A new study suggests that electronic cigarettes may be contaminated with bacteria and fungi associated with lung disease.
The researchers, who tested 75 disposable devices and fluid cans used in electronic cigarettes, found signs of malfunction in more than a quarter of these products.
Glucan, a glycogen found in most fungi, was discovered in 81% of the products.
The researchers say exposure to toxins is linked to a range of health problems such as asthma, low lung function and inflammation.
A team from Harvard University's School of Public Health studied electronic cigarettes for some of the best-selling brands in the United States and identified a bacterial "intrinsic stem" from Gram-negative bacteria, which had a solid outer layer that made them resistant to antibiotics.
Professor David Christian, who led the team, said: "It has been shown that the airborne negative bacterial endotoxin and glucan derived from fungi cause acute and chronic respiratory effects in occupational and environmental conditions."
"Finding these toxins in electronic cigarette products adds to growing concerns about the potential for harmful respiratory effects on users," Christian said.
Levels of "indoor poison" were higher in fruit-flavored products, suggesting a possible link with the raw ingredients used to produce flavors.