The high levels of chemicals used to flavour some brands of e-cigarette fluid could be respiratory irritants, says a new study.

Analysing e-cigarette fluid in a sample of 30 products in the US, the researchers found that some brands of e-cigarette fluid exceed recommended exposure limits.

"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 flavour chemicals are undoubtedly present in a great number of the thousands of products currently available," the authors noted in the study.

Artificial and other flavourings 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 flavouring.

The researchers therefore set out to find out the levels and type of chemicals used to flavour e-cigarette fluid in a sample of 30 products.

The flavouring chemicals totalled more than one percent by volume in 13 of the 30 liquids analysed, levels greater than two percent by weight in seven liquids, and levels greater than three percent by weight in two products.

And many of the ‘tobacco’ flavoured fluids contained chemicals used to flavour confectionery.

Six of the 24 compounds revealed in the analyses were aldehydes, compounds recognised to be primary respiratory irritants.

Using a consumption rate of around five 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, the researchers pointed out.

The findings were published online in the journal Tobacco Control.