None of the commonly used plastics are biodegradable. As a result, they accumulate, rather than decompose, in landfills or the natural environment. Plastics ar

None of the commonly used plastics are biodegradable. As a result, they accumulate, rather than decompose, in landfills or the natural environment. Plastics are not readily biodegradable. Plastics buried in landfills hardly biodegrade. Plastics that are buried under soil or that sink to the bottom of bodies of water hardly biodegrade. There are a variety of biological processes that degrade plastics that remain exposed to atmosphere or in upper layers of bodies of water, particularly when enhanced by ultraviolet fragmentation of high-molecular-weight polymer chains. I'd say more like "the commonly used plastics exhibit variable and typically low degradation rates in the natural environment, making it difficult to quantify natural breakdown of plastics not recycled or incinerated." Here's one of the most fascinating papers I've ever read, about biological colonization and breakdown of floating plastic trash in the oceans: "Life in the “Plastisphere”: Microbial Communities on Plastic Marine Debris" https://macaulay.cuny.edu/eportfolios/branco2014/files/2014/08/Zettler-2013-Plastisphere.pdf https://macaulay.cuny.edu/eportfolios/branco2014/files/2014/... And an article about polyethylene biodegradation by thermophilic bacteria: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2005.02553.x/full http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2005.... And an article that you may have seen before, about polyethylene degradation by waxworm gut bacteria: http://pubs.acs.org/doi/abs/10.1021/es504038a?journalCode=esthag http://pubs.acs.org/doi/abs/10.1021/es504038a?journalCode=es... If you search Google Scholar for polyethylene biodegradation rates you'll find many articles. Some thermoset plastics and halogenated polymers are effectively immune to UV, biological, and combined-effects breakdown. They'll endure into deep time like ceramics. But the most common plastics do degrade, albeit slowly.