The research, conducted at the State University of New York, took in more than 250 bottles from 11 different brands, sold across nine countries. “A few” of the bottles effectively contained no plastic, while others had thousands. None of the brands came out entirely unscathed.

Two of the bottled water manufacturers cited in the study—Nestle and Gerolsteiner—disputed Orb Media’s results, saying they found much lower quantities of microplastics in their water. A bottle of Nestle Pure Life water showed the highest levels in Orb Media’s study, with 10,390 particles per liter.

Is this a problem? No one’s really sure, because there isn’t enough data on the health effects of ingesting plastic. So, partly in response to Orb Media’s research, the World Health Organization has now told the BBC it is launching its own review into the potential risks.

And don’t think the problem only applies to plastic bottles. “The study mostly focused on plastic bottles, but one batch of glass ones were checked for comparison. It turns out that the glass ones have microplastics too,” Andrew Mayes, the University of East Anglia biochemistry lecturer who developed the particle-spotting technique used in this research, told Fortune.

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