Cigarette smoking remains the leading preventable cause of sickness and mortality, responsible for over 400,000 deaths in the United States each year. The worst health consequences associated with smoking (e.g., cancer and heart disease) are linked to inhalation of tar and other chemicals produced by tobacco combustion; the pleasurable, reinforcing, and addictive properties of smoking are produced mostly by the nicotine contained in tobacco. [Using PET Scans, researchers at the University of Michigan (in the first human study) showed that (like heroin and morphine) smoking cigarettes stimulates the brain's production of dopamine and chemicals known as opioids. Smoking cigarettes triggers the release of addictive feel-good brain chemicals, notably dopamine and opiods. It appears that smokers have an altered opioid flow all the time, when compared with non-smokers.] According to WebMD, e-cigarettes are already a booming, billion-dollar industry, on track to outsell tobacco products within a decade. The number of teens and tweens using these products doubled between 2011 and 2012.