Most plastics in the ocean break down into very small particles. These small pieces of plastic are called microplastics. Other plastics are intentionally designed to be small. They are called microbeads and are used in many health and beauty products.
They pass unchanged through waterways into the ocean. Microplastics are tiny plastic particles that result both from the development of commercial products and from the breakdown of larger plastics. As contaminants, microplastics can be harmful to the environment and animal health. Simply unscrewing the cap of a plastic water bottle releases about 14 to 2400 plastic particles.
Ocean plastic is constantly degrading; even something as big and floating as a milk jug will eventually break off and splinter into microplastics.