Press Room

Will My Plastic Bag Still Be Here in 2507?
How scientists figure out how long it takes your trash to decompose.

By Juliet Lapidos
Posted Wednesday, June 27, 2007, at 6:20 PM ET
www.slate.com

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?

woman carrying plastic bagsActually, 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.

Respirometry tests work perfectly for newspapers and banana peels. (Newspapers take two to five months to biodegrade in a compost heap; banana peels take several days.) But when scientists test generic plastic bags, nothing happens—there's no CO2 production and no decomposition. Why? The most common type of plastic shopping bag—the kind you get at supermarkets—is made of polyethylene, a man-made polymer that microorganisms don't recognize as food.