Horai-yu is a strongly alkaline simple sulfur spring (pH 10.1) — sharply slippery on the skin, sitting squarely in the Japanese 'beauty-water' family. At the wellhead the water carries a faint rotten-egg note of H₂S and a mild salty tang. Mineralization is light at 0.284 g/kg, so the character comes from the sulfur fraction (HS-, thiosulfate) and the high pH rather than from bulk solutes. Classified as a low-temperature spring (31.8 °C) heated for bathing.
Good for
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
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⁺77.5mgP24 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.7mgP6 national2%
- Potassiumminor; matters for drinking limitsK⁺1.4mgP17 national1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻69.0mgP32 national39%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻42.0mgP88 national28%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻33.6mgP19 national11%
- HSiO3HSiO₃⁻35.4mgP75 national9%
- Fluoridetrace; drinking limits applyF⁻7.0mgP85 national7%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.7mgP23 national5%
- Thiosulfateoxidized sulfurS₂O₃²⁻3.5mgP91 national
- HydroxideOH⁻2.0mgP95 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻1.9mgP79 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃10.1mgP2 national
- CadmiumCdNDmg
- 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.
- 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.
- 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