You’d be justified in thinking the ingredients inside your bottle of water are fairly simple—but a safety probe has been launched by the World Health Organization after a study found tiny pieces of plastic in more than 90 percent of bottled water, including in some of the world’s most popular brands. The study from D.C.-based nonprofit journalism organization Orb Media found that one bottle of water can potentially contain thousands of plastic fibers that are small enough to be ingested, with the sizes of the pieces ranging from “the width of a human hair down to the size of a red blood cell.” The study analyzed 259 bottles from 19 locations in nine countries across 11 different brands, and concluded that an average of 325 plastic particles were contained in every liter of water sold, and only 17 of the 259 bottles were free of plastics. The WHO said it understood the study would spark concern among the public and that it would launch a review into the safety of bottled water. A spokesperson said the agency would “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.”