New research suggests that the levels of chemicals used to flavor some brands of e-cigarette fluid exceed recommended exposure limits and could be respiratory irritants.

The electronic cigarette market has developed rapidly in recent years, with global sales in 2014 estimated to be in the region of $7 billion, but the health implications of vaping remain hotly contested.

Artificial and other flavorings in e-cigarettes are mostly the same as those used in food and confectionery manufacture, and are therefore often represented as safe by e-cigarette manufacturers.

But as the US Flavor Extracts Manufacturers Association (FEMA) has pointed out, this safety relates to exposure through eating, and not inhalation. And the ingredients listed on the product labels for e-cigarettes rarely include the chemicals used for flavoring.

For the study, researchers set out to find out the levels and type of chemicals used to flavor e-cigarette fluid in a sample of 30 products.

These included two single use disposable brands in five different flavors of tobacco, menthol, vanilla, cherry and coffee; the same flavors in refill bottles; and additional flavors of chocolate/cocoa, grape, apple, cotton candy and bubble gum in refill bottles.

Using a consumption rate of around 5 ml/day, as commonly reported on online vaping forums, vapers would be exposed to twice the recommended occupational exposure limits of benzaldehyde and vanillin with the products tested, say the researchers.

"And toxic degradation products may be produced by reaction of the flavor chemicals at the high temperatures present during vaping," they caution.

They admit that their sample represents a fraction of the e-cigarette products on the market.

But they say: "Nevertheless, the results obtained are likely to be similar to what a broad survey would have revealed, and in any case, suggest that very high levels of some flavor chemicals are undoubtedly present in a great number of the thousands of products currently available."

Regulations are needed, they argue. Researchers said these should include compulsory ingredient listing, limiting the levels of certain flavorings, and limiting the total permissible levels of flavorings, particularly as there is some concern that flavored products might make e-cigarettes more attractive to young people.

The findings are detailed in the journal Tobacco Control.