Microorganisms in the environment, such as bacteria and fungi, work to break down organic materials into the less complex molecules carbon dioxide and water. Since most disposable plastic bags consist of low-density polyethylene (a non-organic hydrocarbon polymer) the microorganisms do not recognize them as food. Polyethylene has a high molecular weight, and therefore is a very stable three-dimensional structure which will take centuries to biodegrade. Buried in a landfill, the process will take even longer. Multiply these facts by trillions of plastic bags, and it has now become urgent to switch to reusable bags as a planet.
Instead of biodegrading, which as mentioned will take hundreds of years, polyethylene photo-degrades, or breaks down into smaller and smaller pieces with the help of sunlight. Unfortunately, while the plastic seems to disappear, it remains in the environment and can be ingested by small organisms. In this way, it has the potential to enter the food chain, where it can do harm in the form of reproductive and hormonal disturbances in even the tiniest of organisms. These microscopic bits of plastic can also absorb organic pollutants such as PCB's and agricultural chemicals and then those pollutants can enter the food chain. In our oceans, these bits might be ingested by zooplankton-eating animals which are in turn ingested by larger organisms. It is easy to see the potential for damage on a world-wide scale that plastic bags pose.
Plastic bags have been in use for a little over 50 years, and have been available widely in supermarkets for the past 30 years. It is estimated that between 500 billion and 1 trillion plastic bags are produced world-wide on an annual basis, the vast majority of which enter the waste stream. By investing in a few reusable shopping bags, and remembering to use them, each of us can influence the number of plastic bags entering the environment. Pooled together, the effect of people around the world using reusable shopping bags has the potential to be of enormous significance to the future of our planet.