Right now, the composition of the e-liquid and the levels of toxic substances, or contaminants, in an e-cigarette are significantly lower than what is found in a regular cigarette.
From a public health standpoint, there is something people often forget. The issue of dose-response factors into every type of illness, unless the cause is immunological. You have to have had a certain amount of exposure to a product in order for it to make you sick. Taking an aspirin will not give you a stomach ulcer, but if you take the whole bottle, you'll be in trouble. So the dose-response component is always key.
Even though nicotine, which is extracted from the tobacco leaf, contains toxins and contaminants, there should be some reassurance in the fact that the concentrations in e-cigarettes are 400, 500, even 1,000 times lower than those found in regular cigarette smoke.