Our results showed that concentrations of analyzed carbonyls were higher in exhaled e-cigarette breaths than in background breaths in the majority of participants’ sessions. The total carbonyl concentration, on average, was 10.5 times higher in exhaled e-cigarette breaths than in background breaths. Our results clearly showed that high carbonyl concentrations—including those of potentially hazardous formaldehyde, acetaldehyde, and acrolein—were not limited to dry puff conditions [ 13 ], since participants were using their e-cigarettes in their typical “vaping” style. None of the participants using their own or the provided e-cigarette with a flavored e-liquid complained of unpleasant sensations during vaping sessions. The only complaint was received from a participant who was offered unflavored pure PG/VG liquids that were found to be “unpleasant.” High RT uptake of acetaldehyde (mean: 91.6 ± 9.9%) and formaldehyde (mean: 99.7 ± 0.9%) was obtained for all cases, and no significant difference was observed for RT uptake of these aldehydes between male and female participants. High exposure to formaldehyde (1.53–24.4 μg·puff; mean: 7.8 μg·puff) was observed in six (out of 18) cases, and the mean value of these exposure levels is comparable with exposure to conventional cigarette formaldehyde (~5 μg·puff) [ 34 ]. The Acute Exposure Guideline Levels (AEGL-1) for formaldehyde, acetaldehyde, and acrolein are 1.1, 81, and 0.070 mg·m, respectively, for 10 min exposure [ 35 ]. We converted our aldehyde levels into mg·mfor 10 min exposure ( Supplementary Material, Table S4 ) and found that formaldehyde concentrations were above the AEGL-1 for sessions #3 (1.93 mg·m) and #7 (4.44 mg·m) and were close to the AEGL-1 for participants’ sessions #10 (0.76 mg·m) and #12 (0.84 mg·m). Acetaldehyde levels didn’t exceed the AEGL-1 for any participants. In the case of acrolein, the exposure level (0.250 mg·m) was 3.6 times higher than the AEGL-1 for participant session #7.

The observed large variability in aldehyde concentrations was most likely because of differences in e-cigarette conditions (type of e-liquid and e-cigarette, e-cigarette settings) and volunteers’ vaping styles (or vaping topography).

The present study has several limitations. First, the sample size was rather limited, considering the observed variability among participants in their vaping styles, used e-cigarettes, and e-liquid flavors. Twelve e-cigarette users were recruited; one male and one female participant were engaged seven and two times, respectively. Thus, 19 experimental sessions were performed during the study ( Table 1 ). The sample size was sufficient, however, to detect a significant increase in aldehydes and MEK concentration in exhaled e-cigarette breaths relative to background breaths. Second, the puff duration of individual participants was measured with a timer as no topography devices were available, making puff duration measurements less accurate (±1 s). Among all participants, the puff duration varied from 2 to 4 s. Given a linear dependence of carbonyl emissions on puff duration and that the mean puff duration was 3 s, our estimates of inhaled carbonyls could be up to 50% uncertain. In order to reduce this uncertainty during the sampling of mainstream e-cigarette emissions, we asked participants to manually depress the e-cigarette power button for the duration they use when vaping. This way, the puff duration during e-cigarette use by a participant is expected to be close to the puff duration for direct e-cigarette emissions generation, thus significantly reducing the uncertainty. We need to emphasize that in future studies, it is important to use a vaping topography device to minimize the uncertainty in carbonyl generation during e-cigarette use. Third, no losses of breath aerosols onto sampling bag walls ( Figure S1a ) or chemical transformations undergone by carbonyls during the sampling were evaluated. To avoid the chemical transformation of unsaturated carbonyls [ 21 ], the samples were eluted within two hours after the sampling and analyzed within 24 h. Another limitation in relation to overall health impact assessment was that this study focused only on analysis of aldehydes, while other chemicals (e.g., toluene, lead, naphthalene, flavorings) have also been found in e-cigarette vapors [ 36 37 ] and may have a substantial impact on human health. In addition, our recent experiments with DNPH cartridges and DNPH impregnated filters showed that even though the DNPH-cartridge is an effective medium to collect gas-phase carbonyls [ 38 ], levels of particle phase carbonyls can be underestimated (~30%). More details on efficiency of different sampling media to collect gas and particle phase e-cigarette carbonyls will be presented in a following paper.