I see you still don't say microplastics are rare. Violently agreeing with each-other, it seems. ;)

Synthetic textiles (clothes, upholstery, carpet, dryer exhaust, washer drainage) are of course the biggest culprits, with most of that trapped indoors with us, or co-located with human activity. If you have a dog that may change the mass fraction, but the MP exposure remains the same (or worse due to additional wear).

Road and tire wear is the other big contributor, again co-localized with population density. That's one of those nuanced cases, because a large fraction of the tire mass is actually natural rubber. The synthetic additives make it categorized as 100% plastic, but this may not accurately reflect reality in terms of the chemistry or hazard-based analysis.