microplastics from plastic pepper grinders?
If you go small enough, you're always inhaling/ingesting something foreign.

Your concern, although logically valid, is nearly impossible to regulate or even measure. We're talking about amount that is, literally, microscopic.

You're definitely eating it, but discussion of the consequences (if any) is off topic on the site. 

The environmental concerns are valid, because even though they're small, they do not decompose and collects.