Pink mist is what the bomb squad calls a person who has been blown apart by a bomb. Dr. Milton told Hannah Davies that when someone is blown up, there is nothing left of them but specks of blood and it is rare that they would even find a finger.
Dylan Young and one of his colleagues turned into pink mist when the bomb that Meredith had taken out of a patient exploded in their hands.
Creating Pink Mist
After the episode, an official behind the scenes video was released to show how the special effect of the pink mist was created.