A new study of 259 water bottles has found tiny pieces of plastic in more than 90 percent of them, prompting a review by the World Health Organization (WHO).

As Graham Readfearn reports for the Guardian, the study was conducted by scientists at the State University of New York (SUNY) in Fredonia on behalf of Orb Media, a non-profit journalism organization based in the United States. Researchers analyzed water bottles from nine countries—the U.S., China, Brazil, India, Indonesia, Mexico, Lebanon, Kenya and Thailand—and found that 93 percent showed at least some sign of contamination from microplastics, or plastic debris less than five millimeters in length.

To identify the microplastics, scientists used a dye called Nile Red, which binds to free-floating pieces of plastic and causes them to fluoresce, according to David Shukman of the BBC. The researchers discovered an average of 10 plastic particles per liter of water, and another 314 particles per liter that they assume to be plastic, but cannot positively identify because they are too small.

Out of all the bottles tested, only 17 had no traces of microplastics. Some had numbers ranging into the thousands. A total of 11 different water bottle brands were tested, among them Nestlé Pure Life, Evian, Dasani and San Pellegrino. There were big differences within brands and even within the same pack of bottles.

As David Common and Eric Szeto of CBC News point out, it is not clear how the microplastics are getting into the water: The debris could be coming from water sources, from the manufacturing or bottling processes, or even from opening bottle caps, which might cause plastic chips to fall into drinks.

A spokesperson for the WHO told the Guardian’s Readfearn that in response to the study, the organization plans to “review the very scarce available evidence with the objective of identifying evidence gaps, and establishing a research agenda to inform a more thorough risk assessment.”

People today drink a tremendous amount of bottled beverages—nearly 500 billion plastic bottles were sold across the globe in 2016 alone—but whether or not microplastics have an adverse effect on human health remains uncertain. “[T]here's just not the research there to tell us,” Bruce Gordon, coordinator of the WHO’s global work on water and sanitation, tells Shukman of the BBC. “We normally have a ‘safe’ limit but to have a safe limit, to define that, we need to understand if these things are dangerous, and if they occur in water at concentrations that are dangerous.”

Professor Sherri Mason, a microplastics researcher and lead author of the new study, said in an interview with Shukman that some of the plastic particles discovered in the water bottles are large enough to simply pass through the body. But these particles, she added, “can release chemicals that cause known human health impacts.” Researchers are also concerned that some of the particles are small enough to travel beyond the gastro-intestinal tract and into the rest of the body.

“We don’t know the implications of what that means on our various organs and tissues,” Mason said.

Though the new study has sparked concerns among scientists and health officials, there are a number of significant drawbacks to the research. While Orb consulted toxicologists and microplastics experts throughout the research process, the study was not published in a scientific journal and has not been subjected to a peer review.

Additionally, as Common and Szeto of the CBC point out, it is possible for Nile Red dye to adhere to substances other of than plastic. Several of the brands that were tested in the study have in fact said that their internal research showed much lower concentrations of microplastics than the ones discovered by the SUNY researchers. Speaking to the CBC, Nestle suggested that the Nile Red dye might “generate false positives.”

If nothing else, the new report highlights the need for further investigation into microplastics exposure and its effect on human health, which is still very much an emerging field of scientific research. Dr. Stephanie Wright of the King’s College Centre for Environment and Health tells the BBC’s Shukman that microplastic particles might “stay within an immune cell in the gut lining, or be passed into our lymphatic system ending up in the lymph nodes, or there is a small potential for them to enter the blood stream and possibly accumulate in the liver.” And because plastic is not biodegradable, these particles might cause harm to human tissue.

“But at the moment,” Shukman adds, “we don't know."