Plastic dust

Researchers studied Plastic particles in bottled water, using Stimulated Raman Scattering:

The researchers found that, on average, a liter of bottled water included about 240,000 tiny pieces of plastic. About 90% of these plastic fragments were nanoplastics. This total was 10 to 100 times more plastic particles than seen in earlier studies, which mostly focused on larger microplastics.

and

The method identified millions of additional particles that did not match the seven categories of plastic. It’s not yet clear if these tiny particles are nanoplastics or other substances.

Why would there be plastic "dust" inside a water bottle?

So far I haven't found any definitive answer to that. However, water does do some damage even to PET (polyethylene terephthalate), which is quite stable in water. Possibly stress points in the plastic bottles are more liable to "corrosion." Possibly the manufacturing process liberates some microstrands of plastic as it blows up the plastic. UV can accelerate degradation. I haven't thought of everything. (Different manufacturers had different quantities: maybe their processes differed or their water sources differed.)

At any rate, using their estimates I get something like a tenth of a part per billion mass of known plastic bits. And probably a hundred times that of unknown stuff--maybe other plastics, maybe bacteria; not known yet.