Check the air handling units. A belt set on a fan will produce a lot of black dust. Are the filters in good shape. Make up air from outside may be laden with dust and if not filtered will be distributed thruout bldg. Outside contamination wind born thru open dock doors. I was reciently in a building that had 2 doors open on bldg. Dust storm outside put.25 in dust over 10000sq ft of wharehouse in about 2 hours. Took customer 3 weeks to clean up. What was stored in bldg before? How long has contamination been there? Over head steel may be laden with dirt and it is slowly falling to floor.