Larger plastic items degrade to form microplastic, but some of the particles also are being put directly into the sea, because bits of cosmetic beads and clothing fibers are small enough to pass through wastewater treatment systems.
Once in the oceans, the particles are transported far and wide in a complex pattern that is difficult to predict. However, scientists have found very high concentrations in the subtropical gyres -- that is, areas where currents rotate rapidly - and in basins such as the Mediterranean.
Microplastics are themselves toxic, but they also soak up harmful chemicals that contaminate the ocean, such as DDT and PBDEs, so that they deliver a concentrated dose to the animals who ingest them. Marine scientists also worry that microplastics will end up in seafood-eating humans as well.
Video: How Much Trash is in the Ocean?
Microplastics are just one of the environmental woes afflicting the world's oceans, and pushing them perilously close to ecological collapse, according to an article published last week in Foreign Policy, a political science journal.