That was always the thing I was wondering about. Where was the origin of these microplastics. Surely it wasn't all just those little beads that were in soap and shampoo
The other source aside from tires and those beads is all the other plastics that we use. Especially when exposed to the elements, all(?) plastics eventually break down into tiny particles. This includes all synthetic fibers, by the way. I've seen studies that show how much synthetic clothes release microplastics each time we wash them.