A strongly acidic, sulfur-bearing spring at Zao Onsen: aluminium and sulfate lead the mineral profile, with a chloride edge alongside. The water is colorless and clear but tastes sharply astringent and carries an unmistakable rotten-egg note of H₂S — acidic enough to prickle on contact with the skin. A generous 223.1 mg/kg of metasilicic acid leaves a smoothing silica film once the sting fades. The house guide printed for guests warns bathers to remove all jewellery and metal before soaking, since this water corrodes and discolours metal.
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⁺12.4mgP94 national34%
- Aluminumshows up in acidic volcanic waterAl³⁺95.3mgP98 national30%
- Magnesiummild; a laxative if you drink itMg²⁺66.3mgP88 national15%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺84.0mgP67 national12%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺52.4mgP14 national6%
- Potassiumminor; matters for drinking limitsK⁺20.2mgP63 national1%
- Iron (II)Fe²⁺6.6mgP83 national0.7%
- ManganeseMn²⁺3.1mgP94 national0.3%
- ZincZn²⁺0.20mgP84 national0.0%
- LithiumLi⁺0.03mgP14 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,014mgP95 national62%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻283mgP63 national23%
- HSO4HSO4420mgP91 national13%
- Fluoridetrace; drinking limits applyF⁻12.6mgP95 national2%
- IodideI⁻1.2mgP83 national0.0%
- BromideBr⁻0.80mgP55 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃223mgP90 national
- H2SO4FreeH2SO4Free13.1mgP91 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.6mgP38 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂369mgP93 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S14.0mgP95 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