A typical one-liter (33-ounce) bottle of water contains some 240,000 plastic fragments on average, according to a new study. Many of those fragments have historically gone undetected, the researchers determined, suggesting that health concerns linked to plastic pollution may be dramatically underestimated, Bloomberg reported.

The peer-reviewed study, published Monday in the journal Proceedings of the National Academy of Sciences, is the first to evaluate bottled water for the presence of “nanoplastics”—plastic particles under 1 micrometer in length, or one-seventieth the width of a human hair. The findings show that bottled water could contain up to 100 times more plastic particles than previously estimated, as earlier studies only accounted for microplastics, or pieces between 1 and 5,000 micrometers.

Nanoplastics pose a greater threat to human health than microplastics because they’re small enough to penetrate human cells, enter the bloodstream and impact organs. Nanoplastics can also pass through the placenta to the bodies of unborn babies. Scientists have long suspected their presence in bottled water, but lacked the technology to identify individual nanoparticles.

To overcome that challenge, the study’s co-authors invented a new microscopy technique, programmed a data-driven algorithm and used both to analyze roughly 25 1-liter bottles of water purchased from three popular brands in the US. (The researchers declined to specify which brands.) They found 110,000 to 370,000 tiny plastic particles in each liter, 90% of them nanoplastics.

The researchers targeted seven common plastic types, including polyethylene terephthalate (PET), which many water bottles are made from, and polyamide, often used in filters to purify water before it’s bottled. But they also discovered many unidentified nanoparticles in the water. If any of those are also nanoplastics, the prevalence of plastic in bottled water could be even higher.

The co-authors of the latest study say their research won’t stop at bottled water. They also plan to investigate nanoplastics in tap water and snow samples collected from western Antarctica.