Nanoparticles are tiny molecules that have been nanonized (ground into fine particles). They could therefore enter the body through the outer layers of the skin. There are still many studies to be done to determine the direct impact of nanoparticles on health. But in the meantime, many scientists are playing it safe and not using nanoparticle ingredients in products that come into direct contact with the skin.
In mineral sunscreens, nanoparticles minimize the formation of a white film after application and allow for a more transparent application, which is more appealing to consumers. Sunscreens containing nanoparticles are therefore easy to apply, non-whitening and lighter.
According to the experts at EWG, the safest choice is a non-nano zinc oxide mineral sunscreen to create a physical barrier.11
11 https://www.ewg.org/sunscreen/report/nanoparticles-in-sunscreen/#.WwqMgy8ZNPU