One paper I read ages ago suggested these nanoparticles had similar behaviors subdermal as asbestos but drew no conclusions if they're carcinogenic. Then there's a few references floating about that I can't verify the veracity of that zinc oxide nanoparticles exposed to UV breakdown into free radicals.

Google search: zinc oxide nanoparticles free radical

The larger particle, and thus opaque versions of these sunblocks seem to behave as you'd expect, they stay on the surface of the skin pretty much entirely wash off and don't interact with the environment. But they're visible light opaque so you look like you've been painted.