In the form of bottles, tires, packaging, and piping, millions of tons of plastic waste are dumped every year in the world's waterways, often ending up in the oceans.

And their amount could almost double by 2060 unless strong measures are taken against the pollution, the Organisation for Economic Cooperation and Development (OECD) warns.

Due to mass production of the material from the 1950s to 2019, 140 million tons have already accumulated in the rivers, lakes, and oceans, the OECD said in a 2023 report.

Some 22% of this forms a "plastic soup" in the oceans and 78% is found in freshwater ecosystems.

Plastic burned in open pits or tossed in uncontrolled or unauthorized dumpsites is the main source of pollution in the aquatic environment.

Most of this plastic waste ends up in freshwaters, with a large part, including bottles and plastic used in the construction sector, sinking in waterways and lakes.

The rest, including food packaging and closed bottles, floats for "years, even decades," before ending up in the oceans, the OECD says.

Waste from shipping activity, including nets and fishing gear, is to a much lesser extent another source of plastic waste in the oceans, as well as so-called microplastics, pieces of plastic that measure less than five millimeters.

So-called macroplastic, which is bigger than five millimeters, has an average life cycle of six months to 35 years and slowly decomposes to become microplastic, which is "more likely to be ingested by aquatic species," the OECD says.

The risk of plastic moving from land to the waterways, and then into the sea, differs from location to location.

Out of some 100,000 waterways, only 1,000 are responsible for four-fifths of the macroplastic waste in the oceans, according to a 2021 study by researchers for NGO Ocean Cleanup published in the Science Advances journal.

The remaining fifth comes from 30,000 other rivers.

Out of the 50 main rivers carrying plastic to the oceans, including small urban waterways, 44 are in Asia, "due to population density and bad waste management," said Laurent Lebreton, Ocean Cleanup's director of research.

The Philippines, which has thousands of islands, dumps the most plastic into the sea. Its Pasig River, which flows into Manila Bay, is "the most [plastic] polluted" in the world.

With the Philippines' Tullahan and Meycauayan Rivers, India's Ulhas River, and Malaysia's Klang River, it is one of the top five carrying plastic into the oceans.

Driven by rising population and economic growth, the global use of plastics should almost triple between 2019 and 2060, to 1,231 million tons (Mt) per year, according to the OECD.

That is a gloomy outlook for the aquatic environment where 493 Mt of plastic could pile up by 2060, of which more than half are from sub-Saharan Africa, China, India, and other developing Asian countries, it says.

In Europe and the US, however, plastic industrial waste in the aquatic environment should decrease due to improved waste management, the OECD forecasts.