A new study suggests a type of e-cigarette called a “tank system” can produce some carcinogens also found in regular cigarettes, and at similar levels.

While e-cigarettes do not burn tobacco to create new chemicals, including cancer-causing ones, some brands get so hot they can produce carcinogens, The New York Times reports.

The study found tank systems produce formaldehyde, known to be a carcinogen. Formaldehyde is formed when liquid nicotine and other ingredients are subjected to high temperatures. The findings will appear later this month in Nicotine and Tobacco Research.

Tank systems are heated with batteries, and look like fountain pens or small flashlights. They are filled with liquid nicotine, which is vaporized.

While scientists have not found any evidence e-cigarettes cause cancer or heart disease, the new study suggests much is not yet known about the products’ health risks.

Last month, the Food and Drug Administration (FDA) announced new rules that would allow the agency to regulate e-cigarettes. The proposed rules would ban the sale of e-cigarettes, cigars and pipe tobacco to anyone under age 18. The rules focus on what goes into e-cigarettes, instead of the vapor that comes out of them, the article notes. The FDA said it could consider adding regulations about e-cigarettes emissions in the future.

“Looking at ingredients is one thing, and very important,” said Maciej L. Goniewicz of the Roswell Park Cancer Institute in Buffalo, New York, who led the new study. “But to have a comprehensive picture, you have to look at the vapor.” A second study scheduled to be submitted to the same journal had similar findings.