A new study finds many teens who use e-cigarettes mistakenly think they are only vaping non-nicotine products, HealthDay reports.

Researchers surveyed 517 people ages 12 to 21 about their use of e-cigarettes, traditional cigarettes and marijuana. They then compared the survey results against the results of urine tests that detect a chemical called cotinine, which is a marker for the presence of nicotine.

About four of every 10 participants who said they vaped only non-nicotine products were found to have cotinine markers in their urine, the researchers report in Pediatrics. In many cases, the teens used Juul vaping pods. The researchers say the pods “have the highest nicotine concentration to date.”

“Many of our participants were unaware of the nicotine content of the e-cigarette products they were using,” the researchers concluded.