18.6.6.1 Respiratory infections and defences of the respiratory tract
The continual exposure of the airways to chemicals from e-cigarettes leads to subclinical effects on the respiratory system (see Section 18.6.5) and may cause long-term inflammatory changes that have the potential to increase the risk of infections. A number of laboratory and animal studies show that e-cigarette emissions affect the immune protection of the respiratory tract. However, there is little evidence to date of an increase in the risk of respiratory infections for people who use e-cigarettes. Longer-term studies are necessary to test the risk of respiratory infections in people who use e-cigarettes over a sustained period.
With one of the largest surface areas of the body that is exposed to the environment, the lung membrane is an important barrier to pathogens such as viruses, fungi and bacteria. Protecting the airways and lungs from infection are innate defences and adaptive immune responses. Innate defences include 1) the mucociliary clearance system (see Section 3.2.1.1), 2) innate immune cells such as macrophages and neutrophils that directly fight against bacteria, and 3) components of alveolar fluid that have anti-microbial and anti-viral properties.1 Laboratory and animal experiments investigating exposure to e-cigarette emissions have detected changes to the immune defences of the airways that may increase the risk of respiratory infections.2 , 3
Exposure of human respiratory tract cells to e-cigarette emissions leads to impairment of the functions of innate immune cells— the first line of immune defence against pathogens.2-5 Experiments characterising immune cells in the sputum (phlegm) of e-cigarette users showed similar changes.6 Flavoured e-liquids containing cinnamaldehydes exert a dose-dependent suppression of the activity of innate immune cells7 and mucosal ciliary clearance, which protects the lungs from pathogens.8 Cells from the human respiratory tract were more likely to become infected with human rhinovirus (a cause of the common cold) if exposed to e-cigarette emissions in laboratory experiments.9
Animal experiments have demonstrated that exposure to e-cigarette emissions damages the immune system and increases the risk of infection. Exposure to e-cigarette emissions or chemicals found in these emissions disrupted the normal immune system function10 and worsened respiratory bacterial infections in mouse experiments.3 An increased adherence of bacteria to respiratory lining cells has been shown in laboratory experiments that may be contributing to increased bacterial infections in these pre-clinical experiments.3 , 5 , 11 Mouse experiments have also shown an increase in lung inflammation with bacterial infection and increased weight loss and mortality from influenza after exposure to e-cigarette emissions.3 , 12 , 13
Exposure to e-cigarette emissions may also affect respiratory pathogens themselves. Laboratory experiments have shown that e-cigarette emissions increased the virulence (harmfulness) and inflammatory potential of bacteria that commonly infect the lungs.14
18.6.6.2 Risk of COVID-19 infection and disease severity
COVID-19 disease is caused by infection with the SARS-CoV-2 virus. See Section 3.9.12.2 for more details about the COVID-19 pandemic. Many large studies indicate that cigarette smoking is associated with a lower risk of COVID-19 diagnosis but a higher risk of more severe disease in those who are diagnosed (see Section 3.9.12.2). However, there are considerably fewer studies examining the risk of COVID-19 diagnosis and disease severity associated with e-cigarette use. Most of these studies indicate no association between e-cigarette use and infection with the SARS-CoV-2 virus, with few addressing the severity of disease.
Six studies failed to detect any change in the risk of contracting COVID-19 disease for people who used e-cigarettes compared to non-users.15-20 One early study found a five-fold increased risk of COVID-19 diagnosis for people who had ever used e-cigarettes, but not for current users. But the design and statistical analysis of this study have been criticised.21 , 22 Larger studies with longitudinal measures are necessary to address this question more comprehensively.
A case-control study of COVID-19 patients in the US found that e-cigarette users had a higher risk of chest pain, chills, muscle aches, headaches, loss of smell/taste, nausea/vomiting, abdominal pain, diarrhoea and non-severe light-headedness.23 However, three other studies16 , 17 , 24 did not find similar increases in COVID-19 disease severity and one study showed unclear results.25 One study of dual use (e-cigarettes and combustible cigarettes) and poly-use (e-cigarettes, combustible cigarettes and cigars) found a higher risk of severe outcomes from COVID-19 disease for dual and poly-users than people who only used e-cigarettes.26
18.6.6.3 Risk of other infectious diseases
Laboratory experiments with gut lining cells indicate that chronic e-cigarette use could disrupt the lining of the gut. These experiments used non-nicotine e-cigarette emissions, showing the effects of the other chemicals present. The study showed that repetitive, but not acute, exposure to e-cigarette aerosols disrupted the cellular barrier of the gut, increased the susceptibility of the gut lining to bacterial infections (for laboratory-grown cells) and triggered gut inflammation.27 There is, however, a lack of studies addressing infection of the gastrointestinal tract in people who use e-cigarettes.
Infections of the oral cavity are discussed in Section 18.6.7.3.
Relevant news and research
For recent news items and research on this topic, click here. ( Last updated January 2023)
References
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