what is the science behind ‘masks help stop you spreading germs , but don’t stop you catching them’

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This is something I’ve heard a lot, and assuming it’s true, what’s the science behind it? Surely holes in masks are the same size no matter whether the germs are heading in or out, but I’m guessing it’s not about the holes and I’m missing something.

Obligatory disclaimer: I’m not an anti-masker, I’m still wearing it in shops for example. This is just me wanting to understand.

In: Biology