Summary: The proposed research, ?Understanding the Association Between Electronic Cigarette Aerosol Emissions, Tobacco Product Characteristics and User Topography and Consumption Behavior,? is comprised of three specific aims: (1) conduct screening design of experiments for 24 e-cig products and 8 e-liquid compositions; (2) evaluate the dependence of total particulate mass concentration as a function of product characteristics and user topography behavior, and (3) evaluate the dependence of nicotine mass fraction of emissions present in the whole aerosol as a function of product characteristics and user behavior characteristics. The Food and Drug Administration (FDA) acknowledges that previous protocols for testing tobacco products, influenced by the tobacco industry, did not represent actual use behavior and yielded emissions that did not represent actual exposure. Although new protocols have been adopted in FDA guidances for reporting harmful and potentially harmful constituents (HPHC) in cigarettes, informed protocols for alternative tobacco products such as electronic cigarettes (e-cigs) are needed. This project addresses this gap, emphasizing the metrics used by Tobacco Regulatory Science (TRS) investigators when reporting emissions from electronic cigarettes. Analysis of TRS literature demonstrates lack of consensus regarding metrics for reporting emissions and report various, sometimes apparently conflicting, dependence of emissions upon User Topography Behavior (UTB) and Product Characteristics (PC). We propose a theoretical framework defining HPHC mass concentration, CHPHC [mg/mL], as the product of the total particulate mass concentration, CTPM [mg/mL], of whole aerosol emissions delivered to a user and the mass ratio of HPHC constituents, fHPHC [mg/mg], present in the whole aerosol emission. The proposed framework permits experimental decoupling of investigations and provides a means for consolidating results from independent TRS researchers into a comprehensive understanding of emissions from e-cigs and e-liquids. This project is related to the ?Behavior? scientific domain. This investigation will (1) propose a standardized test protocol for e-cigs and e-liquids; (2) proposed standardized emissions outcome measures; and (3) inform guidelines for substantial equivalence products and modified risk criteria to distinguish low- and high-dose ENDS.