In 34% of the articles the authors had a conflict of interest.

Serious methodological problems were identified and there is no long-term follow-up.

The findings in the 76 studies were often inconsistent and contradicting.

No firm conclusions can be drawn on the safety of electronic cigarettes.

Abstract

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. Serious methodological problems were identified. In 34% of the articles the authors had a conflict of interest. Studies found fine/ultrafine particles, harmful metals, carcinogenic tobacco-specific 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 short-term exposure. Reports on short-term adverse events were often flawed by selection bias.