Good idea, I edited my answer a couple times (see bold and italics).
I hope someone mentions that many of us use ZERO nicotine in our ecigs.
The overall effect of e-cigarettes haven't had long-term studies because they are a relatively new product. Most experts acknowledge a reduced amount of relative risk compared to tobacco cigarettes, but the level of risk of e-cigs is still debatable.
An e-cigarette heats e-liquid that contains mostly Propylene Glycol (PG) or Vegetable Glycerin (VG) to produce vapor. Both have been tested and found okay for inhalation and PG is also used in fog machines and some inhalers.
The nicotine in e-liquid is extracted from tobacco plants and that's why it's possible for trace amounts of the carcinogens in tobacco plants to be in the e-liquid, but they are also present at similar levels in nicotine gums or patches. There are, on the other hand, over 60 known carcinogens in tobacco cigarettes. Tobacco cigarettes are burned, creating additional carcinogenic free radicals, whereas e-liquid is vaporized like steam and does not have any of those. Since this is in the Respiratory Diseases category, it's important to note that e-cigs do not create tar that is caused by burning tobacco, which coats the lungs and greatly reduces oxygen absorption. Nicotine itself is not carcinogenic, and while it is a vasoconstrictor,
research has shown that it's the carbon monoxide that is more responsible than the nicotine for heart problems related to smoking. However, there is an option to use zero nicotine in the e-liquid, which removes practically all dangers.
