Human breast milk has been sold online for years, and health experts have warned about possible dangers. Now they have a new warning: Some of the milk for sale isn’t strictly human — it’s been topped off with cow’s milk.

That milk could be dangerous for some babies, says Sarah Keim, a researcher at Nationwide Children’s Hospital in Columbus, Ohio.

Keim led a study published Monday in Pediatrics that shows 10 out of 102 breast milk samples purchased online contained at least 10% cow’s milk. The added milk could have come straight from cartons or from baby formula, Keim says.

More: Some Online Breast Milk Might Contain Cow’s Milk