Most plastics are not readily biodegradable, especially when buried in the ground away from sources of oxygen. Many can be decomposed partially by photodecomposition in the presence of sunlight, however. This results in the plastic materials breaking up into tiny fragments.
Although standard polyethylene bags don’t biodegrade, the molecules in them become brittle and start to crack when the bags are exposed to ultraviolet light. But it could take hundreds to thousands of years for this to happen under normal sunlight conditions.
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