Sharp, prickly acidic water from the Takamiya No. 2 source at Zao Onsen, classified on the certificate as an acidic, sulfur-bearing sulfate-chloride spring. The acidity is strong enough to sting slightly on contact, and free hydrogen sulfide adds the unmistakable rotten-egg note of a sulfur-bearing spring. Moderately mineralized at roughly 2,600 mg/kg of dissolved solids, it has noticeable body without the heaviness of a brine. Metasilicic acid at 204.8 mg/kg leaves the kind of thin silica film that makes skin feel smoother afterwards. The sample behind these figures was drawn at the bath inlet rather than the 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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺96.8mgP72 national
- Aluminumshows up in acidic volcanic waterAl³⁺88.4mgP97 national
- Magnesiummild; a laxative if you drink itMg²⁺75.9mgP89 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺60.4mgP17 national
- Potassiumminor; matters for drinking limitsK⁺25.2mgP70 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺15.9mgP96 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺11.8mgP90 national
- Manganesetrace; companion to iron springsMn²⁺3.7mgP96 national
- ZincZn²⁺0.30mgP87 national
- LithiumLi⁺0.04mgP15 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,072mgP96 national
- HSO4HSO4572mgP96 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻343mgP69 national
- Fluoridetrace; drinking limits applyF⁻23.3mgP99 national
- Bromidetrace, sea-originBr⁻0.70mgP52 national
- Iodideyellows in air; helps blood lipidsI⁻0.20mgP51 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃205mgP86 national
- H2SO4FreeH2SO4Free22.9mgP95 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.40mgP7 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂336mgP92 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S5.5mgP85 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