"Some e-cigarettes have as much nicotine as a pack of 20 cigarettes," says anti-smoking campaigner

"Many of the e-cigarettes on the market right now have large doses of nicotine, some have as much nicotine as a 20-pack of cigarettes," says Vince Willmore of the Campaign for Tobacco-Free Kids.

According to US health authorities, more than 2.5 million young people use these electronic cigarettes, which is an urgent public health problem.