A saleswoman helps a customer as she shops for an electronic cigarette at the Vapor Shark store in Miami.

U.S. health officials are flagging the increasing popularity of flavored e-cigarettes as an emerging public threat.

Fruity, sweet and menthol electronic cigarettes started dominating the market over the last few years, making up roughly 56 percent of the industry's sales in 2016, up from 42 percent in 2012, according to a new study from the Centers for Disease Control and Prevention published in the journal Preventing Chronic Disease.

Flavored cigarettes are particularly worrisome to public health officials because of their appeal to teenagers.

"We know these products contain nicotine, which can harm the adolescent brain," said Brian King, one of the study's authors and deputy director for research translation in the CDC's Office on Smoking and Health. While there could be benefits for adults wanting to quit smoking, he said, "there's no redeeming quality for kids."

Israel outlawed the import and sale of e-cigarettes made by Silicon Valley start-up Juul Labs earlier this week, citing public health concerns given their high nicotine content.

The CDC's data are two years old and don't capture current trends. If they did, the numbers would likely be even higher given how popular newer brands like Juul have become.

So far this year, the entire industry has raked in $2.31 billion in sales, up from $1.3 billion in the same period last year, according to Nielsen data compiled by Wells Fargo analyst Bonnie Herzog. Juul has fueled the growth, with its $1.29 billion in sales, accounting for more than half of the market so far this year.

Juul's vaping device, the size of a flash drive, delivers more nicotine than regular cigarettes and accounts for about 70 percent of all e-cigarette sales. With flavors such as creme bruleé and mango, Juul's sales have soared 728 percent over the 12 months ended Aug. 11, according to Nielsen.

Research shows flavors draw adolescents to tobacco products, causing the Food and Drug Administration to ban flavors, excluding menthol, from tobacco cigarettes in 2009. This did not include e-cigarettes, which tend to boast fruity flavors.

Public health officials and anti-tobacco advocates worry these are attracting — and addicting — a new generation to nicotine.

"I always like to say advertising brings the horse to water, flavors get them to drink and nicotine keeps them coming back for more," King said.

Researchers examined Nielsen scanner data for the study, which exclude sales from vape shops and online stores, which they said may underestimate the percent of flavored e-cigarette sales.

Sales of flavored e-cigarettes, excluding menthol, made up 19.8 percent of the market in 2016, up from 2.4 percent in 2012. Menthol e-cigarette sales declined to 36.6 percent in 2016 from 39.9 percent in 2012.

These data measured sales, not who purchased the products, so they cannot conclusively say there's been an increase in young people using e-cigarettes. However, anecdotal evidence suggests these devices, particularly Juul, are becoming more popular among teens.

Within three years, Juul has dominated the e-cigarette market, representing nearly two-thirds of all sales in the past month, according to Nielsen data compiled by Wells Fargo analyst Bonnie Herzog. The brand's popularity has terrified anti-tobacco advocates who for years have pushed cigarette smoking rates to historic lows.

Critics say Juul's fruity flavors attract young people, including kids, who have never smoked, not adults who want to quit smoking. Supporters say flavors help adults who are trying to kick cigarettes and the taste associated with them.

Regulators are now looking to tighten the rules governing e-cigarettes. The FDA is considering banning menthol in cigarettes and fruity flavors in e-cigarettes, asking for research and feedback on the role flavors play in attracting young people and helping adults switch. The agency received 525,302 public comments and is expediting its review.

Some local governments have banned flavored e-cigarettes. Providence, Rhode Island, prohibited most options in 2012 but pardoned menthol.

Rhode Island was the only state reviewed that saw a decrease in flavored e-cigarette sales, but it had the second-highest rate of menthol sales, or 44 percent. King said this reinforces the importance of examining all flavors when crafting public health policy.