A forthcoming study previewed by the New York Times suggests that some electronic cigarettes may not be as harmless as they appear. The study, which will published in the journal Nicotine and Tobacco Research, says that some e-cigs reach high enough levels of heat that they can begin producing some of the same carcinogens that are found in cigarettes.

According to the study's authors, some tank-style e-cigarrettes, which use liquid nicotine, get so hot that formaldehyde is created along with the nicotine vapor that users inhale. The ultimate effects of vapor are unknown, though it is largely thought to be less toxic than cigarettes. 

The FDA recently announced plans to regulate e-cigarettes, though its focus will reportedly be on the ingredients that go into e-cigs rather than on the vapor that they create.

“Looking at ingredients is one thing, and very important,” Maciej L. Goniewicz, who led the first study, told the newspaper. "But to have a comprehensive picture, you have to look at the vapor.”

The report will be published in full on May 15.

[via The New York Times]