The solar UV radiation level needed for cutaneous production of vitamin D3 in the face. A study conducted among subjects living at a high latitude (68 degrees N).
Edvardsen K, Brustad M, Engelsen O, Aksnes L.
Photochem Photobiol Sci. 2007 Jan;6(1):57-62. Epub 2006 Nov 10.
PMID: 17200737