The bath is fed by the Zao Onsen south-district supply cooperative, which pools six separate wells into a single blended line. What arrives is a sharply acidic, sulfur-bearing aluminium sulfate-chloride water — acidic enough to prickle on contact, with a pronounced astringent bite if tasted and an unmistakable rotten-egg note of H₂S coming off the surface. It still runs colorless and clear, and carries a generous 226.5 mg/kg of metasilicic acid, the kind of silica load that leaves a thin film and noticeably smoother skin. The house notice states that no water is added to the source; only in winter, for temperature control, may it be recirculated and reheated.
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, nettō 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.
- Aluminumshows up in acidic volcanic waterAl³⁺108mgP99 national34%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺8.8mgP86 national25%
- Magnesiummild; a laxative if you drink itMg²⁺73.1mgP89 national17%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺94.9mgP72 national14%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺57.6mgP16 national7%
- Potassiumminor; matters for drinking limitsK⁺22.1mgP66 national2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺10.7mgP88 national1%
- ManganeseMn²⁺3.2mgP95 national0.3%
- ZincZn²⁺0.20mgP84 national0.0%
- LithiumLi⁺0.03mgP14 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,153mgP97 national66%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻287mgP62 national22%
- HSO4HSO4338mgP87 national10%
- Fluoridetrace; drinking limits applyF⁻14.4mgP96 national2%
- IodideI⁻1.2mgP82 national0.0%
- BromideBr⁻0.90mgP57 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃227mgP91 national
- H2SO4FreeH2SO4Free7.4mgP88 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.8mgP41 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂265mgP91 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S11.3mgP93 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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