Has the mist been peered through? Revisiting the building blocks of human health risk assessment for electronic cigarette use

Electronic cigarettes, battery-powered nicotine delivery devices, have been increasingly used in the past decade. This critical review provides a qualitative research synthesis of the human health risks associated with E-vapor inhalation in the peer-reviewed literature and our own preliminary experimental results. E-cigarettes may be as efficient as traditional cigarettes in nicotine delivery, especially for experienced users, and studies suggest lower emissions of air toxics from E-cigarette vapor and lower second- and third-hand vapor exposures. Some toxic emissions may however surpass those of traditional cigarettes, especially under high voltage vaping conditions. Experimentally, E-vapor/E-liquid exposures reduce cell viability and promote pro-inflammatory cytokine release. User vulnerability to concomitant environmental agent exposures, such as viruses and bacteria, may potentially be increased. While evidence to date suggests that E-cigarettes release fewer toxins and carcinogens compared to cigarettes, E-vapor is not safe and might adversely affect human immune functions. Major knowledge gaps hinder risk quantification and effective regulation of E-cigarette products including: lack of long-term exposure studies, lack of understanding of biological mechanisms associated with exposure, and lack of integration of exposure and toxicity assessments. Better data are needed to inform human health risk assessments and understand the public health impact of E-vapor exposures.