Commonly sold e-cigarette liquids contain at least one substance classified as a health risk, such as those capable of causing asthma and allergy symptoms, researchers have found.
The researchers analysed each sample to find out exactly which chemicals were present and in what quantities.
Every liquid they tested contained at least one substance that has some level of risk to health according to the United Nations classification system.
These included methyl cyclopentanolone (found in 26.3 per cent of samples) and a-ionone (found in 8.7 per cent of samples), both of which "may cause allergy or asthma symptoms or breathing difficulties if inhaled", according to their classification.
Other substances, such as menthol (42.9 per cent of samples), ethyl vanillin (16.5 per cent of samples) and acetyl pyrazine (8.2 per cent of samples), are classified as "able to cause respiratory irritation".
"Our research reveals that e-cigarette liquids on sale have ingredients that are potential respiratory irritants," said Constantine Vardavas from the University of Crete who presented the research.
"This means that they are substances which irritate the airways according to previous research and international classification standards."
For the research on the content of e-cigarette refills, a random sample of e-cigarette liquids was selected from popular brands on sale in nine European countries, which included a variety of flavours and nicotine strengths.
Another study by Linnea Hedman from Umea University in Sweden revealed that those who smoke e-cigarette may experience respiratory symptoms, such as a persistent cough, wheezing and coughing up mucus as much as those who smoke conventional cigarette.
Researchers surveyed more than 30,000 people, who were randomly selected from the general population, about their smoking habits, use of e-cigarettes and respiratory symptoms.
Both the studies are presented at the European Respiratory Society International Congress in Milan.