## 2 Background
### Inhaled and Exhaled E-Cigarette Aerosol (ECA)  
ECA is generated by various physicochemical processes: self-nucleated condensation in a super saturated medium initiates immediately once the e-liquid vapor leaves the coil, and the nucleated centers generate small nanometer-scale droplets that grow through coagulation and diffusion (see detailed explanation in [35]). The particulate phase is \begin{table}
\begin{tabular}{c c c c c c} \hline \multicolumn{6}{c}{**Parameters of vaping topographies.**} \\ \hline  & \multicolumn{4}{c}{**Mouth to Lung (MTL)**} \\ \hline Intensity & \(m_{b}\) & \(V_{b}\) & \(\Phi_{b}\) & \(t_{p}\) & \(V_{T}\) \\ \hline Low & 2–10 mg & 20–100 & 20–40 & 2–5 & 500–1500 \\ \hline  & \multicolumn{4}{c}{**Direct to Lung (DTL)**} \\ \hline Intensity & \(m_{b}\) & \(V_{b}\) & \(\Phi_{b}\) & \(t_{p}\) & \(V_{T}\) \\ \hline High & 10–40 mg & 300–500 & 100–300 & 3–6 & 1000–3000 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of vaping topography for vaping styles. Puff topography parameters: \(m_{b}\), \(V_{b}\), \(\Phi_{b}\) are, respectively, mass (mg) (aerosol yield), volume (mL), flow (mL/s) per puff of ECA bolus (aerosol yield). Notice that tidal volume \(V_{T}\) listed in the table is not the tidal volume for quiet rest breathing (400–600 mL), since vaping involves suction of ECA through a mouthpiece (see Sections 3.1 and 3.2). Puff time (s) is \(t_{p}\). Values taken from from rough representative averages from data in Figures 1 and 3 of [PERSON] et al. [31] and also from [PERSON] et al. [32]. made of liquid droplets whose chemical composition closely matches that of the e-liquid: propylene glycol (PG), vegetable glycerin or glycerol (VG), nicotine and water [36], together with a trace level contribution of nanometer-sized metal particles [37]. The gas phase is chemically similar. The aerosol contains nicotine and residues produced from the pyrolysis of the glycols and the flavorings (mainly carbonyls), which can be in either the gas or particulate phase depending on their vapor pressure and volatility [38], with most of the PG evaporating into the gas phase and VG tending to remain in the droplets [36]. Count mean diameter (CMD) distributions of mainstream ECA droplets vary depending on the device, puffing style of users, flavors and nicotine content [35; 39]. Droplet number count is heavily dominated by submicron droplets with CMD distributions having either single modes below 100 nm or bimodal forms (one mode well below 100 nm and one in the range 100-300 nm) [35; 39; 40; 41; 42; 43]. However, particle size grows with increasing coil power [39] and even in low-powered devices the mass distribution is dominated by droplets larger than 600 nm [35]. In fact, [PERSON] et al. [35] found a third mode around 1 \(\upmu\)m that becomes more prominent at increasing power of the tested device while the nanometer-sized modes decrease, likely because higher power involves larger vaporized mass that favors coagulation and scavenging of nanometer-sized droplets by larger droplets. The inhaled aerosol mass yield depends on the topography parameters given in Table 1. At inhalation of mainstream ECA, instrument measured droplet density numbers are in the range \(n_{p}=1-5\times 10^{9}\)/cm\({}^{3}\) (see [39; 40; 41; 42; 43]). Total average ECA droplet numbers of \(N_{p}=7.6\times 10^{10}\) were reported by [PERSON] et al. [44] for a tank system using e-liquids with high nicotine content in a 2 s machine puff regime with \(V_{b}=50\) mL puff volume (\(N_{p}\) decreases 25% with nicotine-free e-liquids). (These machine puff time lapses are different from those reported in Table 1. The former correspond only to inhalation times as instruments aim at simulation of a mouth inhalation, the latter are time lapses in human vapers and thus include inhalation and exhalation.) Using the same experimental design, [PERSON] et al. [43] reported an increase of up to 30% for the 4 s machine puff regime.