A simple sulfur spring carrying the unmistakable rotten-egg note of H₂S, weakly acidic and hypotonic. It is unusually rich in metasilicic acid at 173 mg/kg, which leaves a thin silica film that makes skin feel noticeably smoother afterwards. A moderate 264 mg/kg of free CO₂ adds a faint freshness to the feel of the water.
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
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⁺66.8mgP21 national73%
- Potassiumminor; matters for drinking limitsK⁺15.7mgP54 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.4mgP22 national9%
- Magnesiummild; a laxative if you drink itMg²⁺2.6mgP37 national6%
- LithiumLi⁺0.30mgP44 national1%
- ManganeseMn²⁺0.20mgP45 national0.3%
- StrontiumSr²⁺0.06mgP15 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻116mgP46 national50%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻54.5mgP41 national30%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻26.1mgP19 national19%
- ThiosulfateS₂O₃²⁻1.4mgP78 national0.8%
- FluorideF⁻0.50mgP32 national0.5%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃173mgP77 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂264mgP91 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.80mgP75 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.
- 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