Many teens who use e-cigarettes aren’t aware they are inhaling nicotine when they vape, even though they are often taking in high levels of the addictive substance, a new study suggests.

Researchers who collected both survey data and urine samples from more than 500 adolescents found that 40 percent of teens who thought they were using nicotine-free products had positive urine sample tests, according to the study published Monday in Pediatrics.

“This is one of the first studies showing the amount of nicotine kids are getting from e-cigarettes,” said the study’s lead author Dr. Rachel Boykan, a clinical associate professor of pediatrics in the Renaissance School of Medicine at Stony Brook University. “They’re getting a lot — as much or more than they would with traditional cigarettes.”

The analysis of urine samples showed that vapers are getting plenty of nicotine even when they think they weren’t getting any, Boykan said.

Boykan and her colleagues also found that vaping teens were often using their devices with cannabis.

Experts worry that use of e-cigarettes with nicotine will produce a generation of addicted young people who may stick with vaping, or later turn to more dangerous traditional cigarettes.

Between April 2017 and April 2018, the researchers asked 517 adolescents, aged 12 to 21, to complete a questionnaire on tobacco, e-cigarette and marijuana use. Of the volunteers, 284 were also asked to provide a urine sample. The surveys and samples were collected anonymously, with the researchers connecting the two types of data through numbers assigned to each of the volunteers, who received a $5 Starbucks coupon for participating.

Nicotine levels were measured indirectly through the amount of a metabolite called cotinine found in volunteers’ urine.

Among the 517 volunteers: 13.9 percent reported ever smoking cigarettes; 36 percent reported trying e-cigarettes; and 31.3 percent reported they had tried marijuana. Only 2.9 percent reported smoking in the previous week; 14.3 percent reported e-cigarette use during the past week; and 11.4 percent reported using marijuana in the past week.

For their analysis, the researchers used data from 265 of the volunteers who had both survey and urine sample data. Among those who said they thought they were using nicotine-free products, 40 percent had significant levels of cotinine in their urine.

Cotinine levels were highest among adolescents using a newer vaping technology: pod mods. A pod mod is an e-cigarette that uses a disposable pod, or cartridge. The levels of cotinine in pod users was nearly the level seen in tobacco smokers. People who vaped older versions of e-cigarettes had much lower levels of cotinine in their urine samples.

'A bigger buzz'

“Now we have this huge problem with lots of kids using these products with many not understanding what they are getting into,” said pediatrician Dr. Sharon Levy.

“Nicotine is a highly addictive substance and it’s being delivered at much higher concentrations than before," Levy, director of the Adolescent Substance Use and Addiction Program at Boston Children’s Hospital and an associate professor of pediatrics at the Harvard Medical School, told NBC News. "So they’re getting a bigger buzz.”

Levy’s own son asked her if e-cigarettes contained “just juice flavoring” when he was in the eighth or ninth grade, she said.

Dr. Andrew Stokes, an assistant professor of global health at the Boston University School of Public Health, is alarmed by the new study.

Stokes conducted an e-cigarette study, published in February, that found an increased risk of teens who vape to eventually smoke traditional cigarettes.

In fact, vaping teens who didn’t see themselves as ever smoking at the beginning of the study were nearly nine times as likely as those who didn’t use e-cigarettes to later smoke traditional cigarettes.

“What is most alarming is the finding of significantly higher cotinine levels in those who used pod mods, a new class of cartridges on the market,” Stokes said. “And the finding that many of the kids were not aware the products contain nicotine is concerning. It suggests that this may be a pathway into nicotine addiction that the kids were not anticipating.”