Packages of Juul electronic cigarettes are seen on display at Urge, a smoke shop in Hoboken, N.J., Thursday, Dec. 20, 2018. Altria, one of the world's biggest tobacco companies, is spending nearly $13 billion to buy a huge stake in the vape company Juul as cigarette use continues to decline. (AP Photo/Julio Cortez)

HUNTINGTON - As quickly as it appeared, vaping and e-cigarettes uncorked a new batch of questions and health concerns across the country.

Introduced in the United States in 2007, an estimated 3.6 million U.S. teens are now using e-cigarettes, representing 1 in 5 high school students and 1 in 20 middle-schoolers, according to the latest federal figures provided to the Associated Press. In just over a decade, it's developed into a $6.6 billion industry.

Its popularity, particularly among teens and young adults, is undeniable, but many questions remained unanswered. It's cultivated a whirlwind of new concerns from health and prevention professionals.

That includes the government's top doctor, U.S. Surgeon General Dr. Jerome Adams, who advised parents, teachers and government officials to take "aggressive steps" to keep children from using e-cigarettes.

For young people, "nicotine is dangerous and it can have negative health effects," Adams said in an interview with the Associated Press. "It can impact learning, attention and memory, and it can prime the youth brain for addiction."

Most devices heat a flavored nicotine solution into an inhalable vapor. They have been pitched to adult smokers as a less-harmful alternative to cigarettes, though there's been little research on the long-term health effects or on whether they help people quit. Even more worrisome, a growing body of research suggests that teens who vape are more likely to try regular cigarettes.

Federal law bars the sale of e-cigarettes to those under 18.

Adams singled out Silicon Valley startup Juul. The company leapfrogged over its larger competitors with online promotions portraying their small device as the latest high-tech gadget for hip, attractive young people. Analysts now estimate the company controls more than 75 percent of the U.S. e-cigarette market.

"We do know that these newer products, such as Juul, can promote dependence in just a few uses," Adams said.

By design, e-cigarettes are appealing to teens and young adults, explained Teresa Mills, prevention coordinator at the Cabell-Huntington Health Department. They're sleek (the popular Juul resembles a flash drive), and offer sweet flavors like candy, fruit or chocolate. The exhaled vapor likewise not only smells sweet instead of traditional tobacco smoke, it also dissipates quicker - making it easier to use discreetly inside.

The surgeon general's advisory notes that each Juul cartridge, or pod, contains as much nicotine as a pack of cigarettes. Additionally, Adams states that Juul's liquid nicotine mixture is specially formulated to give a smoother, more potent nicotine buzz.

"What we're learning is that most middle- and high-schoolers do not realize the level of nicotine that are in the Juul product," Mills said. "The kids really don't understand or have a perspective of the harm it can do."

While there's a public notion that vaping is safer than traditional cigarettes, the aerosol byproduct and high nicotine exposure are far from harmless, advised Sarah Lawver, American Lung Association director for advocacy for West Virginia. The mid-to-long term effects of e-cigarette use are not yet defined as the products are not subject to scrutiny by the Food and Drug Administration.

"Because there's no FDA review of these products, we just don't know what is in these e-cigarette products," Lawver said.

Moving forward, prevention experts are looking toward the West Virginia Legislature to take action, chiefly by passing legislation that would raise the legal age to purchase tobacco and e-cigarettes from 18 to 21. A bill to do such was introduced in the 2018 session, only to die in committee without a vote.

"We're very concerned we're at risk of losing another generation to tobacco products through these e-cigarettes," Lawver said.

Additionally, advocates are calling for West Virginia to reinstate funding for tobacco prevention and cessation program, which was completely eliminated in 2017. West Virginia is one of only two states to allocate zero state dollars toward these efforts.

That, compared to the skyrocketing use and millions spent on marketing these devices, is a metaphorical mountain for prevention experts, Mills said.

"We specifically need prevention funding because we want to get them before they start," she said. "Because once they do, it's hard to quit."

Advocates will rally at the West Virginia State Capitol on Jan. 25 during the legislative session for Tobacco-Free Day, a public push to raise the legal age to 21.

The Associated Press contributed to this report.