How long does plastic take to decompose in the ocean?
The short answer: most plastics never truly decompose. They fragment into microplastics, then nanoplastics — persisting for centuries. Here's the timeline by item, and why 'decompose' is the wrong word.
"Decompose" is the wrong word
When we ask how long plastic takes to decompose, we usually mean: how long until it's gone? The honest answer is that most ocean plastic doesn't get to "gone." Plastics are made of long-chain polymers that have no natural pathway in marine environments — there are no microbes that evolved to break them down, no enzymatic shortcuts, no compost-like end state. What happens instead is photodegradation (UV light breaking polymer chains) combined with mechanical fragmentation (waves, sand, animal bites).
The result: a plastic straw doesn't disappear after 200 years. It becomes thousands of microplastic particles — too small to see, too small to clean up, chemically just as persistent.
Decomposition timelines
| Item | Time to fragment | Note |
|---|---|---|
| Cigarette butt (filter) | 10 years | Cellulose acetate; the most-littered item on earth. |
| Plastic grocery bag | 10–20 years | LDPE film; UV-fragments faster than rigid plastics. |
| Plastic straw | ~200 years | Polypropylene (PP). |
| Coffee cup lid | ~50 years | Polystyrene; foam pieces first. |
| Six-pack ring | ~400 years | Photodegradable variants now mandated in some U.S. jurisdictions. |
| Plastic water bottle | ~450 years | PET; most-recovered cleanup item. |
| Disposable diaper | ~450 years | Mixed plastics + cellulose. |
| Toothbrush | ~500 years | Nylon bristles + PP handle. |
| Fishing line | ~600 years | Nylon monofilament. |
| Fishing net | ~600+ years | "Ghost gear" — largest source of ocean plastic by mass. |
| Microplastic fragments | Centuries to indefinite | Below ~5mm, fragmentation continues but pieces stay plastic. |
Why these numbers vary across sources
Most-cited timelines come from a small number of source studies and government materials, with numbers extrapolated from controlled UV-aging experiments rather than long-term ocean observation. Conditions vary enormously — a bottle floating at the equator degrades faster than one in cold deep water. And "decomposition" isn't well-defined: some sources count an item as "decomposed" when it's no longer visually recognizable; others count it only when polymer chains have broken down to monomer.
What this means for the everyday choice
Two practical implications:
- Choose marine-degradable options for unavoidable single-use scenarios. Paper, hay, and bagasse straws break down in weeks rather than centuries.
- Reusables compound their advantage over decades. A stainless-steel straw used for ten years displaces ~7,000 disposable straws — a 99.99%+ reduction in plastic mass that could enter the ocean.
Sources and further reading
- NOAA Marine Debris Program — fact sheets on common items and degradation rates.
- U.S. National Park Service "Trash Talk" infographic series.
- Andrady, A. L. (2011) "Microplastics in the marine environment." Marine Pollution Bulletin, 62(8).
- Chamas, A. et al. (2020) "Degradation Rates of Plastics in the Environment." ACS Sustainable Chemistry & Engineering, 8(9).