Plastic bags are non-biodegradable because they are made of long chains of polymer molecules, typically polyethylene. These polymer chains are highly stable and resistant to breakdown by natural processes such as bacteria, fungi, and sunlight. Unlike organic materials such as food waste or paper, which can be broken down by microorganisms into their basic components, plastic bags do not easily decompose.

As a result, plastic bags can persist in the environment for hundreds of years. When they are discarded improperly, they can accumulate in landfills, litter the streets and waterways, and harm wildlife. Some plastic bags can also break down into small particles called microplastics, which can contaminate the soil and water and be ingested by animals and humans.

To reduce the impact of plastic bags on the environment, it is important to reduce their use and promote the use of reusable bags made from more sustainable materials. Additionally, some companies are exploring the development of biodegradable plastics, which can break down more easily in the environment, although these materials also have their own challenges and limitations.