- CalciumCa²⁺0.30mgP1 national0.0%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,812mgP85 national79%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻124mgP99 national6%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻235mgP60 national6%
- Hydrogen phosphateHPO₄²⁻145mgP99 national5%
- Fluoridetrace; drinking limits applyF⁻27.8mgP99 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻24.3mgP99 national1%
- BromideBr⁻4.3mgP79 national0.1%
- HydroxideOH⁻0.30mgP74 national0.0%
- IodideI⁻0.30mgP62 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂85.8mgP95 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃33.1mgP15 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0009mgP56 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —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.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official