The safety of e-cigarettes has come under further scrutiny after a Harvard study found traces of bacteria and fungi contaminants in a number of popular brands.

The study, which examined 75 popular products, found that 27% contained traces of endotoxin, a microbial agent found on gram-negative bacteria, and that 81% contained traces of glucan, which is found in the cell walls of most fungi.

‘Airborne gram-negative bacterial endotoxin and fungal-derived glucans have been shown to cause acute and chronic respiratory effects in occupational and environmental settings,’ said David Christiani, Elkan Blout Professor of Environmental Genetics and senior author of the study.

‘Finding these toxins in e-cigarette products adds to the growing concerns about the potential for adverse respiratory effects in users.’

The researchers are calling for the findings to play a part in developing regulatory policies for e-cigarettes.

Earlier studies at Harvard have revealed that some e-cigarette flavours contain chemicals that are linked to serious respiratory diseases.