E-cigarette users in U.K. have tripled since 2010

The number of people who use electronic cigarettes in the U.K. has tripled over the past two years to 2.1 million, a health charity estimates. It says just over half of current or ex-smokers have now tried electronic cigarettes, compared with 8% in 2010. Action on Smoking and Health (Ash) surveyed more than 12,000 adult smokers.
Background:
E-cigarette users in U.K. have 'tripled' since 2010 [BBC]
Marketing rules too lax on e-cigarettes, critics say [USA Today]