A record 4.3 million people are actively vaping in Britain after a fivefold increase in a decade, according to a report.
Some 8.3% of adults in England, Wales and Scotland are now believed to be regular vapers - up from 1.7% (about 800,000 people) 10 years ago. Action on Smoking and Health (ASH), which compiled the report, said a "revolution" had occurred.
E-cigarettes allow people to inhale nicotine rather than smoke.
The NHS says they carry "a small fraction of the risk of cigarettes" because they don't produce tar or carbon monoxide.