Standard garbage bags do not decompose in the way most people understand the term. This issue touches on a global environmental challenge. These products, used daily by billions, contribute significantly to the accumulation of plastic waste. Discarded plastics persist in the environment for centuries, highlighting a fundamental mismatch between the materials we use and the natural systems designed to break them down.
The Chemical Composition of Standard Garbage Bags
The majority of standard garbage bags are manufactured from polyethylene, a synthetic polymer derived from petroleum and natural gas. This includes low-density polyethylene (LDPE) and high-density polyethylene (HDPE), chosen for their durability, flexibility, and resistance to water. Polyethylene is characterized by long chains of carbon atoms linked together in a highly stable structure.
This strong molecular structure prevents natural decay. Microbes, such as bacteria and fungi, do not possess the necessary enzymes to effectively break the strong carbon-carbon bonds of these polymer chains. The material is also highly hydrophobic, meaning it repels water, which limits the ability of microorganisms to colonize the surface and begin biological breakdown.
The Difference Between Decomposition and Degradation
To understand the fate of plastic bags, it is important to distinguish between decomposition and degradation. Decomposition is a biological process where microbes consume organic matter, converting it into simple, natural substances like water, carbon dioxide, and nutrient-rich biomass. Standard polyethylene bags are not recognized as a food source by these microbes, so they do not decompose.
Degradation, conversely, is a physical or chemical process that breaks a material into smaller pieces. Polyethylene undergoes an extremely slow form of abiotic degradation, primarily through photo-oxidation when exposed to sunlight’s ultraviolet (UV) radiation and oxygen. This process fractures the long polymer chains into increasingly smaller fragments.
The result of this slow physical breakdown is the creation of microplastics. These tiny plastic particles, less than five millimeters in length, retain the original polymer chemistry and persist in the environment, contaminating soil and water. Degradation means the material has fragmented, while true decomposition means the material has been fully metabolized.
Why Landfill Conditions Prevent Breakdown
Most waste, including plastic bags, ends up in modern sanitary landfills that are engineered to preserve waste, not break it down. Landfills are designed as containment vessels to prevent the leakage of contaminants into the environment. The waste is tightly compacted and covered daily, which creates an environment almost entirely devoid of oxygen.
This oxygen-limited, or anaerobic, environment prevents the conditions necessary for aerobic microbial decomposition and photo-degradation. Microbes that require oxygen cannot survive, and the lack of sunlight prevents UV-induced breakdown. As a result, plastics and even organic materials can remain essentially unchanged for many decades. The stability of the polyethylene combined with the landfill design ensures long-term preservation of plastic waste.
Understanding Certified Compostable and Biodegradable Alternatives
Alternatives to traditional plastic bags exist, but their labels require careful interpretation. Products labeled as “compostable” are designed to break down biologically, but they must meet specific standards, such as those set by the Biodegradable Products Institute (BPI). These certified bags, often made from plant-based polymers like polylactic acid (PLA), require the high heat and specific microbial balance of an industrial composting facility to fully convert into carbon dioxide, water, and biomass within a set timeframe.
These specialized requirements mean that most certified compostable bags will not break down in a typical home compost pile or a landfill, where the conditions are too cool and lack the necessary oxygen. Another category is “oxo-degradable” plastic, which contains chemical additives that accelerate fragmentation. These bags are controversial because they quickly break down into microplastics through oxidation, but they do not fully biodegrade, leading to environmental concern and bans in several regions.