The Ōmiya No. 3 source at Zaō is a strongly acidic spring — at pH 1.7 it is sharp enough to prickle on contact — and it carries enough free hydrogen sulfide to count as a sulfur-bearing water, with aluminium, sulfate and chloride giving the bath its body. Moderately mineralized at 2,894 mg/kg of dissolved solids, it also delivers a generous 175.8 mg/kg of metasilicic acid, the kind of thin silica film that leaves skin feeling noticeably smoother afterwards. The sample was taken at the bath inlet rather than at the wellhead, so these figures describe the water as it actually reaches the tub.
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⁺96.0mgP72 national
- Aluminumshows up in acidic volcanic waterAl³⁺95.8mgP98 national
- Magnesiummild; a laxative if you drink itMg²⁺54.5mgP85 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺53.9mgP14 national
- Potassiumminor; matters for drinking limitsK⁺23.3mgP68 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺20.1mgP99 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺6.8mgP84 national
- Manganesetrace; companion to iron springsMn²⁺2.7mgP93 national
- ZincZn²⁺0.20mgP84 national
- LithiumLi⁺0.02mgP12 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,194mgP98 national
- HSO4HSO4802mgP98 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻312mgP66 national
- Fluoridetrace; drinking limits applyF⁻11.7mgP94 national
- Bromidetrace, sea-originBr⁻1.8mgP71 national
- Iodideyellows in air; helps blood lipidsI⁻0.20mgP51 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃176mgP78 national
- H2SO4FreeH2SO4Free40.4mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP26 national
- Phosphoric acidH₃PO₄0.40mgP100 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂397mgP94 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S8.0mgP90 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