If you overheat it (which might happen if you leave the pan on heat with no contents, or you’re trying to cook something like steak at high heat) then it breaks

If you overheat it (which might happen if you leave the pan on heat with no contents, or you’re trying to cook something like steak at high heat) then it breaks down into a fairly nasty gas which is fatal to birds and gives humans flu like symptoms (exact health effects of this aren’t known). And as other people have mentioned, the manufacturing process involves some really nasty chemicals, which often end up leaking out into the environment.