What it is
Onsen-sui is naturally sourced hot spring water that carries dissolved minerals characteristic of its geothermal source. In cosmetics it is used in place of, or alongside, purified water as a mineral-bearing aqueous component with a conditioning, soothing positioning. It appears in Japanese sun gels and essences as part of the water phase.
Safety & warnings
Hot spring water used in cosmetics is generally very well tolerated and is often positioned as suitable for sensitive skin. Mineral content varies by source.
Origin & sourcing
Natural; geothermally heated mineral-bearing water drawn from Japanese hot springs (onsen).
History
Bathing in onsen has a long cultural tradition in Japan, and the perceived skin benefits of mineral spring water carried over into the use of hot spring water in cosmetic products.
Questions everyone asks
What is onsen-sui?
It is Japanese hot spring water containing naturally dissolved minerals, used as a mineral-rich aqueous base in cosmetics.
How is it different from regular water in a formula?
Unlike purified water, onsen-sui carries source-specific dissolved minerals and is used for a conditioning, soothing positioning.