A team of researchers at Harvard T.H. Chan School of Public Health have found a chemical linked to severe respiratory disease in more than 75% of tested flavored electronic cigarettes.

For their research, published online in the journal Environmental Health Perspectives, the team tested 51 flavored electronic cigarettes and refill liquids for Diacetyl, a flavoring chemical linked to the rare and life-threatening lung disease bronchiolitis obliterans, and the compounds acetoin and 2,3-pentanedione, which may also pose a risk to respiratory health and are listed as "high priority" by the Flavor and Extract Manufacturers Association.

The test results showed levels of Diacetyl above the laboratory limit of detection in 39 of the flavors tested, acetoin in 46 of the flavors and 2,3-pentanedione in 23.

Overall, at least one of the three chemicals was detected in 47 of the 51 flavors tested, which included a selection whose names and fruity and sweet flavors could be potentially appealing to younger smokers.

In another study published last week, a team of researchers from Penn State College of Medicine in the US looked at e-cigarettes for their levels of free radicals, molecules that can cause cell damage and cancer.

The results, published in the journal Chemical Research in Toxicology, found that although the level of free radicals is less than what is found in traditional cigarettes, it is still higher than the level found in heavily polluted areas, and the long-term effects are still unknown.

Despite there being more than 7,000 varieties of flavored e-cigarettes and refills on the market, with their popularity increasing, little is known about the long-term health effects.