Okazakiya Spring is a strongly acidic water — pH 2.0, sharp enough to prickle on contact — carrying sulfur, aluminium, sulfate and chloride in the concentrated style typical of Zao. Free hydrogen sulfide at 14 mg/kg gives it the characteristic rotten-egg note, while sulfate lends quiet body and chloride a light salty edge, all at a moderate 2,406 mg/kg of dissolved solids. Metasilicic acid runs to 267.8 mg/kg, leaving the kind of thin silica film that makes skin feel noticeably smoother once the acid bite fades. The posted sample was drawn at the bath rather than the wellhead, so the figures describe the water as it is served.
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⁺90.9mgP97 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺83.3mgP67 national
- Magnesiummild; a laxative if you drink itMg²⁺70.6mgP88 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺50.5mgP14 national
- Potassiumminor; matters for drinking limitsK⁺17.8mgP59 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺11.0mgP93 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺5.9mgP82 national
- Manganesetrace; companion to iron springsMn²⁺3.4mgP95 national
- ZincZn²⁺0.20mgP84 national
- LithiumLi⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,089mgP96 national
- HSO4HSO4402mgP89 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻283mgP62 national
- Fluoridetrace; drinking limits applyF⁻12.0mgP94 national
- Bromidetrace, sea-originBr⁻0.80mgP54 national
- Iodideyellows in air; helps blood lipidsI⁻0.50mgP69 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃268mgP96 national
- H2SO4FreeH2SO4Free11.1mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.1mgP37 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂380mgP94 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S14.0mgP96 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