*The following is excerpted from an online article posted on HealthDay.
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.
Dr. Karen Wilson, chief of general pediatrics for Mount Sinai Health System in New York City, said the stronger nicotine hit produced by dripping also could do harm to the developing brains of teenage users.
"Adolescents should not be using nicotine at all," Wilson said. "It changes the brain chemistry, and adolescents are uniquely susceptible to the addictive properties of nicotine."
Out of 7,045 high school students surveyed, almost 1,100 had used e-cigarettes, the researchers found. One out of four e-cigarette users had tried dripping.
Reasons the students gave for dripping included producing thicker clouds of vapor (64 percent), which suggests these users may engage in smoke tricks or vape competitions, the study authors said.
Better flavor was the reason cited by nearly two out of five students who dripped, and simple curiosity attracted 22 percent.
About 28 percent said the practice produces a stronger "throat hit," or the feeling produced on the back of the throat during inhalation.
White students overall and boys were more likely than others to have tried dripping, the researchers said.
The U.S. Food and Drug Administration has announced its intention to regulate e-cigarettes as tobacco products, but hasn't yet rolled out its new rules.
The study appears online Feb. 6 in the journal Pediatrics.