Just guessing here, but I doubt PEX piping is a major contributor, seeing as it's solid plastic that doesn't shed and is rarely exposed to the elements. The most common sources of micro plastics appear to be things like synthetic fibers in clothing, particles coming off of car tires as they wear down, manufactured plastic particulate like micro-beads/glitter, and plastic objects in the ocean that break down into smaller and smaller pieces due to physical abrasion, sun, damage, etc.
