While zinc oxide had been used for decades in sunscreen products — remember the white coating on lifeguards’ noses? — it was only approved for this use by the FDA approximately 10 years ago. Prior to that, it was actually considered to be (and still is) an approved skin protectant material.
When we think of zinc oxide as a sunscreen we normally only consider one of the three mechanisms that it employs, blocking the UV energy. Zinc oxide also scatters the UV energy. We refer to this as increasing the optical path length. This phenomenon was well explained by Dr. Robert Sayre.