"Microplastics" is a term used to describe the tiny particles of plastic waste. The problem is that these don't break down organically - they just become smaller (to the molecular level). There's famously the "plastic soup [wikipedia.org]" in oceans that contains such particles.
A recent Norwegian* study looked into the originators of these microplastic. Surprisingly enough: car tyres [environment.no]. There are other sources, but they contribute significantly less. According to the infographic [environment.no], it breaks down as follows:
- Car tyres: 2250 tons
- Paint/maintenance of ships: 650 tons
- loss from plastic production: 400 tons
- painting/maintenance of buildings+infrastructure: 310 tons
- laundry: 110 tons
- waste treatment: 100 tons
... and some small change.
This means that car tyres alone, by themselves, account for a staggering 55% of microplastic waste.
To put this in perspective: Germans and Norwegians (both) use up about 2 kilos of car tyres per person per year.
Note: These numbers seem particular to Norway - overall yearly production of microplastics seems (unfortunately) vastly greater, see the below-linked German report (table on page 33) for some EU estimates.
* There's apparently a German study corroborating this. The only one I could find is here [bio-based.eu] (English, downloads a PDF).
PS: For the pedantics [grammarist.com]