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, Medical Express reported.
These included methyl cyclopentanolone (found in 26.3 per cent of samples) and a-ionon ne (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, Greece 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.