The researchers from Tokyo University of Science and Ehime University checked water samples from 29 rivers. They were looking to see whether the waterways contain plastic particles 5 millimeters long or smaller. Tiny pieces of plastic are potentially harmful to ecosystems.

Researchers say they found microplastics in most of the 29 rivers they tested across Japan. They are calling for measures to reduce plastic waste in inland areas.

The researchers say they found microplastic debris from plastic bags, polystyrene foam and other materials in 26 rivers. They found that the more people who are living upstream, the more particles the river water contains.

Microplastics have also been found in the seawater around Japan. It is believed that ultraviolet light and wave action break plastic waste into tiny particles.

Assistant Professor Tomoya Kataoka of the Tokyo University of Science says the latest finding shows that plastic waste is broken down before it is washed into the ocean.