I cannot check right now owing to time constraints, but I can tell you that in the past few months a comparatively large number of studies seem to have emerged.

One article I met earlier in the year was about creating systems to tag microplastic, in order to follow its path: https://news.ycombinator.com/item?id=30020879 ("New measuring method reveals there may be more plastic “on” than “in” your salad" (phys.org))