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 exposed to these emissions. 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. The studies of secondhand and thirdhand e-cigarette emissions are relatively new and incomplete. However, there is little doubt that those nearby people using e-cigarettes can be exposed to emissions from these products, which may 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 therefore 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 (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
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.5-9 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.10 Another study found that by one minute after 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.6 Increases in PM2.5 have been demonstrated for e-cigarette use in rooms, homes, workplaces, convention halls and cars.9-13 E-cigarettes using higher power or a higher ratio of glycerol compared to propylene glycol as a solvent produced higher PM2.5.14, 15 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.15
18.6.9.2 Chemicals detected in secondhand e-cigarette emissions
Secondhand e-cigarette emissions consist of 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.16 These mostly occur at lower concentrations than those in secondhand emissions from combustible tobacco products.16
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.10 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.10 Other substances detected in secondhand emissions include volatile organic compounds and metals, such as chromium, manganese and nickel.17, 18 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
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 The Australia National University (ANU) review published in 2022 similarly concluded that there is “limited evidence that e-cigarette use results in increased concentrations of airborne nicotine …”.3
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 in a test that will indicate exposure to specific chemicals from these emissions. Testing biomarkers in non-e-cigarette users can be used to 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, and the existing studies have produced mixed results.
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.19 A second study found that the levels of blood cotinine in 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.20 Exposure to secondhand emissions may have increased the amount of acrolein in the blood of non-users who attended a vaping convention.21
18.6.9.4 Health effects from exposure to secondhand e-cigarette emissions
There are known short term effects to the health of people who use e-cigarettes, including increases in heart rate, blood pressure, throat irritation and nausea (see Section 18.6.3 and 18.6.8). However, there are relatively few studies addressing short term health effects for people exposed to secondhand emissions.
In one study of vital signs in e-cigarette users compared to bystanders, the effects measured in users were 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.22 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 potential confounding factors.23 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.24 Exposure to secondhand e-cigarette emissions may increase the desire for bystanders to use e-cigarettes themselves.25 These are preliminary results based on a few studies, so considerably more research is necessary to confirm these findings.
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.26 A second cross-sectional survey found an association with mental health effects.27 However, cross-sectional studies cannot be used to imply cause and effect, so 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 do not yet exist; nor do studies on how these emissions might impact on the health of vulnerable populations, including children, pregnant women, and people with chronic lung or heart disease.
18.6.9.5 Thirdhand e-cigarette exposure
Thirdhand e-cigarette emissions 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.28 Nicotine itself is a common surface contaminant.28
There is very little research on thirdhand e-cigarette emissions. A 2015 study of thirdhand exposure found that nicotine levels in the homes of e-cigarette users did not differ to those of non-users.29 However, a later study found an accumulation of nicotine on surface and clothing after e-cigarette use.30 More studies are needed to characterise the levels of exposure and potential risks from thirdhand e-cigarette emissions, particularly from contemporary devices and e-liquids.
Relevant news and research
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References
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