polyhydroxystearic acid
what it is
A polymer built from hydroxystearic acid, a fatty acid derived from castor oil — INCI name Polyhydroxystearic Acid. It is a dispersant: its entire job is keeping particles apart.
why we use it
It wraps each coated zinc oxide particle and stops them clumping together. This is quality control you cannot see: clumped filter particles leave microscopic gaps in coverage, and gaps are unprotected skin. The most anonymous ingredient in the sunscreen is doing some of its most important work.
where you'll find it
In our mineral sunscreen (SPF 50+ PA++++), in the emulsifiers & stabilisers group (12.25% total). Per-ingredient percentages within the sunscreen groups are not yet published; the group totals are exact.
evidence
Functional — dispersion quality is measurable, and it shows up directly in SPF test results, which is the only place it needs to show up.
safety
A large polymer that stays bound to particle surfaces, with no meaningful skin penetration; assessed as safe in cosmetic use.