熱海331号泉Atami Spring No. 331
A calcium-sodium chloride spring drawn from a hot 54.1 °C source by the sea. Richly mineralized at 19.14 g/kg dissolved solids, it is a dense, salt-forward chloride water that leaves a mineral film on the skin and keeps the body warm long after the bath. Weakly alkaline, so it bathes soft on the skin. At the bath it is diluted with water and disinfected with monochloramine.
Good for
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
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
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⁺3,629mgP100 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺3,268mgP94 national
- Magnesiummild; a laxative if you drink itMg²⁺135mgP93 national
- Potassiumminor; matters for drinking limitsK⁺36.6mgP79 national
- Manganesetrace; companion to iron springsMn²⁺1.5mgP86 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10,570mgP97 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,365mgP98 national
- Bromidetrace, sea-originBr⁻32.5mgP95 national
- Iodideyellows in air; helps blood lipidsI⁻0.20mgP52 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃50.0mgP29 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂13.5mgP63 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.9mgP47 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.
- 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.