An acidic, sulfur-bearing spring at Manza: pH 2.4, sharp enough to prickle on the skin, with magnesium and sodium leading a sulfate-dominated water of moderate mineralization. At the wellhead it runs colorless and clear but carries white floating flecks and an unmistakable rotten-egg note of hydrogen sulfide. It is among Japan's hottest natural sources, and the posted analysis states plainly that water may be added so the bath is comfortable to enter. Metasilicic acid at 118 mg/kg leaves a thin silica film, and the bathing indications lead with chronic skin conditions and cuts, the classic territory for strongly acidic sulfur water.
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
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
- Acute illness (especially with fever)
- Active tuberculosis
- Malignant tumor
- Severe heart disease
- Respiratory failure
- Renal failure
- Bleeding disorders
- Severe anemia
- Any other illness in an actively progressing stage
- Pregnancy (especially the early and late stages)
- People with sensitive skin or mucous membranes, especially those with photosensitivity
- Dry skin in the elderly
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.
- Magnesiummild; a laxative if you drink itMg²⁺98.8mgP92 national39%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺129mgP36 national27%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺4.3mgP76 national20%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.7mgP48 national8%
- Potassiumminor; matters for drinking limitsK⁺22.6mgP67 national3%
- Aluminumshows up in acidic volcanic waterAl³⁺3.9mgP83 national2%
- Manganesetrace; companion to iron springsMn²⁺6.0mgP98 national1%
- Iron (II)Fe²⁺0.11mgP29 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻748mgP93 national79%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻104mgP41 national15%
- HSO4HSO4108mgP70 national6%
- FluorideF⁻1.2mgP49 national0.3%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃118mgP60 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.5mgP60 national
- H2SO4FreeH2SO4Free1.2mgP72 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S41.7mgP97 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- ArsenicAs0.01mgP29 national
- CopperCuNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
Measured at the source
On the certificate
- Diabetes
- Chronic digestive disorders
- Cold sensitivity
- Hypertension
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.
- 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