Hokobo Spring is a radon-bearing, weakly radioactive simple thermal water (hypotonic, alkaline, low-temperature). Mineralization is light at 224.5 mg/kg — gentle on the body — and the pH of 8.7 puts it squarely in Japan's alkaline beauty-water family with a slippery feel on the skin. At the source the water is colorless and clear with a faint rotten-egg note from hydrogen sulfide, and radon at 144 Bq/kg places it in the so-called hormesis tradition.
Good for
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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
From a certificate analyzed in 2023.
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, severe anemia or other marked physical weakness
- Severe heart or lung disease causing shortness of breath on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Acute exacerbation of a chronic illness
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⁺53.8mgP14 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.0mgP26 national17%
- Magnesiummild; a laxative if you drink itMg²⁺1.6mgP31 national4%
- Potassiumminor; matters for drinking limitsK⁺2.1mgP22 national2%
- CopperCu²⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻44.1mgP25 national40%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻72.7mgP38 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.5mgP27 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.2mgP42 national8%
- Fluoridetrace; drinking limits applyF⁻2.4mgP62 national4%
- Hydrogen sulfidesulfur character — antibacterialHS⁻1.1mgP70 national1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃9.6mgP1 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.4mgP37 national
- ArsenicAs< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2023.
- Chronic muscle or joint pain or stiffness (the chronic phase of osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like)
- Cold sensitivity
- Peripheral circulation disorders
- Reduced gastrointestinal function (bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Hypertension (bathing)
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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.