Cigarettes Leaves

Graphic explains how electronic cigarettes work. (Image via AP)

E-cigarettes appear on display at Vape store in Chicago, Wednesday, April 23, 2014. The federal government wants to ban sales of electronic cigarettes to minors and require approval for new products and health warning labels under regulations being proposed by the Food and Drug Administration. (AP Photo/Nam Y. Huh)

Electronic cigarettes purchased by Cliff Phillips, a 61-year-old retiree and former smoker, and his wife, Vali, are seen at their home in Cuba, Ill., Tuesday, May 31, 2011. Electronic cigarettes like the one used by Phillips are at the middle of a social and legal debate over whether it’s OK to "light up" in places where regular smokes are banned. E-cigarettes, which are gaining popularity and scrutiny worldwide, are plastic and metal devices that heat a liquid nicotine solution in a disposable cartridge, creating vapor that the "smoker" inhales. (AP Photo/Seth Perlman)

FILE - In this Feb. 20, 2014 photo, Talia Eisenberg, co-founder of the Henley Vaporium, uses her vaping device in New York. Soon, the Food and Drug Administration will propose rules for e-cigarettes. The rules will have big implications for a fast-growing industry and its legions of customers. (AP Photo/Frank Franklin II, File)

In this Feb. 20, 2014 photo, Peter Denholtz, a former smoker and now co-owner of the Henley Vaporium in New York, explains his use of e-cigarettes. E-cigarettes are usually made of metal parts combined with plastic or glass and come in a variety of shapes and sizes. They heat the liquid nicotine solution, creating vapor that quickly dissipates when exhaled. The vapor looks like tobacco smoke and can feel like tobacco smoke when taken into the lungs at varying strengths, from no nicotine up to 24 milligrams or more. (AP Photo/Frank Franklin II)

LONDON, ENGLAND - AUGUST 27: E-Cigarettes are sold at the V-Revolution E-Cigarette shop in Covent Garden on August 27, 2014 in London, England. The Department of Health have ruled out the outlawing of 'e-cigs' in enclosed spaces in England, despite calls by WHO, The World Health Organisation to do so. WHO have recommended a ban on indoor smoking of e-cigs as part of tougher regulation of products dangerous to children. (Photo by Dan Kitwood/Getty Images)

LONDON, ENGLAND - AUGUST 27: Different flavours for E-Cigarettes are sold at the V-Revolution E-Cigarette shop in Covent Garden on August 27, 2014 in London, England. The Department of Health have ruled out the outlawing of 'e-cigs' in enclosed spaces in England, despite calls by WHO, The World Health Organisation to do so. WHO have recommended a ban on indoor smoking of e-cigs as part of tougher regulation of products dangerous to children. (Photo by Dan Kitwood/Getty Images)