The major problem with plastics is that current knowledge suggests that plastics may never biodegrade. A study reviewing plastic pollution in 2020 suggests that
- Plastic will first break down into smaller pieces known as microplastics (typically defined as <5mm in size) and then likely into even smaller pieces known as nanoplastic particles. The rate of this process known as fragmentation is controlled by various natural factors: sunlight (UV radiation), mechanical breakdown (by wind or water erosion) and the chemical structure of the plastic in question. For background on the chemistry of plastics, see What is the process for making plastic?.
- Theoretically, it may be possible that plastic will degrade to an atomic size where it will biodegrade. This a process called mineralization. However, this process is likely on the timescale of thousands to millions of years.
- Current estimates suggest that all plastic waste produced ever is still in a form too large to biodegrade. Unless, this plastic has been burnt first.
Ultimately, a lot more research is required to be able to categorically state the long-term fate of plastic in the natural environment. But the current state of knowledge suggests that plastic biodegradation occurs on very long timescales. This differs to traditional materials such as paper, metals, wood and glass which on long timescales, are incorporated into natural recycling processes. Therefore, humans are taking a considerable risk by using such a long-lasting material as a single-use, disposable product. The outcome is likely a long-term environmental pollution problem.
Napper, I.E. and Thompson, R.C., (2020). Plastic Debris in the Marine Environment: History and Future Challenges. Global Challenges, p.1900081.