3.3. Volatile Organic Compounds (VOCs; Including Nicotine, Propylene Glycol and Glycerol) and Low Molecular Weight Carbonyls
Table 1 summarizes the results for VOCs, including nicotine, propylene glycol and glycerol (the three principal components of e-cigarette base liquid) and low molecular weight carbonyls. Nicotine is present in most e-liquids and e-cigarettes, and several studies have investigated its presence in the ambient air following product use. After the generation and release of e-cigarette aerosol using a smoking machine into an exposure chamber, McAuley
et al. [
11] reported airborne nicotine concentrations ranging from 0.725 to 8.77 µg/m
3 following use of rechargeable e-cigarettes with refillable cartomisers containing 24 mg/mL or 26 mg/mL nicotine. Similarly, Czogala
et al. [
12] used three different e-cigarette products containing 16 mg/mL or 18 mg/mL nicotine and found airborne concentrations in an exposure chamber ranging from 0.82 to 6.23 µg/m
3. Both these studies (and others) used a machine approach to simulate the use of e-cigarettes for estimating potential bystander exposures to exhaled e-cigarette aerosol [
11,
12,
26]. Such an approach does not account for consumer behavior nor the retention of nicotine by the e-cigarette user and so is likely to overestimate airborne nicotine concentrations and potential bystander exposures. In a volunteer study conducted by Schober
et al. [
13], it was found that the nicotine concentration in the ambient air ranged from 0.6 to 4.6 µg/m
3 during a 2 h vaping session using a rechargeable e-cigarette with refillable tank (“open” system).
Table 1.
Average indoor air concentrations of VOCs (including nicotine, propylene glycol and glycerol (principle components of the e-liquid)) and low molecular weight carbonyls (µg/m3) measured before, during and after use of e-cigarettes from two independent sampling sites.
Table 1.
Average indoor air concentrations of VOCs (including nicotine, propylene glycol and glycerol (principle components of the e-liquid)) and low molecular weight carbonyls (µg/m3) measured before, during and after use of e-cigarettes from two independent sampling sites.
Chemical Compound | Background (before Participants Enter Room) | Room Occupied (No Vaping) | Room Occupied (Vaping Permitted) | Room Unoccupied (after Participants Leave Room) | Air Quality Guidelines or UK Workplace Exposure Limit as Published (WEL; 8 h Average) (mg/m3) | Air Quality Guidelines or UK Workplace Exposure Limit * (WEL; 8 h Average) (µg/m3) |
---|
Measurement 1 (µg/m3) | Measurement 2 (µg/m3) | Measurement 3 (µg/m3) | Measurement 4 (µg/m3) |
---|
Propylene glycol | <0.5 | <0.5 | 203.6 | 10.2 | UK WEL: 474 | 474,000 |
Glycerol | <150 | <225 | <250 | <200 | UK WEL: 10 | 10,000 |
Nicotine | <7.0 | <7.0 | <7.0 | <7.0 | UK WEL: 0.5 | 500 |
Isoprene | <0.5 | 6.2 | 9.5 | <0.5 | Not established | Not established |
Acetone | 1.3 | 9.2 | 10.7 | 1.2 | UK WEL: 1210 | 1,210,000 |
Propan-2-ol | 55.3 | 13.6 | 8.0 | 29.2 | UK WEL: 999 | 999,000 |
Hexamethylenecyclotri-siloxane | 5.3 | 29.1 | 13.3 | 4.4 | Not established | Not established |
Octamethylcyclotetra-siloxane | <0.5 | 14.2 | 3.6 | 0.9 | Not established | Not established |
Limonene | 2.2 | 2.1 | 2.9 | 1.5 | Not established | Not established |
Octanal | 2.1 | 3.5 | 5.4 | 4.6 | Not established | Not established |
Decamethylcyclo-pentanesiloxane | 6.3 | 307 | 460.8 | 107.5 | Not established | Not established |
Nonanal | 6.3 | 7.9 | 10.6 | 11.0 | Not established | Not established |
Decanal | 2.8 | 5.7 | 9.5 | 11.6 | Not established | Not established |
2,2,4-Trimethyl-1,3-pentanediol monoisobutyrate | 7.7 | 16.1 | 17.3 | 18.0 | Not established | Not established |
2,2,4-Trimethyl-1,3-pentanediol diisobutyrate | <0.5 | <0.5 | 1.5 | 2.2 | Not established | Not established |
Di-isobutyl phthalate | 3.5 | 4.4 | 2.3 | 2.8 | UK WEL: 5 | 5000 |
Formaldehyde | 32.0 | 31.0 | 37.6 | 21.0 | WHO: 0.1 | 100 |
Acetaldehyde | 9.0 | 6.5 | 12.4 | 6.0 | EU Indoor Air Quality: 0.2 | 200 |
Acrolein | <2.0 | <2.0 | <2.0 | <2.0 | UK WEL: 0.23 | 230 |
Total VOC | 65.0 | 237.0 | 379.8 | 129.0 | UK Building Regulations: 0.3 (8 h average) | 300 |
These levels are in general agreement with the theoretical maximum level determined in a recent publication which used a mathematical model to assess the concentration of nicotine in the indoor air following e-cigarette use [
27]. However in our volunteer study presented here, there was no measurable increase in nicotine airborne concentrations with vaping when compared with either the no vaping control session or background measurements
i.e., all measurements were found to be <7.0 µg/m
3. By way of context, the published UK WEL for nicotine is 500 µg/m
3 [
28]. The low level measured in this study may be attributable to the high retention rate of nicotine in the body, which has previously been reported following inhalation of tobacco smoke [
29], as well as some potential loss by deposition [
30]. Further research in these areas will be informative.
