The tests, carried out at the State University of NY in Fredonia and co-ordinated by US-based journalism organisation Orb Media, found an average of 10 plastic particles per litre of water, each larger than a human hair.
Using an infrared microscope, the researchers found that, for particles around 0.1 millimeters in size, there's roughly 10.4 per liter (2.3 gallons) of bottled water in various major brands.
Mr Bruce Gordon, coordinator of the WHO's global work on water and sanitation, said that he did not want to alarm anyone, and also emphasised that a far greater waterborne threat comes in countries where supplies can be contaminated with sewage.
"I think it is coming through the process of bottling the water".
Microplastics are pieces of plastic less than five millimeters long.
What is undeniable is that, for many people who lack access to safe drinking water from taps, bottled water remains a necessity.
The study identified plastic particles in the water by staining them with a fluorescent Nile Red dye, which made them visible under a blue light.
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The result of this eye-opening research was published simultaneously in 12 worldwide news media on Thursday, March 15, and Tempo was one of the ones included, and the only Indonesian media granted the access to publish the research as an exclusive piece.
Bottled water was about 1000 times more expensive than tap water in the USA, and sold under the pretence of being purer and safer, he said. The analysis was performed by Professor of Chemistry Sherri Mason and her team at her lab at the State University of NY.
The scientists tested 259 individual bottles in nine countries around the world, and 11 different brands. There were an average of four plastic particles per liter. Dr Mayes told Orb Media he was "satisfied that it has been applied carefully and appropriately, in a way that I would have done it in my lab".
CBC News also asked multiple experts to review Orb's study; while similar questions came up with the Nile Red dye, they were convinced there was some level of microplastics in the water and further research was warranted.
Nestle and Gerolsteiner have responded to the results of the study.
Danone, the company behind Evian and Indonesian brand Aqua, told Orb it is "not in a position to comment as the testing methodology used is unclear".
It looked at popular brands of water including Aquafina, Dasani and Nestle Pure Life.
"Orb's findings suggest that a person who drinks a liter of bottled water a day might be consuming tens of thousands of microplastic particles each year", it said. Fiji Water had 12 plastic fibres per litre.