there is none, you still get the nicotine fix that you are addicted to when it is vaporized and you inhale it. also part of the cigarette addiction is the actual physical process of puffing and inhaleing, with an electronic cigarette you are replicating that process.
there are about 212 ingredients in a tobacco cigarette. some of them are known to cause cancer, also the tar in a cigarette is what messes you up. all i know is that since i have been vaporizing liquid nicotine in the electronic cigarette and inhaling that , i no longer crave tobacco cigarette's, i don't wake up hacking and coughing in the morning, and i can now run to the corner without being out of breath.