Some types of e-cigarettes contain enough alcohol to affect motor skills, a new study concludes. E-cigarettes deliver nicotine by vaporizing liquids, which may contain alcohol and other chemicals.

Yale University researchers tested people who used two commercially available e-cigarettes with either high or low amounts of alcohol. Neither group said they felt differently after they inhaled the vapor. But those who used e-cigarettes with high alcohol levels performed more poorly on psychomotor tests. In some cases, they had detectable levels of alcohol in their urine.

“They didn’t actually know they were under the influence of alcohol,” lead researcher Dr. Mehmet Sofuoglu told CNBC. “It still influenced their performance.”

About three-quarters of the commercial e-cigarette liquids tested contained less than 1 percent alcohol. Some e-cigarette users create their own liquids with high alcohol content, the researchers note in the journal Drug and Alcohol Dependence. Almost nothing is known about the prevalence of using e-liquids that contain alcohol, they said.

Sofuoglu said the findings are worrisome, especially in light of a recent government report that found e-cigarette use among teens tripled from 2013 to 2014. An estimated 13 percent of high school students used e-cigarettes in 2014—compared with 9 percent who smoked traditional cigarettes.

“Given the widespread and unregulated use of e-cigarettes, especially by youth and other vulnerable populations, further studies are needed to evaluate both the acute safety and long-term health risks of using alcohol-containing e-cigarettes,” he said in a news release.