Polysilicone-15

Polysilicone-15 (trade name: Parsol SLX) is a silicone-based UVB filter that soaks up UVB light (~280-320nm) with a peak around 310nm.

It's a fairly modest sunscreen filter so formulators mostly use it as an SPF booster and "light stabilizer". This just means it helps keep other light-sensitive ingredients from breaking down in sunlight.

A 2007 study measured the in-vitro SPF of 18 filters and found Polysilicone-15 contributed an SPF of about 3.64 at its maximum legal concentration (though worth noting its maker points out that in-vitro tests tend to underestimate its real performance because of its large polymeric structure).

Usage levels vary; it's approved up to 10% as a UV filter in the EU, China, Japan, Australia, ASEAN, and Mercosur countries. In the US, it's only permitted up to 1% as a light stabilizer because the FDA hasn't reviewed it as a sunscreen active.

Typical use levels range from 1-3%.

The EU's scientific committee concluded that this ingredient is safe for use as a UV absorber up to 10%, and tests came back negative for irritation, phototoxicity, and photoallergy.

There is one case of allergic lip inflammation from a lip balm with this ingredient, though this is rare.