The debate is over safety, and should rightly be given a great deal of attention. The nano question boils down to two critical questions for zinc oxide (and indeed all nano materials).
Does nano zinc penetrate the skin?
Does nano zinc generate Free Radicals and/or cause cell damage and if so where?
World wide many regulatory and independent bodies have surveyed the research and collated what is available to date. Each body needed to make their own safety decision on behalf of the public. Below are the statements from several major agencies around the world, each concluding that, from the research available to date, nano-zinc is safe for use in sunscreen.