E-cigarettes have been on the market for about a decade and are surging in popularity. But there’s still widespread disagreement about their safety or any benefits they may have.

Proponents say that e-cigs, a $2.8 billion market in the U.S., are potentially less harmful than conventional cigarettes and can help smokers quit. Critics say that their safety hasn’t been proven and that it’s too soon to know what the long-term effects of “vaping” (inhaling the vapor) may be. We spoke with medical experts and reviewed more than 50 scientific studies to find out what’s known.

How They Work

A typical battery-operated e-cigarette contains a cartridge of e-cig liquid, or “juice,” which usually contains nicotine and the chemical propylene glycol. They come in an array of flavors including cola and watermelon, which some say are meant to attract younger users. When used, the battery powers an atomizer that vaporizes the liquid in the cartridge for the user to inhale.

Aside from those basics, e-cigarettes vary widely in terms of ingredients, construction, and factors such as battery voltage. The Food and Drug Administration, which regulates regular cigarettes, has proposed regulating e-cigarettes more tightly, but some politicians and many in the industry oppose the plan.

The Nicotine Issue

Juice from e-cigarettes has different amounts of the addictive stimulant nicotine, from zero to about 72 milligrams per milliliter of liquid. (A traditional cigarette has 10 to 15 mg.) “Nicotine has short-term negative health effects, like increasing your heart rate and blood pressure, so it can aggravate heart conditions,” says Marvin M. Lipman, M.D., Consumer Reports’ chief medical adviser. “It also interferes with fetal development, making it unsafe in pregnancy regardless of its source.”

Larger doses of nicotine can be more harmful, especially to children, who may be attracted by the sweet flavors and brightly colored packages. Since January 2015, the American Association of Poison Control Centers has received more than 2,000 reports related to e-cigs, many about children who drank the juice or spilled it on their skin.

Chemical Concerns

When propylene glycol, the liquid in most e-cigs, is heated, it can degrade into formaldehyde, a chemical linked to nose and eye irritation, and an increased risk of asthma and cancer. The Center for Environmental Health recently tested 97 e-cig products and found formaldehyde and the chemical acetaldehyde in more than half of them.

E-cig vapor can also contain lead, cadmium, nickel, tin, and other metals, which can cause nervous system or respiratory problems. And some flavoring chemicals, like those used to create cinnamon flavor, can be toxic.

But unlike conventional cigarettes, e-cigs don’t burn tobacco. So they don’t produce the tar that clogs the lungs or carbon monoxide, which is linked to heart disease.

Overall, experts say more research is needed. But given what we know now, vaping is less harmful than smoking tobacco. “If a patient switches from smoking two packs a day to only using e-cigs, it’s not as good as quitting, but it’s undeniably better,” says Douglas Kamerow, M.D., M.P.H., a former assistant surgeon general and a professor of family medicine at Georgetown University. “But if a nonsmoker starts vaping and gets hooked on nicotine, especially if it leads to tobacco smoking, that’s a problem.”

Can E-cigs Help Smokers Quit?

A recent clinical trial found no real difference between e-cigs and nicotine patches in helping people stop smoking: Neither were very effective. Another study found that teenagers who smoke and vape tend to smoke more conventional cigarettes than those who smoke but don’t use e-cigs. In September, the U.S. Preventive Services Task Force said the evidence was insufficient to recommend e-cigs as a smoking-cessation device.