Footnotes

• Contributors RS, ST, EK, NK and AS designed the experiments. RS, EK and NK performed the measurements. RS, ST, EK and AS analysed the data. AS, ST, RS, EK, NK and DLA wrote the original draft.

• Funding This research is supported by grant number U54DA036105 from the National Institute on Drug Abuse of the National Institutes of Health and the Center for Tobacco Products of the US Food and Drug Administration.

• Disclaimer The content is solely the responsibility of the authors and does not necessarily represent the views of the NIH or the FDA.

• Competing interests The authors declare the following competing financial interest: AS is named on a patent application for a device that measures the puffing behaviour of electronic cigarette users and is a paid consultant in litigation against the tobacco industry.

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