According to the Daily Mail, experts warn that the level of chemicals used to flavor some e-cigarettes may exceed recommended limits and irritate the respiratory system.

Artificial flavorings in e-cigarettes are largely the same as those used in the manufacture of certain foods and confectionery. As a result they are often represented as being safe by e-cigarette manufacturers.

But, the US Flavor Extracts Manufacturers Association notes, the measure of safety is related to exposure through eating, rather than inhalation.

And critics add that the ingredients listed on the product labels for e-cigarettes rarely include the chemicals used for flavouring.