Propylene glycol and glycerol are principal components of e-liquids and their presence in exhaled e-cigarette aerosol is expected. Concentrations of propylene glycol in the range of 110–215 µg/m
3 and glycerol in the range of 59–81 µg/m
3 in the gas phase of emissions have been reported previously [
13]. In other studies, McAuley
et al. [
11] observed airborne concentrations of propylene glycol that ranged from 2.25 to 120 µg/m
3 and Romagna
et al. [
15] reported airborne glycerol concentrations of 72 µg/m
3.
In our study, during
ad libitum use of the ‘closed’ system e-cigarettes, propylene glycol in the air of the meeting room increased from <0.5 µg/m
3 during the no vaping control session to 203.6 µg/m
3 during vaping. At the end of the vaping session, there was a substantial and rapid decrease in the levels detected (down to 10.2 µg/m
3). The levels of propylene glycol determined within our study design were below the UK WEL of 474,000 µg/m
3 set for this chemical [
28]. Glycerol, while also expected to be present in the indoor air during the vaping session, could not be detected with satisfactory precision due to the limit of detection (LOD) for this compound (<350 µg/m
3). Further methodological refinement is required in future work. Nonetheless, it can be established that glycerol in the indoor air did not exceed 350 µg/m
3 during consumption of the e-cigarettes which is below the UK WEL of 10,000 µg/m
3 set for this chemical [
28].
Total volatile organic compounds (TVOCs) is an analytically based classification for a range of organic chemical compounds present in ambient air or emissions and is used for reporting purposes. In evaluating TVOCs, consideration of the individual compounds is also necessary (
Table 1). The background concentration of TVOCs observed in the meeting room ambient air in our study rose from 65 µg/m
3 to 237 µg/m
3 upon occupation of the room. While not components of e-liquids, this increase was likely due to the contribution of siloxane compounds arising from the five volunteers. It is well known that siloxanes are widely used in toiletries, deodorants and other personal care products [
31]; with increasing room temperature during the study session, release of these and other cosmetic components would likely to have increased. A number of other commonly used aroma compounds (e.g., octanal, nonanal) were also detected at lower levels during the study period. During the vaping phase the TVOC concentrations rose to 379.8 µg/m
3, conceivably due to further release of siloxanes and exhalation of propylene glycol from the active consumption of the e-cigarettes (see above). Following participant exit from the office, the TVOC concentrations returned to pre-vaping levels. While a WEL has not been established, UK Building Regulations recommend an 8 h average TVOC level of 300 µg/m
3 [
32].
Previous studies have detected the presence of the low molecular weight carbonyls formaldehyde and acetaldehyde in exhaled e-cigarette aerosols [
10,
13]. It has been reported that potential sources of these compounds in e-cigarette aerosol may arise from the heating or pyrolysis of propylene glycol [
33].
Schripp
et al. [
10] evaluated emissions from e-cigarettes after asking a volunteer user to consume three different refillable “open” e-cigarette devices in a closed 8 m
3 chamber. The authors reported formaldehyde and acetaldehyde in the air of the chamber albeit at significantly lower levels than emissions from a conventional cigarette. Schripp
et al. [
10] concluded that the presence of formaldehyde in the ambient air may be explained by human contamination and not from e-cigarette emissions; it has been previously reported that low amounts of both formaldehyde and acetaldehyde of endogenous origin can be detected in exhaled breath [
34]. In addition, it is widely reported that formaldehyde is released from some furniture and fittings, an effect which increases with room temperature and humidity [
35]. Taken as a whole, this highlights the importance of appropriate control sampling during air quality studies.
