This section summarises the current research describing the nature of secondhand and thirdhand e-cigarette emissions, their chemical constituents and their potential to affect the health of people who are exposed.
The studies of secondhand and thirdhand e-cigarette emissions are relatively new and incomplete. Experiments attempting to quantify emissions and pollution have yielded variable results, perhaps due to the wide range of available e-cigarette devices, e-liquids and modes of use. However, there is little doubt that people in the vicinity of others who are using e-cigarettes can be exposed to chemicals emitted from these products, which might increase their risk of health effects.
18.6.9.1 Secondhand e-cigarette emissions and particulate matter pollution
E-cigarette emissions are aerosols, which consist of gases mixed with tiny droplets containing chemicals, as described in Section 18.5.1. As with tobacco smoke, people in the vicinity of e-cigarette use can inhale secondhand e-cigarettes emissions and may also be exposed to these chemicals from residues left on surfaces, known as thirdhand exposure.
E-cigarettes are assumed to produce emissions only when a user draws (inhales) from the device, meaning that secondhand emissions are likely to arise from exhalation by users alone.1 However, empirical research supporting this proposal is difficult to find. Secondhand e-cigarette emissions are likely to differ from secondhand cigarette smoke, which arises from 1) smouldering of cigarettes between draws, 2) emissions escaping from the mouth-end of the cigarette during a draw and 3) smoke exhaled by the user (see Sections 4.1 and 12.4.1).2 Most studies of secondhand e-cigarette emissions have detected them in rooms, cars or purpose-built chambers after normal e-cigarette use, where the secondhand emissions are exhaled by users. These methods have not characterised the exact source of these emissions, in terms of exhaled aerosols compared to any emitted directly from the device, but they do accurately reflect the exposure of bystanders.
The aerosols emitted by e-cigarettes contain chemicals and contribute to particulate matter pollution (PM).3 Fine particulate matter, known as PM2.5 due to the particles having <2.5 mm diameter, can penetrate into the gas exchange regions of the lungs, known as alveoli. Chemicals that reach this region may damage the lungs, as well as enter the circulation and cause damage to other parts of the body. Exposure to ambient PM2.5 is a global risk factor for mortality and associated with respiratory and cardiovascular events.4 Experiments using representative human airway replica machinery have found that e-cigarette respiratory depositions are likely to occur in the alveoli following secondhand exposure to aerosol.5,6
A major Australian review has found conclusive evidence that e-cigarette use results in increased airborne particulate matter in indoor environments.3 Specifically, e-cigarette use has been shown to increase the levels of indoor PM2.5 pollution above background levels.7-11 One study of secondhand emissions produced in a room-like chamber detected particles in the size range of 20 nm to 300 nm diameter, which constantly increased during e-cigarette use, reaching a final peak concentrations of 7 × 106 particles per litre of air.12 Another study found that after one minute of e-cigarette use, the levels of PM2.5 increased 160-fold above background at a distance of 0.5 m, and 103-fold at 1 m.8 A study of five vape shops showed a 21-fold increase in PM2.5 during opening hours when people presumably used e-cigarettes inside the stores, compared to when the shops were closed.13 Increases in PM2.5 have been demonstrated for e-cigarette use in rooms, homes, workplaces, convention halls and cars.11,12,14-17 E-cigarettes using higher power or a higher ratio of glycerol compared to propylene glycol as a solvent produced higher PM2.5.18,19 There has been a progressive increase in the PM pollution emitted by e-cigarettes from the original to the fourth generation of products, which use increased power.19 These studies clearly show that people in the vicinity of e-cigarette use are exposed to PM2.5 pollution above background levels, which might increase their risk of health effects.
The 2018 review from the National Academies of Sciences, Engineering and Medicine concluded that there is “conclusive evidence that e-cigarette use increases airborne concentrations of particulate matter and nicotine in indoor environments compared with background levels”.1
18.6.9.2 Chemicals detected in secondhand e-cigarette emissions
Secondhand e-cigarette emissions contain water vapour as well as a range of chemicals in the gas and particle phases. A systematic review has reported numerous chemicals detected in exhaled e-cigarette emissions including nicotine, glycerol, propylene glycol, formaldehyde, acetaldehyde, polycyclic aromatic hydrocarbons (PAHs) and metals.20 These mostly occur at lower concentrations than those in secondhand emissions from combustible tobacco products.20
Numerous studies have detected nicotine in the secondhand emissions from e-cigarettes.1 In a study using a room-like chamber, peak concentrations of 0.6 mg per cubic metre of nicotine, 2,200 mg per cubic metre of propylene glycol, and 136 mg of glycerol were detected.12 Nicotine was detected in the gaseous phase of secondhand emissions, along with 1,2-propanediol, 1,2,3-propanetriol, diacetin, flavourings and nicotine. Carbonyl compounds, such as formaldehyde and other aldehydes, were not detected in this study.12 Other substances detected in secondhand emissions include volatile organic compounds and metals, such as chromium, manganese and nickel.21,22 Volatile organic compounds detected in secondhand emissions include toluene, benzene and isoprene.1 Most of the chemicals detected in secondhand emissions are present at very low levels, but above background levels in households.1
Another study has measured the chemicals in secondhand emissions in homes and in bars. This study found that the levels of carbonyls, such as formaldehyde and acrolein, in home settings exceeded the 8-hour exposure limits established by the California Office of Environmental Health Hazard Assessment, but only when the highest powered devices were used.23 These levels were even higher when detected in bars.23
18.6.9.3 Biomarkers of exposure to secondhand e-cigarette emissions
A biomarker, in the case of secondhand e-cigarette emissions, is a substance in the human body that can be measured to indicate exposure to specific chemicals from these emissions. Testing biomarkers in non-e-cigarette users can imply exposure to secondhand e-cigarette emissions. However, there are limitations to this approach; people can be exposed to many of the chemicals in e-cigarettes via other sources, such as tobacco smoke and other types of pollution. Currently there are few biomarker studies for non-users exposed to secondhand e-cigarette emissions.
