From what I understand, most microplastics in the environment are from washing clothing made of synthetic fibers[1] instead of natural products like cotton. It's rare to find something made from 100% cotton - it's usually a blend of mostly synthetic and sometimes natural fibers. Every time you wash them millions of microfiber plastics are released into the sewer system and there is no filter system capable of removing them so they end up permanently in the water cycle. They even end up in rain and snowfall[2], and have been found in organs of the human body[3] and of course wildlife.

> Microplastic pollution caused by washing processes of synthetic textiles has recently been assessed as the main source of primary microplastics in the oceans.

[1]https://www.nature.com/articles/s41598-019-43023-x

[2]https://www.nationalgeographic.com/environment/article/micro...

[3]https://www.theguardian.com/environment/2020/aug/17/micropla...