Polymethylsilsesquioxane

Polymethylsilsesquioxane is an inert, film-forming silicone powder used in makeup and skincare (often a few percent to ~10%+) to improve slip, blur, and wear; it is not chemically reactive and is generally non-sensitizing in patch testing. True irritation is uncommon, but in severely compromised skin the occlusive/film-forming effect and mechanical friction from powdery textures can occasionally aggravate dryness or stinging, so it is best classified as exceptionally gentle rather than perfectly inert. Safety Notes: Polymethylsilsesquioxane is used as a silicone resin microsphere/texturizer for slip, soft-focus, and oil-absorption; in many emulsions, sunscreens, and primers it appears at low levels (~0.05–1%) for feel and finish. In high-performance makeup/primer and blurring powder-cream hybrids available OTC, it can be pushed into the mid-to-high single digits and into double digits (typically ~5–15%) and, in extreme silicone-resin heavy anhydrous/primer systems, up to about 20–25% while maintaining spreadability. Rinse-off products generally sit toward the low end because the sensorial benefit is less persistent, whereas leave-on complexion products and sunscreens most commonly reach the upper end.