E-cigarettes may be a popular alternative to the traditional smoke sticks, despite contrasting opinions on how safe they are. Adding to the list of harmful effects, the American Thoracic Society published research at its annual conference that certain flavors used in vaping liquid may alter cells in lung tissue. During the study, researchers exposed human airway epithelial cells to doses of 13 e-cig flavors for periods of 30 minutes and 24 hours. 5 of the 13 displayed "adverse effects to cells in a dose-dependent manner." Of those five, three flavors -- Hot Cinnamon Candies, Banana Pudding (Southern Style), and Menthol Tobacco -- were toxic to lung cells at higher doses in the 30-minute test. What's more, when cells were exposed for 24 hours, the same trio of flavors stunted cell growth as the dosage increased. Also of note: the flavorings' negative effects didn't occur with nicotine or the e-liquid vehicle on its own (propylene glycol and vegetable glycerin).