Inert earth mineral used as a thickening, lubricating, and sunscreen ingredient in cosmetics. Along with titanium dioxide, zinc oxide is considered to have no risk of skin sensitization. It can also be a soothing agent and, potentially, an antioxidant. Nano-sized zinc oxide is not believed to be a safety concern for skin.
References for this information:
Particle and Fibre Toxicology, August 2016, page 44
Indian Journal of Dermatology, September-October 2012, pages 335-342
Photodermatology, Photoimmunology, & Photomedicine, April 2011, pages 58-67