Objective: To provide a systematic review of the existing literature on health consequences of vaporing of
electronic cigarettes (ECs).
Methods: Search in: PubMed, EMBASE and CINAHL. Inclusion criteria: Original publications describing a
health-related topic, published before 14 August 2014. PRISMA recommendations were followed. We identified
1101 studies; 271 relevant after screening; 94 eligible.
Results: We included 76 studies investigating content of fluid/vapor of ECs, reports on adverse events and
human and animal experimental studies. Seriousmethodological problemswere identified. In 34% of the articles
the authors had a conflict of interest. Studies found fine/ultrafine particles, harmfulmetals, carcinogenic tobaccospecific
nitrosamines, volatile organic compounds, carcinogenic carbonyls (some in high but most in low/trace
concentrations), cytotoxicity and changed gene expression. Of special concern are compounds not found in conventional
cigarettes, e.g. propylene glycol. Experimental studies found increased airway resistance after shortterm
exposure. Reports on short-term adverse events were often flawed by selection bias.
Conclusions: Due to many methodological problems, severe conflicts of interest, the relatively few and often
small studies, the inconsistencies and contradictions in results, and the lack of long-term follow-up no firm conclusions
can be drawn on the safety of ECs. However, they can hardly be considered harmless.
© 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license