Is this mould or dust?

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My parents' books are covered in this. They say it's dust but it doesn't brush off. It looks more like mould to me. They've offered the Dc some books, but if they take them, will it spread to their books at home? Or is it genuinely dust?

It’s mildew- which is mould. It will spread if put with other books. If they are really good books that you want you can put a bit of chlorine bleach on an anti bac wipe and wipe down the books and then set them in the sunshine (flipping pages every si often) to kill it.

By wipe down books, I mean every page and the ends. Not just the cover.

I believe it might be what is called foxing which is a kind of mould.

https://en.m.wikipedia.org/wiki/Foxing

It's "foxing" which is mould not dust

Dust jacket is very mouldy, that needs to be binned.