In our study, using a 38.5 m
3 environment, we observed slight changes in formaldehyde levels from an empty meeting room background value of 32.0 µg/m
3, to 31.0 µg/m
3 with occupancy, to 37.6 µg/m
3 during e-cigarette use. The level fell rapidly to 21.0 µg/m
3 following vacation of the office by study participants. The WHO has established a guideline indoor air value of 100 µg/m
3 for formaldehyde [
36]. While indicated as a short-term (30 min) guideline to prevent sensitivity or sensitization in both adults and children, WHO has stated that this value is sufficient to prevent long-term health effects, including cancer, since two distinct long term risk assessment models in the review arrived at proposed guideline values of around 210 and 250 µg/m
3 [
36]. The levels of formaldehyde determined within our study design were below WHO Indoor Air Quality guideline value of 100 µg/m
3 set for this chemical and comparable to range of values typically found in domestic or public spaces [
36,
37]. Schripp
et al. [
10] and Schober
et al. [
13] both reported formaldehyde levels below the WHO Indoor Air Quality Guideline.
When compared with the non-vaping session, we found acetaldehyde levels changed from a background of 9.0 µg/m
3 to 6.5 µg/m
3 after occupation to 12.4 µg/m
3 during the vaping session. These values and those reported by Schripp
et al. [
10] and Schober
et al. [
13] were well within the EU Indoor Air Quality guideline for acetaldehyde which is set at 200 µg/m
3 [
38].
A further finding in our study was the absence of a measurable increase in acrolein, the pyrolysis product of glycerol [
33], in the office air with use of e-cigarettes when compared to control measurements (<2.0 µg/m
3). This finding is consistent with those findings from Romagna
et al. [
15], who did not detect acrolein in air quality measurements in a 60 m
3 room during
ad libitum use of e-cigarettes.
By way of context, it has been reported by the US Environmental Protection Agency (EPA) and others that the burning of candles indoors resulted in a measureable increase of benzene, toluene, formaldehyde, acetaldehyde and acrolein [
39]. In air quality measurement studies following their use, formaldehyde levels in the air ranged from 1.0–323.5 µg/m
3 and acetaldehyde from 1.0 to 74.95 µg/m
3; reported levels of these two carbonyls measured in our study were substantially less than the maximal values in these studies [
9].
For acetone and isoprene, both exhaled breath components [
40], there was an increase from baseline during the occupied non-vaping session and active vaping sessions. Isoprene increased from a baseline measurement of <0.5 µg/m
3 to 6.2 µg/m
3 during room occupation to 9.5 µg/m
3 during active vaping. Acetone increased from a baseline measurement of 1.3 µg/m
3 to 9.2 µg/m
3 during room occupation to 10.7 µg/m
3 during active vaping. Following participant exit from the room, the concentrations of both compounds returned to background levels. This indicates that the occupants were the primary source of isoprene and acetone. A UK WEL has not been established for isoprene; acetone levels in all measurements were substantially lower than the UK WEL which is currently 1,210,000 µg/m
3 [
28].
3.7. Study Limitations and Strengths
The key aim of our study design was to replicate a real-life scenario with unrestricted use of a disposable “closed” system product by the vaping volunteers. In doing so, overhead sampling of the ambient air was chosen rather than personal dosimetry approaches to reduce potential confounding of vaping behaviors from intrusive sampling.
Our use of volunteers in conditions designed to replicate those in a real-world situation limited the sample duration and therefore the sensitivity of the some of the methods employed, which were not as sensitive as in some other studies which used a machine generated aerosol. Arguably, if the presence of certain chemicals can only be detected by employment of artificial or atypical conditions, it is reasonable to question the appropriateness of such data. The use of consumers within the study removed many of the issues associated with the use of smoking machine generated aerosols, for example questions around the potential retention of chemicals in the body or that of different machine protocols not replicating product consumption profiles. With regards to the method to measure glycerol in our study, sensitivity was not as low as anticipated. While there could be some scope for reducing the LODs for these and other chemicals further by increasing sampling duration, this would be difficult without introducing other potential confounding factors such as opening and closing meeting doors for refreshment breaks. By excluding opening and closing doors in this study, and bylimiting the air exchange to natural room ventilations, the levels reported in our study are likely to represent an overestimate of normal conditions. The measurement of air exchange and other environmental parameter measurements in the methodology are supportive of this.
Another limitation in this study was the use of a single product; as noted above, other research groups have reported findings that were not replicated in this present study. Such studies used different products which may reflect variations in e-liquid or device quality, sufficient details of which are often not reported. Additionally, given the focus on ambient air, the primary emissions of the analyzed product were not determined in this study, which may be of interest in future work focusing on consumer rather than bystander exposures. Further air quality studies could also investigate other product types as well as different settings and volunteer groups.
The potential issue of cross contamination with cigarette smoke has been noted previously [
2]. Given the sensitivity of the methods employed in this study, potential confounding from recent tobacco smoking was minimized. A strength of this study was that the rooms used here had never been smoked in nor were they used for any prior tobacco research.