Named simply after the cave-bath itself, Tsubo-yu is a sulfur-bearing sodium bicarbonate-chloride spring — the soft, skin-smoothing 'jūsō-sen' bicarbonate frame layered with a salty, heat-retaining chloride finish, all carried on a quiet H₂S note. At pH 6.9 it sits in neutral territory; the water emerges colourless with a faint rotten-egg whisper and a mild sulfur taste on the lips. Metasilicic acid is generous at 95.5 mg/kg, leaving the kind of fine silica film that makes skin feel noticeably smoother afterwards. The source rises naturally at 53.5 °C and feeds the small rock-pool tub that gives the spring its name along the Kumano Kodō pilgrim route.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺426mgP72 national90%
- Potassiumminor; matters for drinking limitsK⁺24.6mgP69 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺24.8mgP43 national
- Magnesiummild; a laxative if you drink itMg²⁺2.5mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻793mgP86 national67%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻200mgP54 national29%
- Fluoridetrace; drinking limits applyF⁻8.4mgP88 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.70mgP63 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃95.5mgP54 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.5mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂71.5mgP74 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.70mgP74 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