In the studies that have been done to date, depending on the generation of the device, you'll see early e-cigarettes did not generate as much because they didn't reach the temperature. The newer ones, the second generation, tend to generate a lot more of the vape and the aerosolizing of compounds. For the most part, it is nowhere close to what you would get out of a cigarette or an equivalent. But the nicotine in some of them, because they are not regulated, can sometimes spike even higher than what you would get out of a cigarette. The science can only tell you about what is on the market, but that same product today may be very different from the same product a week later. It's shifting. Because of the lack of regulations of this market, we have no idea whether what is true today in what we are studying will be true tomorrow. So you have to take that with a grain of salt.
On October 28th, 2014. See this statement in context.