It occurs to me that it would depend on what functions were carried on there.

Piano making is a complicated beast, and has lots of components. The part that comes to mind as most likely to be polluting is casting the iron (steel?) frame. Foundry operations are notoriously prone to pollution.

However, generally frame casting is, as I understand it, carried on at different types of facilities than the wood functions, such as making the case and of course the assembly functions. I've been given to understand that, of modern times, only Steinway makes their own frames, while all other piano companies(at least in America) have their frames made by a very limited number of specialized foundries.
If the piano factory turned out finished pianos, it's almost certain, to my mind, that the foundry work was not done there. Even Steinway, who makes their own, does so at different sites than final assembly.

I can't guarantee, of course, that there are NO polluting functions in a final assembly plant.

Dave Oesterreich