They are marketed as being healthier than conventional cigarettes, but new research suggests people who smoke electronic cigarettes could still be inhaling a host of dangerous chemicals.
People who smoke e-cigarettes may inhale higher concentrations of nicotine and of other toxins, say U.S researchers.
Nicotine is both addictive and, in large quantities, toxic - and some of the other chemicals found in e-cigarettes are believed to be carcinogenic.
Researchers at New York University found that due to the "frequency of puffing" and "depth of inhalation" e-cigarette smokers absorb higher levels of harmful chemicals than those who smoke traditional cigarettes.
Initially, e-cigarettes were seen as a healthier, tobacco-free alternative for people who were addicted to nicotine.