Albert Barnes Commentary 2 Chronicles 29:5

"and said unto them, Hear me, ye Levites; now sanctify yourselves, and sanctify the house of Jehovah, the God of your fathers, and carry forth the filthiness out of the holy place." — 2 Chronicles 29:5 (ASV)

Sanctify yourselves now—Following David’s example, Hezekiah gives this command, likely knowing that the priests had contracted many defilements during the preceding time of idolatry.

The “filthiness” or “uncleanness” (2 Chronicles 29:16) might have consisted partly of mere dust and dirt, and partly of idolatrous objects introduced by Ahaz before he finally shut up the temple (2 Kings 16:10–16).