For a decade, scientists have debated the safety of e-cigarettes: Are they safer than tobacco? Can they help people quit smoking? Will they cause nonsmokers, particularly kids, to start?

Even based on normal use of e-cigs, the answers aren't clear. Research published Monday in Pediatrics found that a quarter of teen users of e-cigarettes potentially got higher exposures to toxic chemicals because they had tried "dripping," a practice that creates a stronger hit and thicker  clouds of vapors.

"We did not know what to expect," said lead author Suchitra Krishnan-Sarin, a psychiatry professor at Yale University School of Medicine. She said she conducted the survey at eight Connecticut high schools after hearing teenagers talk about the practice.

E-cigarettes normally work by allowing users to draw a flavored liquid containing nicotine from a miniature tank through a battery-powered heating element, often a coil. The liquid is vaporized and then condenses in the device to form a mist that is inhaled through a filter.

"Dripping," by contrast, involves placing drops of the e-liquid directly on a modified  heating coil and inhaling the vapor directly.

It takes some work to modify the device.  That's one reason that it's "nowhere near as common as it used to be," said Shane King, who was working Monday at Love Vape in Philadelphia's Queen Village section.

Other shops agreed that the practice was in sharp decline locally, for a variety of reasons.

Alex Shanfeld, owner of Funny Monk Emporium in Manayunk, said that a 40 percent state tax had dramatically cut into the vaping business, and that dripping in particular was done by a do-it-yourself niche group that tended to buy online.

At  Vapor Heaven in Northeast Philadelphia, Nate Schwartz said that dripping had plummeted from a majority of e-cig users a few years ago to single digits today "because you can take the same flavor and cloud off of the tanks. The technology has improved" just within the last year or so, said Schwartz, who is training to be a manager.  "They are just building the heating element better."

The Connecticut survey of 7,000 students was completed in 2015. Of the nearly 1,100 who said they had ever used e-cigarettes, 26 percent reported dripping. Of those, 64 percent said they did it to produce thicker clouds of vapor, 39 percent said it made flavors taste better, 28 percent said it produced a stronger throat hit, and 22 percent said they were curious.

The survey did not ask how often or for how long the students dripped, Krishnan-Sarin said in an interview, so it was impossible to determine whether the practice was new or a trend.

She said that she cautions parents about dripping, however, because one study that used machine-generated aerosols to examine nicotine and other emissions from one product had found that "volatile aldehyde emissions (such as formaldehyde) were higher with direct dripping than with conventional e-cigarette or combustible cigarette use."

The risks of short-term and long-term use of e-cigarettes are not known, Krishnan-Sarin said.

"While e-cigarettes may contain less toxicants than cigarettes, e-cigarettes contain many other chemicals like propylene glycol and glycerin, which, when heated at high temperatures (as with 'dripping'), can produce high levels of carcinogenic compounds like aldehydes.

“E-liquids also contain many flavor chemicals, such as aldehydes, vanillins, and alcohols, which are considered safe for ingestion, but little is known about the toxicity of inhaling these chemicals, especially at high temperatures. So, along with encouraging their teen to not use cigarettes, parents should also encourage them to not use e-cigarettes, and especially avoid alternative behaviors like dripping.”