The paradox of the safer cigarette: understanding the pulmonary effects of electronic cigarettes.

Electronic cigarette (e-cigarette) use continues to rise globally. E-cigarettes have been presented as safer alternatives to combustion cigarettes that can mitigate the harm associated with tobacco products; however, the degree to which e-cigarette use itself can lead to morbidity and mortality is not fully defined. Herein we describe how e-cigarettes function; discuss the current knowledge of the effects of e-cigarette aerosol on lung cell cytotoxicity, inflammation, anti-pathogen immune response, mucociliary clearance, oxidative stress, DNA damage, carcinogenesis, matrix remodeling, and airway hyperresponsiveness; and summarize the impact on lung diseases, including COPD, respiratory infection, lung cancer, and asthma. We highlight how the inclusion of nicotine or flavoring compounds in e-liquids can impact lung toxicity. Finally, we consider the paradox of the safer cigarette - the toxicities of e-cigarettes that can mitigate their potential to serve as a harm reduction tool in the fight against traditional cigarettes, and we summarize the research needed in this under-investigated area.