Little plastic fibers from clothing can be that long, but so thin as to be virtually invisible.
https://www.bioplasticsmagazine.com/en/news/meldungen/PHA-all-the-fashion—.php
Polyhydroxyalkanoate clothing is a solution.
Interesting because I started reading about PHA’s in grad school (just out of nerdy interest). That’s been thirty years ago. Now these plastics are finally being marketed.
True, and lots of microplastics are much smaller than 5 mm even in their longest dimension. I was just amused by the thought of people deliberately swallowing easily-detected pieces.