In the June 2014 issue of ASHRAE Journal, Bud Offerman, an expert on indoor air published a well-done risk assessment of active and passive exposure to e-cigarette aerosol using standard methods. (ASHRAE is the American Society for Heating, Refrigeration, and Air Conditioning Engineering, the organization that develops engineering standards for, among other things, building ventilation systems. The tobacco companies spent years keeping ASHRAE from identifying secondhand smoke as a serious indoor air pollutant, something ASHRAE eventually did.)



Here are the first and last paragraphs of Offerman's paper:

The prevalence of the use of e-cigarettes is increasing. E-cigarettes are marketed as an alternative to smoking tobacco that only produces harmless water vapor, with no adverse impact on indoor air quality. However, published literature seems to show that e-cigarettes are not harmless. Photo 1 shows an e-cigarette user exhaling a dense visible aerosol into the surrounding air. This visible aerosol consists of condensed submicron liquid droplets, which contain many chemicals including some that are carcinogenic, such as formaldehyde, metals (cadmium, lead, nickel), and nitrosamines.

We conclude that e-cigarettes emit harmful chemicals into the air and need to be regulated in the same manner as tobacco smoking. There is evidence that nitrosamines, a group of carcinogens found specifically in tobacco, are carried over into the e-cigarette fluid from the nicotine extraction process. There is also evidence that the glycol carriers can by oxidized by the heating elements used in e-cigarettes to vaporize the liquids, creating aldehydes such as formaldehyde. Consumers should be warned that, while the health risks associated with the usage of e-cigarettes are less than those associated with tobacco smoking, there remain substantial health risks associated with the use of e-cigarettes.



This paper almost certainly underestimates the risks because it only considers the chemicals in the aerosol, not the untrafine particle, which have important adverse effects.



The full paper is available here.