The study provides invaluable information to address rising concerns about micro- and nanoplastic toxicity.

Microplastics – ranging from less than a quarter inch (5 mm) down to a micrometer – are formed when plastics break down into progressively smaller bits. And, in recent years, concerns have been raised that microplastics are showing up literally everywhere on Earth – from the bottom of oceans to the top of mountains.

These plastic particles are consumed by humans and other creatures, with research now looking into the potentially harmful effects of this consumption. However, these studies are predominantly concerned with microplastics. Now, researchers from the Columbia Climate School, New York, have used advances in imaging technology to investigate the 'spawn' of microplastics – nanoplastics, less than a micrometer in size – to see how prevalent they are in the bottled water we drink.

"Previously, this was just a dark area, uncharted," said Beizhan Yan, one of the study's corresponding authors. "This [study] opens a window where we can look into a world that was not exposed to use before."

The researchers developed a technique called stimulated Raman scattering (SRS) microscopy, which involves probing samples with two simultaneous lasers that are tuned to make specific molecules resonate. The technique allows micro- and nanoplastic analysis at the single-particle level, providing high chemical specificity and throughput. Focusing on seven common plastics, the researchers created a data-driven algorithm to interpret the results.

Testing three popular brands of bottled water sold in the US (they declined to name them), the researchers analyzed them for the plastic particles present down to just 100 nanometers in size, which had previously been invisible under conventional imaging. They observed 110,000 to 370,000 plastic fragments in each liter (33.8 fl oz), 90% of which were nanoplastics. The rest were the larger – but still very small – microplastics. They determined which of the seven specific plastics they were and charted their shapes, information which could be relevant to biomedical research.