Some e-cigarette products popular in the U.S. are contaminated with bacterial and fungal toxins, Harvard T.H. Chan School of Public Health researchers found in tests for a new study.

Details: Researchers tested 75 products, including single-use cartridges and refillable e-liquids — 27% contained traces of endotoxin, a microbial agent found on Gram-negative bacteria, and 81% had traces of glucan, found in most fungi. Endotoxin concentrations were higher in fruit-flavored products, indicating raw materials used in flavor production may be a contamination source.

"Airborne Gram-negative bacterial endotoxin and fungal-derived glucans have been shown to cause acute and chronic respiratory effects in occupational and environmental settings. Finding these toxins in e-cigarette products adds to the growing concerns about the potential for adverse respiratory effects in users."

— Professor David Christiani, Senior study author

The big picture: The researchers note in the study, published online in the journal Environmental Health Perspectives Wednesday, contamination of the products could have occurred at any point during the production of the ingredients or after the e-cigarette product was finished.

Between the Lines: Cotton wicks used in e-cigarette cartridges may be one potential source of contamination, they hypothesised. Endotoxin and glucan are both known contaminants of cotton fibers.

The other side: A February study found e-cigarettes are an effective way for smokers to quit smoking — twice as effective as other nicotine products like gum and patches.

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