When e-cigarettes were introduced to the U.S. market in 2007, they were billed as a safe alternative to conventional tobacco cigarettes. But since then various studies have found that these e-cigs may not be so harmless after all.

Researchers found that almost all e-cigarette vapor contains two cancer-causing chemicals: propylene glycol and glycerin. The 2016 study was published in the journal Environmental Science and Technology.

“Advocates of e-cigarettes say emissions are much lower than from conventional cigarettes, so you’re better off using e-cigarettes,” said Berkeley Lab researcher and the study’s corresponding author Hugo Destaillats, in a statement. “I would say, that may be true for certain users — for example, long time smokers that cannot quit — but the problem is, it doesn’t mean that they’re healthy. Regular cigarettes are super unhealthy. E-cigarettes are just unhealthy.”

However, previous reports have also found health concerns with e-cigarettes. A report published in the New England Journal of Medicine found that e-cigs give off formaldehyde, which is another carcinogen. And in 2009 the FDA announced that a laboratory analysis of electronic cigarette samples found that they contain “carcinogens and toxic chemicals such as diethylene glycol, an ingredient used in antifreeze.”

Despite these dangers, people are still lighting up electronically. Nearly 13% of all adults (and more than 21% of adults 18 to 24) used e-cigarettes in 2014, according to a report by the Centers for Disease Control and Prevention. E-cigarette use among middle and high school students tripled from 2013 to 2014, with more than 13% saying they had used the devices, says the CDC.

In an interview with the Washington Post, U.S. Surgeon General Vivek H. Murthy called the increasing number of young people using e-cigarettes a "major public health concern." "We know enough right now to say that youth and young adults should not be using e-cigarettes or any other tobacco product, for that matter," Murthy said. "The key bottom line here is that the science tells us the use of nicotine-containing products by youth, including e-cigarettes, is unsafe."

There may be one small silver lining in this cloud. A 2016 study published in the British Medical Journal found that e-cigarettes may have helped more people quit smoking. Researchers found that the number of people trying to quit didn't change, but the number of successful attempts increased. About 18,000 additional people kicked the habit in the U.K. in 2015, the study found.

E-cigarette sales in the U.S. reached $3.7 billion in 2015, and analysts predict that sales could hit $10 billion in five years.

How they work

man vaping in a cloud of smoke E-cigarettes have an atomizer that heats a nicotine liquid that turns to vapor. (Photo: PavelKant/Shutterstock)

Many of them are designed to look like conventional cigarettes, but they are actually battery-operated devices. They contain an atomizer that heats a nicotine liquid that turns to vapor; the vapor is then inhaled and exhaled, much like tobacco smoke.

The liquid — commonly known as e-juice, e-liquid, smoke juice and cig juice — comes in a cartridge and is a mixture of nicotine, water, glycerol, propylene glycol and flavorings, according to the Tobacco Vapor Electronic Cigarette Association.

The amount of nicotine varies; consumers can select cartridges ranging in strength comparable to ultralight cigarettes all the way to regular-strength smokes.

And yes, the liquid comes in flavors. Although e-cigarette manufactures say they don’t market to young people, liquid cartridges come in a nauseating array of flavors — everything from butterscotch and fruit punch to cinnamon bun and milkshake.

E-cig rules

San Francisco will become the first city to ban e-cig sales after city officials voted June 26 to ban stores from selling the items and online retailers from delivering to city addresses, according to CNN. The measure heads to Mayor London Breed's desk, but she has already expressed her support.

San Francisco is home to leading e-cigarette manufacturer Juul Labs.

The new legislation highlights what e-cigarette opponents and health advocates say is the FDA's failure to act on the issue, points out CNN. Officials from San Francisco, New York and Chicago criticized the FDA in a March letter for allowing e-cigs to remain for sale without undergoing a review on their impact on public health.

“San Francisco has never been afraid to lead and we’re certainly not afraid to do so when the health and lives of our children are at stake,” City Attorney Dennis Herrera said in that letter. "By law, before a new tobacco product goes to market, the Food and Drug Administration is supposed to conduct a review to evaluate its impact on public health. Inexplicably, the FDA has failed to do its job when it comes to e-cigarettes. Until the FDA does so, San Francisco has to step up."

The Food and Drug Administration banned e-cig sales to children under 18 in May 2016, but other regulation has been slippery. According to the FDA, currently e-cigarettes that are marketed for therapeutic purposes (like the cessation of smoking) are regulated by the FDA Center for Drug Evaluation and Research (CDER). But the agency didn’t have jurisdiction over the rest of the market ... until recently. The FDA finalized a rule that went into effect in August 2016 that broadens the administration's authority to include e-cigarettes.

The FDA has stated its intent to issue a proposed rule that would extend the agency's authority to products that meet the statutory definition of “tobacco product,” which would include e-cigarettes.

“Before this final rule, these products could be sold without any review of their ingredients, how they were made, and their potential dangers,” explains Mitch Zeller, J.D., director of the FDA’s Center for Tobacco Products, in a statement. “Under this new rule, we’re taking steps to protect Americans from the dangers of tobacco products, ensure these tobacco products have health warnings, and restrict sales to minors.”

Editor's note: This story has been updated since it was originally published in November 2015.

How safe are e-cigarettes?
Even as San Francisco becomes the first city to ban them, electronic cigarettes are surging in popularity.