How close are we to definitively identifying the respiratory health effects of e-cigarettes?

Use of electronic cigarettes (e-cigarettes) is frequently promoted as a less harmful alternative to cigarette smoking. The impact of repeated inhalation of e-cigarette aerosols on respiratory health is not well understood. Areas covered: Using results from laboratory, observational, and clinical studies, we synthesize evidence relevant to potential respiratory health effects that may result from inhalation of e-cigarette aerosols. Expert commentary: Chemical analyses reveal that e-cigarette aerosols contain numerous respiratory irritants and toxicants. There are documented cytotoxic effects of e-cigarette constituents on lung tissue. Studies among ex-smokers who switched to e-cigarettes note reduced exposure to numerous respiratory toxicants, reduced asthma exacerbations, and chronic obstructive pulmonary disease symptoms. Regular exposure to e-cigarette aerosols is associated with impaired respiratory functioning. Potential respiratory health risks resulting from secondhand e-cigarette aerosol exposure have not been sufficiently evaluated. Current evidence indicates that although e-cigarettes are not without risk, these products seemingly pose fewer respiratory health harms issues compared to tobacco cigarettes. Data from prospective studies and randomized controlled trials examining the impact of e-cigarette use on lung health are needed to better understand respiratory health risks tied to use of these products.