Humans have filled the world’s oceans with more than 170 trillion pieces of plastic, dramatically more than previously estimated, according to a major study released Wednesday.

The trillions of plastic particles — a “plastic smog,” in the words of the researchers — weigh roughly 2.4 million metric tons and are doubling about every six years, according to the study conducted by a team of international researchers led by Marcus Eriksen of the 5 Gyres Institute, based in Santa Monica, Calif. That is more than 21,000 pieces of plastic for each of the Earth’s 8 billion residents. Most pieces are very small.

The study, which was published in the PLOS One journal, draws on nearly 12,000 samples collected across 40 years of research in all the world’s major ocean basins. Starting in 2004, researchers observed a major rise in the material, which they say coincided with an explosion in plastics production.

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