Literally the first thing that comes from google search of this. Plus they claim this number when you are actually there in their museum dedicated specifically to this horrible part of their history. Visited last year, some 'jobs' around smelting had survivability in mere weeks because of all the toxicity.

Plus took a trip quite deep into the mines themselves. Imagine mountain that is almost 4800m high, drilled through like a proper swiss emmental cheese (they claim around 500 different entrances to tunnel system).