Further studies I've just read say that there is no evidence that exposure to asphalt (bitumen) fumes is a known carcinogen. But they advise to only be around it in a well ventilated area. HT rooms probably don't qualify as a well ventilated area. But if it's on the in-side of the walls, there would be little exposure, I guess.

https://docs.google.com/spreadsheet/ccc?...amp;usp=sharing