An ultra-acidic, iron-bearing simple thermal spring of the sulfate sub-type — pH 2.8 sharp enough to sting slightly on contact, with a strong astringent bite and an unmistakable metallic iron note. Total iron of 48 mg/kg oxidizes to reddish-brown on air contact, and 150 mg/kg of metasilicic acid leaves a thin silica film for noticeably smoother skin. Emerges from the well at a hot 56 C.
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⁺80.4mgP25 national37%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺1.6mgP70 national17%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺41.0mgP98 national15%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺17.5mgP37 national9%
- Aluminumshows up in acidic volcanic waterAl³⁺5.6mgP84 national7%
- Potassiumminor; matters for drinking limitsK⁺24.0mgP68 national6%
- Iron (III)Fe³⁺7.3mgP96 national4%
- Magnesiummild; a laxative if you drink itMg²⁺3.8mgP43 national3%
- LithiumLi⁺0.60mgP61 national0.9%
- ZincZn²⁺1.0mgP95 national0.3%
- ManganeseMn²⁺0.70mgP72 national0.3%
- AmmoniumNH₄⁺0.50mgP49 national0.3%
- StrontiumSr²⁺0.10mgP26 national
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻379mgP80 national79%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻72.5mgP33 national20%
- FluorideF⁻1.8mgP56 national0.9%
- BromideBr⁻0.30mgP36 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- HydroxideOH⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃150mgP68 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.3mgP39 national
- Metaarsenious acidHAsO₂1.8mgP94 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- ChromiumCrNDmg
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