The safest choice to protect your skin from sunburn?
Zinc oxide (ZnO) is a largely inert (nonreactive), white compound used for bulking or as UV filler. It also has antibacterial and deodorant properties. Hence, it is often in baby powder and anti-dandruff shampoos. It is a strong UV filter and is used in sunscreen lotions, for both adults and children. Standard ZnO microparticles are white pigments. They also make your sunblock white, which may stop some people from using enough of it. But smaller nano particles do not cause this white coloring.
Generally, properties of nanoparticles can be different than their larger counterparts. However, ZnO nanoparticles have the same crystal structure as micro-particles and similar physical and chemical characteristics. Zinc oxide in this form is slightly more soluble than normal.
There is no evidence that ZnO can cause skin senzitisation or allergies, and it has low to very low long term toxicity and mutagenic effects. Nano particles of ZnO are more likely to penetrate the skin, however even with penetration enhancers, this amount is very small and hundred times below the daily intake in food.
When inhaled repeatedly, ZnO nano particles may cause inflammations in the lunge. Because of that, ZnO (nano) is not allowed in spray products in the EU.
There is little evidence that ZnO is harmful to the environment, especially compared to other UV filtering ingredients. ZnO also is a popular choice in reef-friendly sunscreen. However, this is still under investigation. One 2018 study shows that ZnO in its nano form can be very harmful to one particular coral.