We know almost nothing about the health effects of something most of us probably eat every day: plastic.

Scientists just a couple of weeks ago announced the first preliminary evidence that small bits of plastic are getting into us, though this was long suspected given that much of what we eat and drink is stored or served in it.

Plastic was already known to contaminate the marine food chain, getting into plankton, shellfish, fish, turtles and sea birds. It’s easier to study animals: Scientists have found plastic pieces of all sizes by dissecting individual creatures that had died.

But then some enterprising scientists from the Medical University of Vienna looked into the matter of human plastic ingestion by collecting stool samples. Out of eight human subjects – from Europe, Russia and Japan – all had ingested bits of plastic ranging in size from rice grains to sand grains. (The rest of us are all human subjects in a much larger experiment for which we didn’t volunteer.)

The researchers looked for 10 kinds of plastic and found nine. The most prevalent were polyethylene terephthalate, known as PET, and polypropylene – components of food packaging and water bottles. The findings were presented at a gastroenterology conference on Oct. 23.

It’s mildly reassuring that this plastic had exited the subjects’ bodies. And the results are still preliminary. Skeptics in the scientific community urged caution, because it was a small study, not yet peer reviewed, and because the researchers hadn’t ruled out the possibility that the plastic bits had come from laboratory contamination. But then, if microplastic is so ubiquitous that trained scientists can’t keep it from contaminating a lab experiment, how could any of us possibly keep it from contaminating breakfast, lunch and dinner?

If the pieces of plastic were in these people’s digestive tracts, nobody is sure yet how they got there. Some might be coming up the food chain – ingested when we eat creatures that ate plastics, or when we eat creatures that ate creatures that ate plastic, etc. Some may be coming from water bottles and food containers. Other studies have shown that both tap water and bottled water are contaminated with microplastics. A story in National Geographic points out that carpeting and other household items can shed plastic fibers, and that fibers from synthetic clothing are floating around our homes.

Also unknown are the health ramifications, if there are any. In studies of sea birds, ingesting plastic had exposed them to chemicals called phthalates, which are known hormone disrupters.

From an evolutionary standpoint, we know we’re not well adapted to eating plastic, and it’s well known that plastics can kill sea birds and marine life, though they often ingest much more, and in bigger pieces relative to body size, sometimes mistaking plastic trash for jellyfish or other sources of food.

If there were ever a study that called for a follow-up, it’s this one. A bigger sampling of people might give us clues as to where the plastic is coming from and how it might affect us. Maybe if people get worried about our own health, we’ll be more likely to do something about the plastics that are already killing so many other animals.