Will My Plastic Bag Still Be Here in 2507?

How scientists figure out how long it takes your trash to decompose.

By Juliet LapidosPosted Wednesday, June 27, 2007, at 6:20 PM ET

How long will those bags last?Starting July 1, most large grocery stores in the state of California will be legally required to recycle plastic shopping bags. In Europe, even stricter anti-plastic measures are gaining traction. Retailers in Modbury, England, for example, recently committed to an outright plastic-bag ban. News reports have cited a statistic that the ubiquitous receptacles take 500 years to break down in landfills. How do we know?Actually, we don't. Plastic bags have only been around for about 50 years, so there's no firsthand evidence of their decomposition rate. To make long-term estimates of this sort, scientists often use respirometry tests. The experimenters place a solid waste sample—like a newspaper, banana peel, or plastic bag—in a vessel containing microbe-rich compost, then aerate the mixture. Over the course of several days, microorganisms assimilate the sample bit by bit and produce carbon dioxide; the resultant CO₂ level serves as an indicator of degradation.

So, where does the 500-year statistic come from? Although standard polyethylene bags don't biodegrade, they do photodegrade. When exposed to ultraviolet radiation from sunlight, polyethylene's polymer chains become brittle and start to crack. This suggests that plastic bags will eventually fragment into microscopic granules. As of yet, however, scientists aren't sure how many centuries it takes for the sun to work its magic. That's why certain news sources cite a 500-year estimate while others prefer a more conservative 1,000-year lifespan. According to some plastics experts, all these figures are just another way of saying "a really, really long time."

Sometimes, even banana peels don't decompose once they reach the landfill. For sanitary reasons, modern landfills are lined on the bottom with clay and plastic to keep waste from escaping into the soil and are covered daily with a layer of earth to reduce odor. The landfill, then, acts like a trash tomb—the garbage within receives little air, water, or sunlight. This means that even readily degradable waste objects, including paper and food scraps, are more likely to mummify than decompose.