Microplastics are small pieces of plastic. They’re commonly defined as any piece of plastic smaller than 5mm (about 0.2 inches) in size, but they’re often much smaller, even invisible to the naked eye.
Microplastics are formed either directly/intentionally (e.g. microbeads used in cosmetics) or, more commonly, through the gradual breakdown of larger pieces of plastic.
They’re not an ingredient in plastic; they’re just small pieces of plastic.
The concerning feature of microplastics – at least, the ones formed from traditional plastics – is that they never really go away. They just get smaller.
For a microplastic to fully disappear, it needs to biodegrade. And traditional plastics (the ones made from petroleum-derived components) do not biodegrade. Instead – through a combination of UV radiation (which weakens and breaks the chemical bonds holding traditional plastics together), heat, and physical wear – traditional plastics just break down into smaller and smaller pieces over time.