I'm not sure if they have established a threat. I thought it was mostly hypothesised or very locally specific harms.

On the other hand I suspect much of the real science on environmental plastic might avoid the term microplastic since it seems to have a meaning that flows to whatever can make the scariest headline today. I have seen the size range to qualify run from microscopic up to a couple of millimetres. Volumes, quantities, or location stated without regard to individual particle size. I'm relatively certain that they have not discovered 1mm particles inside red blood cells.

Even what counts as a plastic seems to be an easy way of adding vagueness, I saw one table that seemed to count cellulose as a plastic, which makes sense if you are thinking about properties of the material, but unsurprisingly easy to come across that it's not really worth going looking for it.