A study in May, for example, found evidence that e-cigarette flavors had toxic effects -- including poorer cell survival and signs of increased inflammation -- on a type of cardiovascular cell in the lab.
"The use of e-cigarettes is increasing and the data demonstrating potential harm ... is also growing," doctors from the University of Massachusetts Medical School said in a commentary published alongside the May study. "In addition to harm from the nicotine, the additives are a potential source of adverse vascular health and one that is being disproportionately placed on the young."
While experts have long suspected that vaping poses fewer health risks than smoking cigarettes, the doctors wrote that "little is known about the potential toxicology" of flavorings, particles, heavy metals and other components used in e-cigarettes.
"Nobody knows what it does to the human lung to breathe in and out aerosolized propylene glycol and glycerin over and over. It's an experiment, frankly," Dr. Robert Jackler, founder of Stanford Research Into the Impact of Tobacco Advertising, said at a congressional hearing last month.
"We will find out, years from now, the results," he said.
Jackler said a number of chemicals used by the flavor industry may be safe when absorbed through the intestine, but we don't yet know the impact they can have on the lungs over a long period of time.
"There's no doubt in my mind that vaping is safer than conventional smoking," Jackler said, "but that doesn't mean that it's safe."