UPI reports e-cigarette users are inhaling unknown ingredients that are irritating their air passageways.br When people vape, a chemical reaction forms in which vanilla-flavored glycerols turn to acetals.br Researchers created a "vaping machine" to test eight different flavors of Juul refill cartridges.br They found four of those flavors included menthol, which can increase nicotine cravings.br The nicotine content of e-cigarette flavorings are equal to two packs of tobacco cigarettes.