For the study, the researchers from Harvard University examined 75 popular e-cigarette products -- cartridges (single use) and e-liquids (refillable material) -- sold in the US.
The results, published in the journal Environmental Health Perspectives, showed that 27 per cent of these products contained traces of endotoxin, a microbial agent found on Gram-negative bacteria, and 81 per cent 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 senior study author David Christiani, Professor at Harvard University.
"Finding these toxins in e-cigarette products adds to the growing concerns about the potential for adverse respiratory effects in users," Christiani added.
The findings showed that 17 of 75 products (23 per cent) contained detectable concentrations of endotoxin and that 61 of 75 products (81 per cent) contained detectable concentrations of glucan.
The study also found that endotoxin concentrations were higher in fruit-flavoured products, indicating that raw materials used in the production of flavours might be a source of microbial contamination.
The researchers noted that the contamination of the products could have occurred at any point during the production of the ingredients or of the finished e-cigarette product.
"In addition to inhaling harmful chemicals, e-cig users could also be exposed to biological contaminants like endotoxin and glucan," said lead author of the study Mi-Sun Lee.