One-quarter of U.S. teen e-cigarette users have experimented with "dripping" -- a new vaping method that produces thicker clouds of vapor, researchers report.

Regular electronic cigarettes produce inhalable vapor by gradually drawing liquid into a heating coil through an automatic wick, explained lead researcher Suchitra Krishnan-Sarin.

"Dripping" involves placing drops of e-liquid directly onto the exposed heating coil of an e-cigarette or atomizer, and then immediately inhaling the cloud of vapor produced, said Krishnan-Sarin, a professor of psychiatry at Yale University School of Medicine in New Haven, Conn.

"They say it makes the flavors taste better and gives you a stronger hit," Krishnan-Sarin said.

She said she learned about the practice while talking with teenagers, and decided to ask about it in a survey on e-cigarette use among high school students.

The survey revealed that 26 percent of student e-cigarette users at eight Connecticut high schools had tried dripping at least once.

"I didn't know what to expect," Krishnan-Sarin said. "We didn't know what we would find, because we only had anecdotal evidence based on what kids were telling us."

Experts are concerned that "dripping" could expose users to increased levels of toxins and carcinogens created when the liquid in e-cigarettes is vaporized at high temperatures.

Previous research has shown that "the levels of some chemicals like formaldehyde and other aldehydes, which are known carcinogens, are higher with direct dripping than with conventional e-cigarette use," Krishnan-Sarin said.

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