Shutterstock A handful of recent studies are beginning to reveal the health effects of e-cigarette use, and they are not all positive. Some evidence has suggested that e-cigs may help adults quit smoking conventional cigarettes, but other studies have found that they might encourage teens to start. Regulators and health experts are particularly concerned about a device called the Juul, which packs the same nicotine content per pod as a pack of cigarettes. Despite capturing more than 70% of the e-cig market and being recently valued at $15 billion, Juul faces a growing backlash from the FDA and scientists who say the company intentionally marketed to teens. In studies of e-cigarettes that don't include Juul, researchers have found evidence of toxic metals like lead in e-cig vapor.