Nicotine delivered by e-cigarettes may pose a health risk comparable to traditional tobacco products, a researcher at Johns Hopkins University believes.

While the flavor element in e-cigarettes is heated and therefore eliminates the toxins created by burning tobacco, it still delivers nicotine, a highly addictive substance that raises the user's blood pressure and spikes adrenaline. That boosts the user's heart rate and increases the likelihood of a heart attack, Dr Michael Blaha, a researcher at the university in Baltimore, Maryland, said in a research note.

"There are many unknowns about vaping, including what chemicals make up the vapor and how they affect physical health long term," he said. "People need to understand that e-cigarettes are potentially dangerous to your health. You're exposing yourself to all kinds of chemicals that we don't yet understand and that are probably not safe."

Electronic cigarettes are as addictive as tobacco products because both contain nicotine, which may be as habit-forming as heroin and cocaine, the doctor warned.

"Many e-cigarette users get even more nicotine than they would from a tobacco product," Blaha said. "You can buy extra-strength cartridges, which have a higher concentration of nicotine."

Nearly 7 in 10 smokers say they want to quit, and some use e-cigarettes as a way to quit smoking, but vaping products have not been approved by the US Food and Drug Administration for that purpose, the doctor warned.

More research is needed to determine if reducing the amount of nicotine in an e-cigarette helps some users quit smoking. But it appears e-cigarettes can introduce teenagers to tobacco products. Many vaping products are offered in sweet, fruity flavors that appeal to young users.

"What I find most concerning about the rise of vaping is that people who would have never smoked otherwise, especially youth, are taking up the habit," Blaha said.

The reasons may be simple: With no smell created by burning tobacco, the stigma of smoking is reduced, e-cigarettes generally cost less than traditional cigarettes, and many believe vaping is less harmful than smoking, the researcher said.

Through Nov 20, the US Centers for Disease Control and Prevention (CDC) reported 2,290 cases of lung injury associated with e-cigarettes, or vaping, in all states except Alaska.

Researchers have confirmed 47 deaths in 25 states. Those who died ranged in age from 17 to 75 years. Ninety-five percent of those with lung injuries were hospitalized, 68 percent were male and 15 percent were younger than 18.

Earlier this month in Michigan, a teenager underwent what may be the first double-lung transplant after sustaining lung damage from vaping.

Most users who sustained lung injuries or died had a history of using THC-infused vaping products. THC is the psychoactive ingredient in marijuana that gives users a "high" or "buzz". The CDC said users should not buy any vaping products, especially those containing THC, from "informal sources" such as friends, family, in-person or online dealers.

Investigators discovered that THC products obtained illegally often contain vitamin E acetate used as a thickening agent, but caution that more research is needed to establish a conclusive link with reported lung injuries.

A 17-year-old male in Canada developed what appears to be a new type of vaping-related lung damage and nearly died after using TCH-infused and flavored e-cigarettes for five months.

The disease has not been found among users of vaping products in the US. It is officially known as bronchiolitis and generally called "popcorn worker's lung" and named for factory workers who inhaled diacetyl, a chemical approved for food flavoring and once used in buttery popcorn. The problem was first observed in 1985. The chemical is no longer used by most major manufacturers of microwave popcorn, the CDC said.

In the disease, the smallest airways in the lung — the bronchioles — become scarred and constricted. That blocks the movement of air, which makes the bronchioles unable to expel carbon dioxide, which can build up to toxic levels, researchers said.

There are now about 5.3 million US high school and middle school students who use e-cigarettes, compared with 3.6 million student users last year, two surveys published this month in the Journal of the American Medical Association found.

Last week, the Massachusetts state Senate approved a bill to impose a 75 percent tax on e-cigarettes, to impose a first-in-the-nation ban on menthol-flavored cigarettes and to improve access to smoking-cessation programs. The governor has not yet signed the bill into law.