Matthew Poole and continuators on Numbers 19:18

18 And a clean person shall take hysop, and dip it in the water, and sprinkle it upon the tent, and upon all the vessels , and upon the persons that were there, and upon him that touched a bone, or one slain, or one dead, or a grave.

To wit, a Priest; for to such the work of cleansing was appropriated. See Levit. 13.

Even those who were in part purified by the fire. See Numb. 31. 23.