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Chances are good that you’ve seen people “smoking” e-cigarettes: inhaling from cigarette-shaped devices, then puffing out clouds of odorless fog. E-cigarettes, also known as e-cigs or electronic nicotine delivery systems, have been on the market for about a decade and are surging in popularity. But there’s widespread disagreement about their safety or any benefits they may have.

Proponents say that e-cigs, a $2.8 billion market in the United States, are potentially less harmful than conventional cigarettes and can help smokers quit. Critics say that their safety hasn’t been proved and that it’s too soon to know what the long-term effects of “vaping” (inhaling the vapor) may be. Consumer Reports 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. The juices 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 FDA’s 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 milligrams.) “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, 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. And in 2014, poison-control centers responded to 3,783 e-cigarette and liquid nicotine exposure cases. More than half of those involved children younger than 6 who might have ingested or inhaled liquid nicotine or gotten it on their skin or in their eyes. In 2015, those centers received 3,067 nicotine-exposure reports across all age groups.

Chemical concerns

When propylene glycol 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, such as 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, 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 they 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. Last fall, the U.S. Preventive Services Task Force said the evidence was insufficient to recommend e-cigs as a smoking-cessation device.

For more information about e-cigs, go to consumerreports.org/cro/health/e-cigarette-guide/index.htm.

Copyright 2015. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.