Natural biodegradation occurs when microorganisms such as bacteria and fungi break down organic substances into carbon dioxide and water. Most plastic bags are made out of low-density polyethylene, a man-made hydrocarbon polymer that microorganisms do not recognize as food. Polyethylene is unappealing to microorganisms because of its high molecular weight, its highly stable three-dimensional structure and its resistance to water. Plastic bags there before take hundreds of years to biodegrade, and in a landfill the process takes even longer. The environmental problems caused by plastic's slow biodegradation is one of the reasons it is so urgent to stop using disposable plastic bags in favor of reusable bags.
Before polyethylene biodegrades, it breaks down into smaller and smaller pieces through exposure to light in a process known as photodegradation.
Photodegradation is not the same as biodegradation because the microscopic pieces of plastic that result are still in the form of man-made polymers. These microscopic pieces are harmful to the environment because they can be ingested by animals and cause hormonal disturbances, as well as absorb persistent organic pollutants such as PCBs and various pesticides. In the ocean these pieces resemble zooplankton to many animals and in some areas considerably outweigh them, making their ingestion by marine animals unavoidable.
Plastic bags have been in use for just over 50 years and have only been used in supermarkets for the past 30 years, but they already pose significant environmental problems in the ocean, as litter and in landfills. According to the Christian Science Monitor, between 500 billion and 1 trillion plastic bags are produced world-wide each year, most of which enter the waste stream. We can decrease the masses of plastic bags plaguing the planet by regularly using reusable shopping bags.