Background: Electronic cigarettes (e-cigarettes) are novel nicotine delivery devices. E-cigarette user nicotine dependence and secondhand e-cigarette exposure have not been well characterized. Objectives: 1) To characterize secondhand e-cigarette exposures during e-cigarette conventions to formaldehyde, acetaldehyde, acrolein, nicotine, and propylene glycol using environmental monitoring, 2) to characterize secondhand e-cigarette exposures to acrolein, nicotine, and tobacco-specific nitrosamines (TSNAs) during e-cigarette conventions using biomarkers of exposure, 3) to characterize secondhand e-cigarette exposures during e-cigarette conventions using 8-isoprostane, a biomarker of effect and, 4) to assess nicotine dependence of e-cigarette users attending an e-cigarette convention. Methods: Data was collected at five e-cigarette events throughout the Southeast from October 2015 to March 2017. During the first convention, e-cigarette users completed a survey that included questions from the Fagerstrom Test for Nicotine Dependence (FTND) and questions concerning behavioral and addiction characteristics. During the remaining four conventions (September 2016 to March 2017), a secondhand exposure assessment using biological and environmental monitoring was completed using non-smoking volunteers (n=34). Air pumps were placed inside backpacks worn by volunteers (n=22) and used to sample the air in e-cigarette environments. Urine and saliva samples were collected pre- and post-exposure. Urine samples were analyzed for cotinine, trans-3-hydroxycotinine, two acrolein metabolites, four tobacco-specific nitrosamines (TSNAs), and 8-isoprostane. Saliva samples were analyxed for cotinine and trans-3-hydroxycotinine. Results: E-cigarette vapor contained elevated concentrations of propylene glycol (median=305.51 g/m3) and nicotine (median=1.10 g/m3). Airborne concentrations of formaldehyde, acetaldehyde, and acrolein during e-cigarette events were low. Concentrations of creatinine-corrected urinary cotinine, trans-3-hydroxycotinine, S-(3-hydroxypropyl)-N-acetylcysteine (3-HPMA), S-carboxyethyl-N-acetylcysteine (CEMA), and salivary cotinine significantly varied across sampling times. Most users surveyed were classified as moderately dependent using the FTND (average FTND score = 5.0). Most (85.2%) e-cigarette users smoked tobacco cigarettes prior to beginning e-cigarette use. Conclusion: E-cigarette secondhand exposures in public settings with high-concentrations of e-cigarette vapor do not contain elevated concentrations of formaldehyde or acetaldehyde. Secondhand e-cigarette vapor does contain elevated concentrations of nicotine, propylene glycol, and possibly acrolein. E-cigarette use can result in average nicotine dependence higher than previously reported for e-cigarette users and tobacco smokers.