In its usual form, zinc oxide scatters ultraviolet and visible light. Scattering is a process in which a particle absorbs electromagnetic radiation, including light, and then emits it without undergoing a chemical reaction. Scattering is not the same as reflection. In reflection, light bounces off an object without being absorbed. Zinc oxide does reflect light in addition to scattering it, however.
Zinc oxide acts as a barrier to dangerous UV radiation because it sends it back into the atmosphere by scattering and reflection instead of allowing it to enter the skin. It’s considered to be a physical sunblock instead of one that reacts chemically after light exposure.
Since zinc oxide reflects light so well, it often produces a white appearance on the skin. Reducing the ZnO particles to nanoparticle size reduces the reflection but doesn’t stop the scattering of UV light, so the substance continues to act as a sunscreen. Additional processes may occur due to the unusually small size of the particles, however. They may be photoreactive, which means that they may participate in chemical reactions after light exposure. Manufacturers generally coat the nanoparticles in sunscreen to reduce this possibility.
Research into the effects of zinc oxide nanoparticles is continuing. They are generally considered to be safe, but some questions have been raised about their safety in particular situations.