Polyester is a plastic polymer called polyethylene terephthalate ( PET ).  It makes up half of all fibers used to make clothes and 80% of all synthetics. It is made from oil and natural gas and is nonrenewable. It is also nonbiodegradable. A known carcinogen called antimony trioxide is used in the process of making it.

Polyester sheds microfibers when it is washed in the laundry and these particles end up in the ocean. They enter the food chain and people eat them.   Microfibers are from many other plastic sources in addition to clothes. Changing consumer habits will not be sufficient to solve the problems. Government, scientific and industry solutions are needed.
Source: The Conscious Closet by Elizabeth L. Cline