Footnotes

• Contributors The listed authors have been credited with authorship based on the provided authorship criteria as follows: ND: conception, design, analysis and interpretation of the data, drafting of manuscript, final approval; GG: critical revision of manuscript, final approval; JS: acquisition of data, interpretation of data, final approval; SD: critical revision of manuscript, final approval; J-FE: Conception, design, interpretation of the data, critical revision of manuscript, final approval.

• Funding This work was supported by Swiss National Science Foundation (grant 33CSC0-122679).

• Competing interests Dr. Etter reports that he was reimbursed (plane + hotel) by a manufacturer of refill liquids for e-cigarettes, for traveling to London and to China, to visit e-cigarette factories. He was not paid for these meetings.

• Ethics approval Ethics Committee for Clinical the Research of Lausanne University Medical School.

• Provenance and peer review Not commissioned; internally peer reviewed.