New parents know that surprises abound in a baby’s diaper. But according to new research, it’s what you can’t see (or smell) that could be a cause for alarm. Infant fecal matter contains a lot of plastic.

In a study published in Environmental Science & Technology Letters, scientists at the New York University School of Medicine braved the task of examining retrieved baby poop from six 1-year-old infants by running it through a filter to trap microplastics. They discovered it harbored a median of 36,000 nanograms of polyethylene terephthalate, or PET, a common plastic, per gram of feces. That was roughly 10 times the amount found in the 10 samples of adult waste they analyzed. The researchers estimated that a baby is exposed to an average of 83,000 nanograms of PET per kilo of body weight each day.

How does PET wind up in poop? The plastic is practically everywhere, from carpet fibers to the plastic bottles used for baby formula to toys. It’s also found in bedding and clothing. Because babies are explorers, low to the ground and tactile, they seem to be at greater risk of exposure. Eating, drinking, crawling, or even gumming a LEGO could be a potential pathway to either swallow or inhale tiny amounts of the plastic. They even found amounts in the very first feces of newborns.

There are some caveats. The pilot study looked at poop from three newborns, six infants, and 10 adults, which is not an appreciably large sample from which to draw firm conclusions. But considering how pervasive PET is in modern life, it’s safe to say babies are probably being exposed to plenty of it. (And not from diapers, which are made from polypropylene.)

Should it be a cause for worry? Science isn’t sure yet. Microplastics contain over 10,000 chemicals, some of which are known to be harmful and some that are still poorly understood. They can also accumulate heavy metals. It’s possible plastics could interrupt normal endocrine, hormonal, or neurological development, making their presence in baby discharge concerning. Even though it comes out in waste, there’s potential for it to be absorbed by the body.

One easy way to reduce a baby’s exposure is to skip heating plastic bottles for formula. Use a glass bottle instead. You can also avoid food wrapped in plastic. But totally eliminating exposure in the modern world is going to be a tall order.

[h/t Wired]