Developing Evidence to Inform Regulatory Policy on Nicotine Content in E-Liquids

The goal of this observational study is to learn if electronic cigarette users who currently choose to use lower nicotine eliquids have reduced harm compared those who choose higher nicotine eliquids. The main questions it aims to answer are: * Do lower nicotine users actually consume less nicotine compared to higher nicotine users? * Do lower nicotine users have lower amounts of harmful and potentially harmful constituent exposures compared to higher nicotine users? * Do lower nicotine users perceive their choice of product to be less harmful than higher nicotine products? Researchers will monitor ecig users of different nicotine concentrations for two weeks in their natural environment to determine how much eliquid and nicotine they consumed, assess their daily mood and craving, and measure exposure and health effect. Participants will: * Complete surveys on history of tobacco use, nicotine dependence and perception. * Use their electronic cigarette with their usual nicotine eliquid ad lib for two weeks in their natural environment. * Complete daily questionnaires for mood and craving for two weeks in their natural environment. * Collect three salivary samples at home, morning, afternoon and evening, each day for two weeks, in their natural environment. * Use a topography monitor to record puffing behavior for every puff taken during week two, in their natural environment. * Provide saliva, urine and blood samples in the lab at the end of each week.