My apologies, as I have been off the site for a couple of years. I posted this query twice elsewhere before I remembered how best to do it

Does anyone know the composition of the black stuff that forms on welding shop walls? I rented out my old 2000 square foot shop to a guy who claims that he has gotten lung problems since moving into the space. We cleaned the walls and floor, but couldn't get the ceiling as it is 24' high. Any ideas how best to remove the black stuff?