UAB research has characterized in detail how polymer-based commercial tea bags release millions of nanoplastics and microplastics when infused. The study shows for the first time the capacity of these particles to be absorbed by human intestinal cells, and are thus able to reach the bloodstream and spread throughout the body.
Linked Source here. Sample 3 is a round teabag which is a distinct feature of Tetley tea. Although I’m sure other brands may use that design too.