Scientists in Austria say they’ve detected tiny bits of plastic in people’s stool for the first time, but experts caution the study to be too small and premature to draw any credible conclusion.

Presenting their findings at a congress in Vienna on Tuesday, researchers from the Medical University of Vienna and the Environment Agency, Austria said their pilot study detected so-called microplastics in all samples taken from eight volunteers in Europe, Russia and Japan.

“It’s small scale and not at all representative,” said Martin Wagner, a biologist at the Norwegian University of Science and Technology.

He noted that the study wasn’t reviewed by independent scientists and the authors haven’t provided details about the measures taken to prevent samples from becoming contaminated.

Also, according to Wagner; if microplastics are found in stool, this doesn’t mean they have entered the human body, he said. Unlike other substances we eat, microplastics are too large to be absorbed by cells in the gut and simply pass through.

Microplastics — defined as pieces smaller than 5 millimeters — have previously been found in water, animals and food, but so far studies haven’t proved they pose a risk to human health.