Tiny pieces of plastic measuring less than five millimeters in length—or roughly the size of a sesame seed—have become a nearly ubiquitous presence in our world. Scientists have found the particles, better known as microplastics, everywhere from the oceans, the air to tap and bottled water as well as beer, and table salt. But a new pilot study detailed at yesterday’s United European Gastroenterology meeting adds a somewhat surprising hiding place to the list: human stool.

Wired’s Robbie Gonzalez reports that a team of Austrian researchers led by Philipp Schwabl, a physician-scientist at the Medical University of Vienna, asked eight participants from eight different countries—Austria, Italy, Finland, Japan, the Netherlands, Poland, Russia and the United Kingdom—to track their food consumption over the course of one week and provide a stool sample at the end of the testing window.

Back in the lab, the scientists screened the stool for 10 types of microplastics, including polyethylene terephthalate (PET), which is commonly used in plastic bottles and shopping bags, and polypropylene (PP), which is found in bottle caps and rope. Nine of the 10 varieties were ultimately detected, with PET and PP topping the list. All eight of the samples tested positive for plastics.

An average of 20 microplastic particles were present in every 10 grams of feces, but Inverse’s Emma Betuel notes that overall quantities were all over the map, with different samples including between 18 and 172 particles per 10 grams. The microplastics measured between 50 and 500 micrometers; for comparison, a single strand of human hair is about 100 micrometers thick.

As Laura Parker writes for National Geographic, the test subjects’ food diaries offer a thorough list of potential plastic culprits. Of the three men and five women, all aged 33 to 65, two chewed gum on a daily basis, while six ate seafood during the week in question. All dined on plastic-wrapped food and drank from plastic water bottles.

But Schwabl tells Wired’s Gonzalez that it’s unclear which of these items left plastic lurking in the participants’ stool. The seafood, plastic packaging or even traces of table salt could be behind the unwelcome discovery.

Then again, the culprit could be something else entirely: Richard Thompson, a marine scientist at the U.K.’s University of Plymouth who was not involved in the study, tells National Geographic's Parker that it’s possible PET particles simply fell from curtains or clothing onto the subjects’ plates, silently mingling with a tasty seafood dish before hitching a ride to the intestines. (Earlier this year, Thompson and his colleagues published a paper comparing the risk posed by airborne plastic fibers that land on food during preparation with that of eating Scottish mussels exposed to seaborne plastics particles. Interestingly enough, the airborne fibers posed a greater threat to human test subjects.)

Inverse’s Betuel notes that the presence of plastics in participants’ stool suggests some particles are still hiding inside the body. As Schwabl explains, microplastics may build up in the intestine over time, causing inflammation and potentially affecting the gut’s tolerance and immune system. Although the effects of microplastics on human health are still under investigation, previous animal studies have shown the pesky particles can affect the bloodstream, lymphatic system and even the liver.

According to Deutsche Welle, microplastics are either intentionally manufactured (think exfoliating beads seen in facial scrubs) or the product of larger plastic items, such as packaging or clothing fibers, breaking down over time. Researchers have found evidence of microplastics in animals across the global food chain, but this is the first time the particles have been spotted in human excrement.

Still, Schwabl tells National Geographic’s Parker that the team’s findings are far from conclusive, especially due to the pilot study’s small sample size.

“We didn’t study harm,” Schwabl concludes. “We showed there are microplastics in human stool. Up to now, people believed it, but now we know it. That’s important.”