Why do plastic bags stick around?

You know what one of humankind’s longest lasting legacies would be if we got wiped from the face of the planet tomorrow? Our greatest works of literature and art? Our incredible edifice of scientific knowledge? Out finest pieces of engineering? No, no, and no. Aside from some bronze sculptures, the lunar rover, and silver service sets, if aliens visited in a thousand years time, what they’re most likely to come across is our garbage–among the most enduring of which is likely to be our nuclear waste (here’s a brainteaser: how do you design a sign to warn future generations of radioactive landfills?)

But that’s for another post. Today I’m talking about plastic bags.

So, for all the readers who are already one step ahead and asking: How can we know how long plastic bags will stick around when they’ve only be in circulation for fifty years or so? I have one word for you: Respirometry.

Respir-what? Respirometry. Literally “the measurement of respiration”. To make long-term estimates of this sort when there’s no first-hand evidence available, scientists often use respirometry tests. The experimenter places the test material in a sealed vessel containing microbe-rich compost, then aerates the sample. Over the course of a few days the microorganisms will assimilate the test material and produce carbon dioxide as a by-product. Thus, the CO2 level acts as an indicator of decomposition rates.

Using things like bananas and newspapers that have well-known decomposition lifetimes for calibration, scientists can make estimates as to how long other slower degrading objects will likely last. Objects like plastic bags.

And what do the scientists discover for plastic bags? Zero carbon-dioxide production. Since most plastic bags are made of polyethylene–a substance which microorganisms can’t eat (although a 16-year old Canadian made the headlines a few years ago with a very promising discovery)–they’ll never biodegrade. Woah! But wait, doesn’t that mean they’ll never degrade? How do we get the 500-1000 year estimates? Well, plastic bags might not biodegrade, but they do photodegrade.

Polyethylene is made of long polymer chains that when exposed to UV-radiation become brittle and crack. Plastic bags will slowly fragment, eventually turning into microscopic granules. Plastic dust. That doesn’t sound too good, huh? In fact, plastic scraps have been found in the stomachs of creatures as diverse as camels to leatherback turtles. Ingestion that can result in blockages, internal infections, starvation and death.

And that’s only one problem. What other issues can you think of with using plastic bags?

So, next time you’re loading up at the supermarket with a bunch of plastic bags, have a think about the millenia-long journey those sacks are about to embark on. Will they end up in the Pacific Gyre, the intestines of an albatross, or your local landfill?

Maybe it’s time to get a reusable bag.

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2 thoughts on “Why do plastic bags stick around?

  1. Sylvia says:

    Or move to Wales. 🙂

  2. stvgskll says:

    Are plastic bags a thing of the past in Wales? Awesome.

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