How Long Does It Take for Plastic to Decompose?

By: William Harris  | 
Plastic isn't a material that decomposes easily. Plastics tend to break down into smaller particles until they are too small to be seen. Alistair Berg / Getty Images

Drop a ketchup bottle on the floor, and you'll be thankful for polyethylene terephthalate, or PET, the nearly indestructible plastic used to make most containers and bottles. Drop the same bottle into a landfill, however, and you might wonder: How long does it take for plastic to decompose?

This question can be hard to answer because petroleum-based plastics like PET don't decompose the same way organic material does.


Biodegradation and Plastic

Wood, grass and food scraps undergo a process known as biodegradation when they're buried, which is a fancy way of saying they're transformed by bacteria in the soil into other useful compounds.

Are we stuck with that plastic bottle forever if it doesn't make it into a recycling bin?

But bacteria turn up their noses at plastic waste. Load their dinner plates with some plastic bags and bottles, and the one-celled gluttons will skip the meal entirely.


Based on this logic, it's safe to argue that plastic will never biodegrade. Of course, that's not the end of the story. In 2008, Daniel Burd, a student at Waterloo Collegiate Institute, demonstrated that certain types of bacteria can break down plastic. His research earned the top prize at the Canada-wide Science Fair, earning him $10,000 cash and a $20,000 scholarship.

Researchers have confirmed Burd's findings and identified several other plastic-eating bacteria, but none have been effective in practical applications. Until waste treatment plants can implement any new processes, the only real way to break down plastic is through photodegradation.


How Photodegradation Breaks Down Plastic

Photodegradation is a type of of decomposition that requires sunlight, not bacteria. When UV rays strike plastic, they break the bonds holding the long molecular chain together. Over time, this can turn a big piece of plastic into lots of little pieces.

Of course, plastic buried in a landfill rarely sees the light of day. But in the ocean, which is where a lot of discarded grocery bags, soft drink bottles and six-pack rings end up, plastic bathes in as much light as water.


In 2009, researchers from Nihon University in Chiba, Japan, found that plastic in warm ocean water can degrade in as little as a year. This doesn't sound so bad until you realize those small bits of plastic are toxic chemicals such as bisphenol A (BPA) and PS oligomer. These end up in the guts of animals or wash up on shorelines, where humans are most likely to come into direct contact with the toxins.

Biodegradable Plastics

One solution to the environmental disaster of plastic pollution is so-called biodegradable plastic. There are two types currently on the market: plant-based hydro-biodegradable plastic and petroleum-based oxo-biodegradable plastic.

In the former category, polylactic acid (PLA), a plastic made from corn, tops the list as the most talked-about alternative. PLA decomposes into water and carbon dioxide in 47 to 90 days — four times faster than a PET-based bag floating in the ocean.


But conditions have to be just right to achieve these kinds of results. PLA breaks down most efficiently in commercial composting facilities at high temperatures. When buried in a landfill, a plastic bag made from corn may remain intact just as long as a plastic bag made from oil or natural gas.

Biodegradable Plastic FAQ

How long does it take plastic to decompose?
Resistant plastics like PET take much longer to decompose since they require UV light to break down, not bacteria. Estimates suggest that it can take plastic water bottles around 450 years to decompose in landfills.
How long does it take for plastic bags to decompose in the ocean?
Plastic bags traveling in the ocean tear away due to constant motion and UV light. It takes them 20 years to decompose and settle. Plastic bottles can take up to 450 years, while fishing lines take around 600 years.
What is an example of a biodegradable substance?
Biodegradable materials break down into smaller, more usable forms over time. Examples of this include human and animal excretion, plant leaves, wood, roots, fruit, roots and dead humans/animals (except hair and bones).
Does plastic eventually break down?
Plastic isn't a material that decomposes easily, and must be recycled. Plastic tends to break down into smaller particles until they are too small to be seen. This means that every molecule of plastic produced since 1907 is still present in the environment.

Lots More Information

Related Articles

  • Gerngross, Tillman U. and Steven C. Slater. "How Green Are Green Plastics?" Scientific American. August 2000.
  • Kawawada, Karen. "WCI student isolates microbe that lunches on plastic bags." The Record. May 22, 2008. (Nov. 22, 2010)
  • Ransford, Matt. "Why Trashing the Oceans is More Dangerous Than We Imagined." Popular Science. April 1, 2008.(Nov.22,2010)
  • Royte, Elizabeth. "Corn Plastic to the Rescue." Smithsonian Magazine. August 2006.
  • Shukman, David. "Warning on plastic's toxic threat." BBC News. March 27, 2008. (Nov. 22, 2010)
  • Sohn, Emily. "Plastic decomposes quickly at sea, study finds." MSNBC. Aug. 20, 2009. (Nov. 22, 2010)