Sunscreens can block up to 55% of the harmful free radicals from UV exposure. This is where the other preventative products come in. SkinCeuticals antioxidants are clinically proven to provide the most advanced protection from environmental aging. When used together, SkinCeuticals antioxidants and suncreens provide the ultimate protection - preventing the visible signs of photo-aging, and promoting optimal skin health. You'll feel the difference!