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.

Most recently, 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.”

Previous reports have also found health concerns with e-cigarettes. A 2015 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 percent of all adults (and more than 21 percent 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 percent saying they had used the devices, says the CDC.

In May 2016, however, the Food and Drug Administration banned e-cig sales to children under 18.

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

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

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 goes into effect early August 2016 which 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.”