Dust.

It's hard to classify this photo, but I've decided to put it in general portrait genre, as it's only kinda fine artsy.
Shot outdoors, in order to make the clean up easier.
Dust was made with regular flour thrown against the models body from behind the back. Lit with 2 large softboxes and a harder light reflector to the face.
Final picture is a composite of 6-7 takes.