Narrowband uvb is a very effective treatment for some skin conditions. The mechanism is totally different to vitamin D and oral supplementation basically has no effect. UV exposure is a significant health risk in some parts of the world and the public information campaigns to reduce exposure seem well justified. But I wonder if there isn't a case for a narrowband uvb sunscreen that only lets those dangerous but highly effective dna mutating rays through along with some sort of cheap and reliable dosimetry.