Polystyrene, one of the world's most common plastics (one popular product is Styrofoam), degrades in decades when exposed to sunlight, rather than thousands of years as previously claimed. 

Polystyrene has been routinely detected in the world's oceans since the 1970s., though at nowhere near levels it should be. In fact, only 1 percent of the plastic that should be found is detectable. Chemistry may be why.

The idea that sunlight degrades plastics is nothing new, but the new study shows sunlight doesn't just cause the plastics to physically break down--it also causes them to degrade chemically into dissolved organic carbon and trace amounts of carbon dioxide, at levels far too low to impact climate change. Once the plastic undergoes this transformation, its original form disappears from the environment, and it becomes entirely new byproducts that cannot be seen by the naked eye. 


Photo by Jayne Doucette, Woods Hole Oceanographic Institution

Considering how this transformation happens will be an important part of estimating how much plastic is actually out in the environment.

"Right now, policy makers generally assume that polystyrene lasts forever in the environment," says Collin Ward, a marine chemist at Woods Hole Oceanographic Institution and lead author of the study. "That's part of justification for writing policy that bans it. One of our motivations for this study was to understand if polystyrene actually does last forever. We're not saying that plastic pollution isn't bad, just that the persistence of polystyrene in the environment may be shorter and likely more complicated than we previously understood. The chance for injury to the environment over decades is still available."

Microbes may hate Styrofoam but sunlight does not

Previous estimates of how quickly polystyrene breaks down were based on narrow assumptions, like the role of microbes. Plastic may be an organic carbon but the chemical structure of polystyrene is complex and bulky with a ring-based backbone making it very difficult for them to digest. However, that ring-based backbone of polystyrene is a the perfect shape and size to catch certain frequencies of sunlight.

In the lab, the researchers tested whether sunlight could transform polystyrene by exposing five different samples of commercially available polystyrene. The group submerged each of them in sealed glass containers of water and shined light on them from a solar simulator, a lamp that replicates the frequencies of sunlight. The scientists then collected CO2 and compounds that dissolved into the water.

With a variety of chemical tools, including a room-sized accelerator mass spectrometer, Ward and colleagues traced the origins of carbon atoms found both in the CO2 and filtered water. The study also found that additives to polystyrene, which can determine its color, flexibility, and other physical features, play a major role in breakdown. Different additives seem to absorb different frequencies of sunlight, which influences how fast the plastic breaks down.