A simple sulfur spring in the Toyosato district of Nozawaonsen village, sampled at the hot-water distribution box rather than at the wellhead. The water runs colorless and clear, yet it announces itself with a moderate rotten-egg note of H₂S and a distinctly sulfurous taste — hydrogen sulfide ion measures 38.9 mg/kg. Weakly alkaline and only lightly mineralized at 817 mg/kg dissolved solids, it feels soft rather than sharp on the skin. A generous 102.1 mg/kg of metasilicic acid leaves the kind of thin silica film that puts it in Japan's beauty-water family.
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⁺177mgP45 national74%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺49.2mgP56 national24%
- PotassiumK⁺3.8mgP30 national1.0%
- AmmoniumNH₄⁺1.4mgP72 national0.8%
- StrontiumSr²⁺0.30mgP53 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻291mgP71 national56%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻89.2mgP38 national23%
- Hydrogen sulfidesulfur character — antibacterialHS⁻38.9mgP99 national11%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻49.4mgP29 national7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.2mgP43 national2%
- FluorideF⁻1.0mgP47 national0.5%
- Bromidetrace, sea-originBr⁻0.30mgP37 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃102mgP54 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.8mgP44 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.8mgP80 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- 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.
- 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