> what does the evidence say? Afaict, not much.

That's only because we have no way to gather evidence, because almost everybody alive has microplastic exposure, so we don't have a control group.

It's just the nature of poorly understood psychology and the long human lifespan that most scientific research wouldn't be able to establish causality on a whole host of negative things (e.g. look how long it took to even get agreement that football concussions were detrimental in the long-term).