A US-based study has find tiny particles of plastic in most samples of bottled water tested by researchers, but it’s not known whether the so-called microplastics are a health risk.

The study, carried out by non-profit journalism group Ord Media, involved tests on more than 250 bottles from 11 top bottled water brands from countries in Asia, Europe, Africa, and the Americas. It found 93 per cent of bottled water sampled had some sign of microplastic contamination.

For plastic particles in the 100 micron, or 0.10 millimetre size range, tests at the State University of New York revealed a global average of 10.4 plastic particles per litre. These particles were confirmed as plastic using an industry standard infrared microscope, Orb reported.

The tests also showed a much greater number of even smaller particles that researchers said were also likely plastic. The global average for those particles was 314.6 per litre. With the smaller particles there was a possibility some of the particles could be other contaminants as well as plastic, though “rationally expected to be plastic”, Orb reported.

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Some bottles had thousands, a few “effectively” had no plastic. One bottle had more than 10,000 particles per litre. The scientific report said data suggested contamination was at least partially coming from packaging and/or the bottling process.

Bottled water manufacturers had emphasised their products met all government requirements.

A 2016 European Food Safety Authority report said as many as 90% of microplastic particles consumed might pass through the gut without any impact. Of the rest, some can lodge in the kidneys and liver.

Orb quoted one toxicologist saying knowledge about the toxicity of microplastics was limited. As fas as was known there was little health concern.

A World Health Organisation official told BBC News the research had not been done to know what the plastic particles might do in the body. WHO was to launch a review into the potential risks of plastic in drinking water.

Gerolsteiner, a German bottler, and Nestle both said their own tests found much lower quantities of microplastics in their water than the amount found in the Orb study.

Orb was also behind a study last year that showed tap water around the world is also contaminated with microplastics. That study used different methods to identify microplastic but there was room to compare the results, Orb said.

For microplastic debris around 100 microns in size, about the diameter of a human hair, bottled water samples had 4.45 microplastic particles per litre, compared to the 10.4 in the bottled water.

Microplastic was also found in water in glass bottle samples.

University of East Anglia biochemistry lecturer Andrew Mayes, who developed a technique used in the study to identify the microplastic, told Fortune a batch of glass bottles checked for comparison, also had microplastics.

Two of the best-known brands tested in the Orb study were Evian and San Pellegrino. For both brands, there were samples with no microplastics, while for Evian the highest number was 256 particles “rationally expected to be plastic”, while for San Pellegrino the highest number was 74.

Danone, which owns the Evian brand, told BBC News it could not comment on the Orb study because “the methodology used is unclear”.

It said its bottles had “food grade packaging” and highlighted a smaller 2017 German study that found plastic particles in single-use bottles but not above a statistically significant amount.