What is an electronic cigarette, and is it more harmful?

This year, the number of people using e-cigarettes is expected to reach one million. It's been touted as a healthy alternative to smoking, but what exactly is an e-cigarette?

 An electronic cigarette feels like a real cigarette, and even provides a nicotine fix. However, there is no burning tobacco, meaning there are no toxins such as tar, arsenic, and carbon monoxide.

When a person uses an e-cigarette, a sensor detects airflow and triggers a processor to turn on a heater, or "vaporizer." This heats a liquid inside a replaceable cartridge, usually a solution of propylene glycol mixed with flavors and a variable amount of liquid nicotine (some cartridges contain no nicotine at all).

This creates vapor that the user breathes in, while an LED lights up to simulate the end of a lit cigarette. The result is a device that looks like a traditional cigarette, but which its lawyers claim is safer.