###### Abstract  
We examine the plausibility of aerial transmission of pathogens (including the SARS-CoV-2 virus) through respiratory droplets that might be carried by exhaled e-cigarette aerosol (ECA). Given the lack of empiric evidence on this phenomenon, we consider available evidence on cigarette smoking and respiratory droplet emission from mouth breathing through a mouthpiece as convenient proxies to infer the capacity of vaping to transport pathogens in respiratory droplets. Since both exhaled droplets and ECA droplets are within the Stokes regime, the ECA flow acts effectively as a visual tracer of the expiratory flow. To infer quantitatively the direct exposure distance, we consider a model that approximates exhaled ECA flow as an axially symmetric intermittent steady starting jet evolving into an unstable puff, an evolution that we corroborate by comparison with photographs and videos of actual vapers. On the grounds of all this theoretical modeling, we estimate for low-intensity vaping (practiced by 80-90% of vapers) the emission of 6-210 (median 39.9, median deviation 67.3) respiratory submicron droplets per puff and a horizontal distance spread of 1-2 m, with intense vaping possibly emitting up to 1000 droplets per puff in the submicron range with a distance spread over 2 m. The optical visibility of the ECA flow has important safety implications, as bystanders become instinctively aware of the scope and distance of possible direct contagion through the vaping jet. electronic cigarettes; droplet dynamics; bioaerosols; turbulent jets; COVID-19 202129 June 20219 July 2021  
along a continuous spectrum of diameters into two mutually exclusive modalities; thus, we avoid as much as possible the binary \"droplets\" vs. \"aerosols\" terminology. The purpose of the present paper is to fill an important gap in the above-mentioned body of literature on pathogen (including SARS-CoV-2) spread through respiratory droplets, namely to examine the plausibility, scope and risks of this transmission taking place through a different expiratory route: exhaled e-cigarette aerosol (ECA). While there is no factual evidence or proper elaborate research (a search of the literature revealed three opinion pieces [19; 20; 21] that offer a very limited argumentation) on pathogen transmission through this vehicle, we hypothesize that it is entirely plausible that it occurs, simply because vaping (usage of e-cigarettes) is an expiratory activity (as well as smoking). To fulfill this task, we need to develop a comprehensive and interdisciplinary theoretical modeling of this phenomenon, proceeding along the following methodological steps:  
1. The wide variety of vaping styles and puffing protocols bears influence on respiratory parameters. We follow a practical and convenient simplification of this intensity spectrum by considering two principal modes: a low-intensity style (\"mouth to lung\") style and a high-intensity one (\"direct to lung\")
2. Available evidence on the physical and physicochemical properties of ECA show that its particulate phase (submicron liquid droplets) evolves within the Stokes regime, in which viscous forces dominate over inertial forces, so that particle Reynold numbers are negligible (\(Re_{p}\ll 1\)). As a consequence, the particles trace the fluid flow (basically exhaled air) roughly as tracer gases or molecular contaminants. 3. To infer the respiratory dynamical parameters that can be associated with vaping, we need to estimate the volume of exhaled ECA aerosol per puff (vaping tidal volume) and its exhalation velocity. We use for this purpose available data on a very useful proxy: cigarette smoking. Since vaping involves suction through a mouthpiece, we also need to look at the effects of these instruments on respiratory parameters. 4. Given the outcomes of the previous step, we estimate a range of exhalation velocities that can associated with vaping.