- Aim 1 only: 1) smokers who smoke at least 10 cigarettes per day with a stable smoking pattern of at least 6 months and no prior e-cig use within one year; smoking status will be confirmed by salivary cotinine; subjects with at least 10 ng/ml cotinine in their saliva being considered smokers; 2) e-cig users who report using e-cigs daily for 3 months with at least one nicotine-containing cartridge/day or 1 ml of liquid/day; they should have not smoked a cigarette for at least one year; 3) non-smokers who have smoked less than 100 cigarettes in their lifetime and not for at least 1 year, and not have used an e-cig for at least 1 year; lack of regular smoking will be confirmed by salivary cotinine