The source feeding the Kawarayu communal bathhouse in Zao Onsen, Yamagata: a strongly acidic, sulfur-bearing aluminium-sulfate-chloride water. It is acidic enough to prickle on contact — the sharply acidic style associated with stubborn skin conditions — and carries the unmistakable rotten-egg note of H₂S at the spout, with an astringent tang if tasted. Moderately mineralized with noticeable body, it also holds a generous 195 mg/kg of metasilicic acid, which tempers the bite with a smoothing silica film. The posted composition notice records the water reaching the bath at the same temperature as the source, and the certificate's sample was drawn in the bathhouse tub itself rather than at a wellhead.
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
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, netto 熱の湯).
cold sensitivity
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.
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺12.7mgP94 national36%
- Aluminumshows up in acidic volcanic waterAl³⁺77.9mgP97 national25%
- Magnesiummild; a laxative if you drink itMg²⁺71.5mgP89 national17%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺96.6mgP72 national14%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺54.0mgP15 national7%
- Potassiumminor; matters for drinking limitsK⁺19.6mgP62 national1%
- Iron (II)Fe²⁺8.1mgP86 national0.8%
- ManganeseMn²⁺2.7mgP93 national0.3%
- ZincZn²⁺0.20mgP84 national0.0%
- LithiumLi⁺0.03mgP14 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻993mgP95 national61%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻300mgP64 national25%
- HSO4HSO4421mgP91 national13%
- Fluoridetrace; drinking limits applyF⁻6.6mgP84 national1%
- BromideBr⁻1.0mgP59 national0.0%
- IodideI⁻0.20mgP51 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃195mgP84 national
- H2SO4FreeH2SO4Free13.4mgP92 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP33 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂571mgP97 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S12.1mgP93 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.
- 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