Abstract
Background: Since the emergence of e-cigarettes on the market in the early 2000s, the prevalence of e-cigarette use has increased globally. The health risks of using e-cigarettes have been increasingly revealed; however, the health effects on non-users exposed to e-cigarettes are less known. Methods: A systematic review was conducted of peer-reviewed articles from 2004 to October 2024 from PubMed and Embase. We focused on the studies that described health outcome measures among non-smokers/vapers exposed to secondhand e-cigarettes. We excluded animal studies and those that did not include human participants. We also omitted studies with financial conflicts of interest with the tobacco industry. Results: Of the 8635 studies we found in our search, 16 were included in the final review. Study designs included in our review included a case study, a cohort, eight experimental, four cross-sectional studies, and two observational studies. Health outcome measures were self-reported health symptoms and biomarkers. Ten out of fourteen studies examined respiratory health risks, six described immunological effects, two examined cardiovascular risks, and one explored mental health effects. Self-reported health symptoms such as bronchitis, shortness of breath, asthma, throat irritations, ear infections, and mental health disorders were observed among secondhand e-cigarette exposures when compared with controls. Biomarker measures varied among studies, except for cotinine concentrations of non-smokers/vapers exposed to secondhand e-cigarettes, which were likely to be higher than non-exposed. However, all studies encountered potential limitations. Conclusions: Our review found that secondhand e-cigarette exposure is not harmless and may have negative health consequences. However, higher-quality prospective studies remain essential to examine long-term secondhand exposure.