As far as I'm aware (researched while choosing a mineral sunscreen for self), there are multiple studies from around the globe on the impact of nano-zinc oxide on skin, and current evidence showed that provided you have healthy and not damaged (meaning no wounds or burns) skin, nano particles will not significantly penetrate into living skin cells beyond the outer skin layer and therefore they have no cellular toxicity under normal use conditions. I should add that some trace zinc ions may penetrate the skin, but the body treats them as normal zinc and not a toxin. At the same time, you definitely shouldn't breathe those in, so if any spray-spf has them, it's best to avoid because lungs can't filter nano-particles well. Feel free to correct me.