It may be a better idea to just grab a glass from the tap.

A lot of bottled water contains tiny bits of plastic, known as microplastics, according to research conducted by a non-profit journalism organization called Orb Media.

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.

Most microplastics are thought to pass through our systems, although very small particles (such as those found in the surveyed bottled water) may be absorbed into our organs, such as the liver and kidneys. That’s one concern; the other is that the particles, even those we excrete, might give off toxins as they pass through us.

What is undeniable is that, for many people who lack access to safe drinking water from taps, bottled water remains a necessity.

Orb Media was also behind a study last year that showed tap water around the world is contaminated with microplastics. The particles have also been found in beer, honey, table salt, and of course seafood, as our oceans are increasingly polluted with plastic waste.

Some countries, particularly in Europe, have recently started taking the plastic waste threat seriously. The EU has a target of making all plastics recyclable by 2030, while the U.K. has adopted a slightly less urgent target of eliminating all avoidable plastic waste within the next 25 years.

As noted in Orb Media’s piece, EU rules ban contaminants in bottled water, although they don’t specifically regulate microplastic content. The U.S. also lacks specific laws on microplastics in food and drinks.

This article was updated to include the quote from Andrew Mayes.