[dkbrk]

This is a good question and you shouldn't have been downvoted for it. I had a similar concern.

I think the answer is this [0]:

> Many fungi are hyperaccumulators, therefore they are able to concentrate toxins in their fruiting bodies for later removal.

And the linked article alludes to that:

> Heavy metals and other toxins are extracted and captured in the mushrooms that grow, while the substrate leftovers, including the mycelium, are compacted and heated to create clean bricks for new construction.

Presumably they validate that the process results in the substrate having an acceptably low level of toxins before using it as material for new construction.

[0]: https://en.wikipedia.org/wiki/Mycoremediation