Hirayu Onsen

Sulfuric hot springs are among the most studied in Japanese balneology. The sulfur compounds — primarily hydrogen sulfide and thiosulfate — have documented antimicrobial and anti-inflammatory properties. Regular bathing is associated with relief from chronic skin conditions including eczema and psoriasis, as well as joint inflammation and muscle soreness. Sulfuric waters have been prescribed in Japanese medical practice since the Edo period.

The distinctive rotten-egg smell dissipates quickly after leaving the bath. Avoid if you have a sulfur allergy, very sensitive skin, or respiratory conditions. Remove silver jewellery before entering — sulfur will blacken it permanently.

Bicarbonate springs (sodium bicarbonate, calcium bicarbonate, or hydrogen carbonate) are particularly effective for skin conditions. The bicarbonate ions cleanse and soften the skin surface, removing excess sebum without stripping the skin's acid mantle. These springs are traditionally recommended for acne-prone skin and as a gentle option for sensitive skin types. The water has a characteristically clean, soft feel.

Bicarbonate springs are generally among the most gentle and well-tolerated onsen types. Those with very dry skin may wish to apply moisturiser after bathing, as the cleansing effect can temporarily reduce surface oils.

Sodium chloride springs — essentially natural saltwater baths — are celebrated for their warming and moisturising effects. The salt forms a thin film on the skin after bathing that slows moisture evaporation, keeping skin hydrated longer than a freshwater bath. This "heat-retaining" property means bathers stay warm for significantly longer after leaving the water, making these springs especially popular in winter. Salt springs are among the most accessible for first-time onsen visitors.

Those with high blood pressure or heart conditions should consult a doctor before bathing, as the warming effect increases circulation. Avoid immersing open wounds. The salt will sting slightly in eyes — take care when submerging.

Calcium chloride springs share the heat-retaining property of sodium chloride springs but with a stronger warming effect due to the divalent calcium ion. They are prized for muscle and joint relief — the combination of heat retention and calcium's role in muscle function makes them a popular choice for athletes and those with chronic musculoskeletal complaints. The water has a slightly bitter mineral taste.

The strong warming effect means those with cardiovascular conditions, high blood pressure, or pregnancy should limit soak duration and consult a doctor if in doubt. Avoid entering immediately after vigorous exercise — let your heart rate normalise first.

Iron-bearing springs are recognised by their characteristic rust-red or amber colour and metallic taste. The iron content — primarily ferrous bicarbonate or ferric sulfate — is associated with stimulation of red blood cell production and is traditionally recommended for anaemia and fatigue recovery. The distinctive colouring comes from iron oxidising on contact with air and is not a sign of contamination.

Iron springs will stain light-coloured swimwear and towels a persistent brownish-orange. Avoid wearing white or light fabrics into the water. Those with haemochromatosis (iron overload condition) should seek medical advice before bathing.