Japan has over 3,000 onsen resorts producing spring water of dramatically different chemical compositions. The minerals determine the color, smell, feel, and therapeutic benefits of each spring. Here's how to understand what you're bathing in.
Simple Alkaline Springs (単純温泉)
The most common onsen type — water with low mineral content but slightly alkaline pH. The defining characteristic: after bathing, skin feels extraordinarily smooth (bijin-no-yu, "beauty water"). The alkalinity gently dissolves keratin proteins on the skin's surface. No distinct smell or color. Found throughout Japan; Hakone's many springs are predominantly this type. Suitable for all skin types; recommended for first-time onsen visitors.
Sulfur Springs (硫黄泉)
Unmistakable by their rotten-egg smell (hydrogen sulfide) and milky-white water. The sulfur deposits turn water cloudy and coat bathing surfaces in a yellow-white film. Therapeutic reputation: reduces blood pressure, opens airways, treats skin conditions. Best examples: Kusatsu Onsen (Gunma) — Japan's most celebrated sulfur spring, so acidic it can dissolve small metal items; Noboribetsu (Hokkaido) — the dramatic Jigokudani (Hell Valley) steam vents feed multiple spring types including strong sulfur. Not recommended for those with metal implants or sensitivity to sulfur compounds.
Iron Springs (含鉄泉)
Water rich in iron compounds produces a distinctive rusty-orange color and a metallic taste. Contact with air causes rapid oxidation — baths may appear different colors depending on fresh water input. Therapeutic reputation: addresses iron-deficiency related conditions, improves circulation. Best examples: Arima Onsen in Kobe (kinsen, "gold spring" — actually iron-orange color, Japan's most famous iron spring); Tamagawa Onsen in Akita. The visual drama of a rust-red bath is striking and distinctive.
Sodium Chloride Springs (食塩泉)
High salt content produces water that heats the body more efficiently (salt retains heat in the skin post-bath, reducing post-onsen chill). The warm-retention effect is why these springs are popular in winter onsen destinations. No strong color or smell. Therapeutic reputation: cardiovascular benefits, skin hydration. Gero Onsen (Gifu) — one of Japan's three great onsen — has excellent sodium chloride springs.
Acidic Springs (酸性泉)
Very low pH (sometimes pH 1–2, more acidic than lemon juice). The acidity has strong antibacterial properties and is traditionally used for skin conditions. Kusatsu's main spring is strongly acidic; bathing time is limited to 3 minutes (timed bathing, jikan-yu). Sensations during bathing: mild tingling. Not recommended for those with sensitive skin or open wounds.
Choosing Your Onsen Type
For smooth skin: alkaline simple springs (Hakone, Kinosaki). For drama and experience: sulfur springs (Kusatsu, Noboribetsu). For winter warmth retention: sodium chloride springs (Gero, Atami). For therapeutic skin treatment: acidic springs (Kusatsu). For visual impact: iron springs (Arima). Most resort towns have multiple spring types — Beppu alone has eight distinct spring categories.