The systematic review provides an overview of the few “real-life” studies on the seconhand exposureto aerosol of e-cigarettes. These studies indicate that emissions from e-cigarettes do contain potential toxic compounds such as nicotine, carbonyls, metals, and organic volatile compounds, besides particulate matter. While usually these compounds are generally at lower concentrations than those found in secondhand tobacco smoke, these findings made false the popular statement that e-cigarette emissions are “only water vapor,” or that they only include glycerin and propylene glycol beyond nicotine. The number of studies available and the types of e-cigarettes assessed is relatively small, and it is thus unknown if the chemicals and their concentrations vary markedly or not across different e-cigarette types. Moreover, whether secondhand exposure from e-cigarettes poses health risks at short- and long-term is still unknown, and needs further investigation.
Few studies have attempted to investigate e-cigarette aerosols in real-life conditions [8••]. In most of the papers [2••, 5•, 6, 9, 10•, 11, 12], “real-life conditions” refer to simulation of active vaping in a controlled room or chamber, by means of human volunteers actively vaping. Although this approach could serve to control for a number of variables by design, the conditions are so specific that generalization of results are far from satisfactory. Well conducted observational studies in true real conditions, in which the behavior of active vapers and bystanders is registered, together with a valid measurement of environmental markers and personal biomarkers of exposure, should offer new clues about the exposure to e-cigarette emissions.
We have found similar concentrations of PM2.5 in the smoke-free homes and in the e-cigarette user homes, both under 10 μg/m3, which is the threshold concentration for long-term exposures established in the Air Quality Guidelines of the World Health Organization . This is in contrast to the PM2.5 concentrations in the conventional cigarette user’s home, which were 58 times higher than in the e-cigarette user home. The air nicotine concentrations in the homes of smokers of conventional cigarettes were similar to the concentrations that have been observed in hospitality venues when smoking was allowed .
In our observational study, the particulate matter emissions from e-cigarette study were similar to those found in the smoke-free homes. We however observed PM2.5 peaks (over the 10 μg/m3 limit) concurrent with the e-cigarette puffs. This supports past observations that e-cigarettes emit particulate matter [2••, 5•, 6, 10•, 11]. E-cigarettes produce an aerosol with fewer chemical components than those in conventional cigarettes because they do not require combustion, and hence, the temperature reached is lower than that in the conventional cigarettes, as shown in other studies [3, 15].
Some caution in the interpretation of the results of our observational study is needed, because they are based in the homes of four volunteers and only one vaper, using a specific type of vaporizer. Another potential limitation could be related to the possible differences (size and distribution) of the particulate matter from e-cigarettes and conventional cigarettes. An experimental study with aerosol from three e-cigarettes produced by a standard smoking machine  showed that the average particle number concentration and particle size of the aerosol from the e-cigarettes is comparable to that of the fresh mainstream tobacco burning cigarette smoke. However, differences among e-cigarette aerosols, due to differences in the type of devices (i.e., cig a likes, medium-sized vaporizers, and tank vaporizers or “mods”) that operate at different voltages and temperatures are possible. Despite the potential limitations, our observational study is the first attempting to assess the emission of PM2.5 from e-cigarette vapor in real-life use conditions at home, with real e-cigarette and cigarette users and not smoking machines in a laboratory or controlled room, and a long time analyzed (60 min). As shown by the literature review, few studies have attempted to investigate e-cigarette aerosols in real-life or quasi-real-file conditions. In most of the papers, “real-life conditions” refer to simulation of active vaping in a controlled room or chamber, by means of human volunteers actively “vaping”. Although this approach could serve to control for a number of variables by design, the conditions are so specific that generalization of results are far from satisfactory. In addition to further controlled experiments mimicking real-life conditions with using e-cigarette users to produce the aerosols, well designed and conducted observational studies in true real conditions, in which the behavior of not only active vapers but also bystanders is registered, together with a valid measurement of environmental markers and personal biomarkers of exposure, should offer complementary clues about the exposure to e-cigarette aerosols.