A team from Harvard School of Public Health in Boston tested 51 types of flavoured e-cigarettes and liquids. The investigators found that 47 (more than 75 percent) of them contained diacetyl.
The chemical has been linked to a severe lung disease called bronchiolitis obliterans, also known as "popcorn lung" because it was first diagnosed in workers who inhaled artificial-butter flavouring in factories making microwave popcorn.
Two other related flavouring chemicals that may pose a lung hazard were also found in many of the flavoured e-cigarettes and liquids tested, according to researchers led by Joseph Allen, an assistant professor of exposure assessment science at the school.
One such flavouring chemical, acetoin, was detected in 46 of the flavors while another, called 2,3-pentanedione, was found in 23 of the flavours, the findings showed.
The study was published online in the journal Environmental Health Perspectives.
Much we don't know
"Recognition of the hazards associated with inhaling flavouring chemicals started with 'popcorn lung' over a decade ago," Allen said in a Harvard news release. "However, diacetyl and other related flavouring chemicals are used in many other flavours beyond butter-flavoured popcorn, including fruit flavours, alcohol flavours and, we learned in our study, candy-flavoured e-cigarettes."
Study co-author David Christiani, a professor of environmental genetics at the school, added, "Since most of the health concerns about e-cigarettes have focused on nicotine, there is still much we do not know about e-cigarettes. In addition to containing varying levels of the addictive substance nicotine, they also contain other cancer-causing chemicals, such as formaldehyde, and as our study shows, flavouring chemicals that can cause lung damage."
There are currently more than 7,000 varieties of flavored e-cigarettes and liquid, the researchers said, but there's a lack of information on their potential health effects.
Attempts by HealthDay to reach the American Vaping Association, which represents the industry, were unsuccessful.