I've read in many articles that nano titanium dioxide and zinc oxide are photocatalysts, meaning they can form free radicals under influence of UV light. The definition of what would be considered nano varies between countries, we look at the coated clumped together particles, but in Europe they look at the particles seperately. I've seen many "non-nano" sunscreens labeled as "nano" in Europe. So I would rather choose a sunscreen with coated particles, than a "non-nano" sunscreen that isn't coated, not knowing if it would be considered nano under stricter regulations.
I was wondering, coating the particles reduces the photocatalytic capacity, but to what extend? Is it for example 90% or less? How stable are these coatings? Are the particles photostable during a whole day? And are there possible other negative side effects from these sunscreen ingredients?