Nicotine
Biomarker studies have indicated that people exposed to secondhand e-cigarette emissions can absorb chemicals such as nicotine. A study measuring cotinine, a biomarker for nicotine exposure, found this chemical increased in blood, serum and urine samples from non-users after exposure to secondhand e-cigarette emissions.24 Another study found that the levels of cotinine in the blood of non-users, as well as airborne nicotine in the home, were higher when e-cigarettes were used compared to homes where e-cigarettes were not used.25 Children who are exposed to e-cigarette emissions have higher nicotine in their blood compared to unexposed children.26
A longitudinal study of a family experience found nicotine in the blood, urine, breast milk and hair of a non-using mother who was exposed to e-cigarette emissions from the father. Nicotine and cotinine were also found in the urine of a three-year old child from this family.27
Carbonyls
Exposure to secondhand emissions may have increased the amount of acrolein in the blood of non-users who attended a vaping convention.28
N-nitrosamines
Cancer-causing N-nitrosamines are formed from nicotine molecules that undergo chemical reactions. These chemicals that are found in tobacco are also found at lower levels in e-cigarettes, likely due to the nicotine content that is purified from tobacco. A biomarker for NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol), which is only present in tobacco, has been found in the urine of people exposed to secondhand e-cigarette emissions.29
Toxic metals
Biomarkers for toxic metals in people who vape usually take the form of direct detection of these metals in biological samples. In one study, cobalt has been found in the blood of non-using people who are exposed to e-cigarette emissions in their homes – at higher levels than non-users who are not exposed.30 A longitudinal study of one family found metals in the cord blood of the mother (a non-user who was exposed to e-cigarettes) and in the urine of an exposed child.27 The detected metals (chromium, copper, lead, nickel, tin and zinc) matched those found in the vape refill liquid used by the father. These results indicate the feasibility of transfer of toxic metals from a father who uses e-cigarettes to a non-using mother and child.27
18.6.9.4 Health effects from exposure to secondhand e-cigarette emissions
There are known short term effects and potential longer-term risks to the health of people who use e-cigarettes (see Sections 18.6.1 to 18.6.8). However, there are relatively few studies addressing health effects for people exposed to secondhand emissions.
In one study of vital signs in e-cigarette users compared to bystanders, the effects included increases in heart rate, breathing rate and oral temperature, and a decrease in blood oxygenation after 20 minutes of use. Bystanders, however, only exhibited a slight increase in oral temperature.31 A prospective study found evidence that people exposed to secondhand e-cigarette emissions had an increased risk of shortness of breath and bronchitic symptoms, but not wheezing, after accounting for numerous other potentially confounding factors.32 An experimental study of 40 non-users exposed to secondhand emissions for 30 minutes resulted in reporting of symptoms such as burning, dryness, sore throat, cough, breathlessness and headache. Nasal and throat/respiratory symptoms were associated with the amount of volatile organic compounds in the emissions, implying that these chemicals may have played a role in producing symptoms.33 Exposure to secondhand e-cigarette emissions may increase the desire for bystanders to use e-cigarettes themselves.34 These are preliminary results based on a few studies, so considerably more research is necessary to determine the range of short- and long-term health effects that might be caused by e-cigarette emissions.
A cross-sectional survey study has found that adolescents exposed to secondhand e-cigarette emissions in their households were more likely to report having asthma.35 Secondhand aerosol exposure was associated with a higher chance of reporting an asthma attack over the past 12 months in another study of youth (11 to 17 years).36 A second cross-sectional survey found an association with mental health effects.37 However, prospective studies are necessary before any conclusions can be made about for risk of asthma or mental health problems after exposure.
Long-term studies on the health effects of exposure to secondhand e-cigarette emissions have not been published; nor have studies on how these emissions might impact on the health of priority populations, including children, pregnant women, and people with chronic lung or heart disease.
18.6.9.5 Thirdhand e-cigarette exposure
Thirdhand e-cigarette exposure consist of chemicals from e-cigarettes that settle on surfaces and may build up over time. People may be exposed to these chemicals through touching these surfaces or breathing in these chemicals after off-gassing from the surface.38
Numerous studies have shown that thirdhand e-cigarette residues contain nicotine.38-40 Residual nicotine from e-cigarettes persists on glass and cloth surfaces for at least 3 days after e-cigarette use.41 A study conducted in vape shops showed the presence of nicotine and cancer-causing nitrosamines 4-(N-methyl-N-nitrosamino)-4-(3-pyridyl)butanal (NNA), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) on surfaces, that have likely come from secondhand e-cigarette emissions.13
Mice exposed to thirdhand e-cigarette residues displayed lung abnormalities such as increased collagen in the small airways and increased thickening of the lining of large airways. Similar experiments showed an increase in platelet aggregation and other changes that may increase the risk of stroke, in mice exposed to thirdhand e-cigarette residues.42 It is not known whether these effects occur in humans or their clinical relevance if they do.
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References
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