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Harvard University researchers are warning people who use e-cigarettes to be careful with the flavorings that they choose for their refills because a lot of them can lead to lung disease.

For this study, the team at Harvard T.H. Change School of Public Health analyzed 51 types of flavored e-cigarettes to look for diacetyl, acetoin and 2,3-pentanedione, which are chemicals that have been tied to respiratory health issues. Diacetyl, specifically, has been linked to a condition known as "popcorn lung" (bronchiolitis obliterans).

"Recognition of the hazards associated with inhaling flavoring chemicals started with 'popcorn lung' over a decade ago," lead researcher Joseph Allen said in a Harvard news release. "However, diacetyl and other related flavoring chemicals are used in many other flavors beyond butter-flavored popcorn, including fruit flavors, alcohol flavors and, we learned in our study, candy-flavored e-cigarettes."

After testing the e-cigarettes one at a time, they found the presence of at least one of the three chemicals in 47 of the e-cigarettes. Diacetyl was present in at least 75 percent of the e-cigarettes and the levels of this chemical in 39 of the e-cigarettes tested were higher than the limit of detection in the lab. The researchers found acetoin in 46 of the e-cigarettes and 2,3-pentanedione in 23 of them.

"One of three flavoring chemicals was found in 92 percent of the e-cigarettes we sampled and these chemicals are of interest because of what we know about the associations of inhaling these chemicals and severe, irreversible lung disease that occurred in popcorn workers over a decade ago," Allen said to CBS News.

The researchers noted that there are more than 7,000 different flavors of e-cigarettes and liquid. They added that it is highly possible that these flavorings also contain chemicals that can hurt lung health.

"Since most of the health concerns about e-cigarettes have focused on nicotine, there is still much we do not know about e-cigarettes," Study co-author David Christiani said.

The study was published in Environmental Health Perspectives.