What science says about e-cigarettes

Battery-powered e-cigarettes have become wildly popular over the last decade, especially among teenagers and young adults. At least some of this popularity stems from their marketing as safe alternatives to conventional, combustible cigarettes, as well as their many, often fruity flavors.
But how much do we understand about the safety of e-cigarettes, or the more than 7,000 e-liquids on the market?
“The picture is still unclear,” says Aruni Bhatnagar, a professor of medicine at the University of Louisville. “We do not know to what level or to what extent these devices are safe.”
In conventional tobacco cigarettes, the tobacco is burned and nicotine is transferred into the aerosol. In e-cigarettes there is no burning, but there is a solution of a carrier — usually propylene glycol or glycerol which is heated to allow for nicotine to be inhaled. 
“There has been this belief that nicotine by itself is not as particularly harmful and that if you deliver it in a certain way which gets rid o