They are toxic because your body can't deal with them, microplastics are -maybe counterintuitively- still large enough to only cause smaller issues like indiges

They are toxic because your body can't deal with them, microplastics are -maybe counterintuitively- still large enough to only cause smaller issues like indigestion. For the problems with pfas, read any source from the rest of this thread. In the end if your body could get rid of X, X generally isn't a problem. Sometimes it's chemical (i.e. lead/carbonmonoxide/etc) sometimes it's physical (i.e. asbestos/microplastics), accumulating enough of anything not meant to be processed/disposed in your body will be a problem.