Austrian researchers have found evidence that tiny plastic particles – or microplastics – can be found in human feces.

Though the study analyzed the stool samples of only eight people, all of them tested positive for plastic.

Microplastics are harmful to marine life, but their effect on humans is still unknown.

The World Health Organization launched an investigation earlier this year after a separate study found plastic in 90% of water bottles tested.

A group of Austrian researchers have found evidence that microplastics – extremely small particles of plastic beads, fibers, or fragments – accumulate in human feces.

Scientists from the Environment Agency Austria and the Medical University of Vienna analyzed the stool samples of eight participants from all over the world, including Italy, Japan, Poland, the Netherlands, Russian, the United Kingdom, Finland, and Austria.

Throughout the study, the participants recorded what they ate in the week prior to their stool sampling. They all drank from plastic bottles or consumed plastic-wrapped foods in that time, according to the study. All eight stool samples tested positive for microplastics.

According to the study, published Monday in the United European Gastroenterology Journal, up to nine types of plastic were found in the stool samples.

Researchers noted that the microplastics, which can form when larger pieces of plastic break down, could help transmit toxic chemicals and pathogens into the human body. They also noted that the particles could weaken the immune response of the gut.

Plastic manufacturing has skyrocketed around the world in the last two decades; almost half of all the plastic ever made has been manufactured in the 21st century. But only 20% of plastic is recycled, National Geographic reported, and people continue to buy a global average of nearly 1 million plastic bottles per minute.

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About 18 billion pounds of plastic flow into oceans each year. After sea animals consume some of this plastic, humans are likely to ingest it through tuna, shrimp, or lobster, the study said. In addition, humans likely consume plastic that enters products from food processing systems.

Monday’s study may be the first to show the presence of plastics in the human gut, and it comes a few months after the World Health Organization announced it would investigate the potential effects of plastic on human health. The WHO launched the review in March after a separate study found microplastics in 90% of 259 bottles.

Previous studies have shown that plastic is present in the food and drinks we consume, including fish and water, though it remains unclear how microplastics affect our bodies.