In a rare national advisory, the top US public health official warned Americans of the dangers of e-cigarettes like the Juul, a popular device that lets users inhale nicotine vapor without burning tobacco.

US Surgeon General Jerome Adams said in the advisory on Tuesday that e-cigs like the Juul are a particular danger to kids and teens and called for fresh measures to halt their rising popularity.

“We need to protect our kids from all tobacco products, including all shapes and sizes of e-cigarettes,” Adams said in a statement, adding, “We must take action now to protect the health of our nation’s young people.”

The advisory singles out Juul multiple times, saying the sleek devices are popular among teens because they’re easy to conceal and don’t emit much odor. It tells parents, health professionals, and teachers to be on the lookout for all forms of nicotine-delivery devices, including e-cigs.

Adams’ announcement comes on the heels of warnings from several other federal agencies about a rise in e-cig use, including from the Centers for Disease Control and Prevention and the Food and Drug Administration.

In November, after new CDC data pointed to a 78% increase in e-cig use among high-school students, FDA Commissioner Scott Gottliebannounced moves to further restrict sales of e-cigarettes to prevent them from getting into the hands of young people. That included a crackdown on flavored offerings, which researchers say appeal strongly to young people.

Several days before the FDA’s announcement, Juul Labs, the Silicon Valley startup behind the most popular e-cig in the US, temporarily halted sales of its flavored varieties in stores until they agreed to adopt the company’s new age restrictions and a stronger system for making sure customers are at least 21 years old.

‘E-cigarettes and youth don’t mix’

Though smoking conventional cigarettes is uniquely deadly, and vaping appears to be somewhat healthier (especially for adults looking to switch), public health experts are concerned about how e-cigarettes affect young people.

Because of their runaway popularity, e-cigs could create a new generation of Americans hooked on nicotine, one of the world’s most addictive substances and the key ingredient in e-cigs like the Juul, these experts warn. Their concern comes in part from a host of studies suggesting that teens who vape are significantly more likely to go on to smoke regular cigarettes than teens who never vape.

This finding could be related to the way nicotine affects the developing brains of young people. Though the research on e-cigs is still limited because the devices are so new, researchers have a wealth of data on the negative effects of nicotine on teens who start smoking early.

In brain-imaging studies of adolescents who started smoking in their teens, researchers have found signs of reduced activity in the prefrontal cortex, the part of the brain tied to planning and decision-making. The same teens performed worse on memory and attention tasks than teens who didn’t smoke.

Nicholas Chadi, a clinical pediatrics fellow at Boston Children’s Hospital, spoke about the Juul at the American Society of Addiction Medicine’s annual conference this spring. He described some anecdotal effects of nicotine vaping that he’d seen among teens in and around his hospital.

“After only a few months of using nicotine,” the teens “describe cravings, sometimes intense ones,” Chadi said, adding that “after only a few hundred cigarettes — or whatever the equivalent amount of vaping pods — some start showing irritability or shakiness when they stop.”

Most e-cigs contain toxic metals, and using them may increase the risk of a heart attack

Beyond the effects of e-cigs on the developing brain, a host of health issues related to e-cigs is beginning to emerge.

This spring, scientists looked at the compounds in several popular brands of e-cigs aside from the Juul and found some of the same toxic metals that are in conventional cigarettes, such as lead.

A study published this month found that people who vape tended to have high concentrations of some of these toxic chemicals in their bodies.

In another study published this summer, scientists concluded that there was substantial evidence tying daily e-cig use to an increased risk of a heart attack. And this fall, a small study with rats suggested that vaping could have a negative effect on wound healing that’s similar to the effect of regular cigarettes.

But many teens may not be aware of these health risks. Researchers say that could be because so little research has focused on the Juul, which has captured a nearly 80% market share in the US.

So for a study published in October, researchers from Stanford University’s School of Medicine surveyed young people who vaped and asked them whether they used the Juul or another e-cigarette.

From their sample of 445 high-school students, the researchers observed that teens who used the Juul tended to say they vaped more frequently compared with those who used other devices. Juul users also appeared to be less aware of how addictive the devices could be, compared with teens who used other e-cigs.

“I was surprised and concerned that so many youths were using Juul more frequently than other products,” Bonnie Halpern-Felsher, a professor of pediatrics who was a lead author of the study, said in a statement.

“We need to help them understand the risks of addiction,” she added. “This is not a combustible cigarette, but it still contains an enormous amount of nicotine — at least as much as a pack of cigarettes.”