A preliminary study in the New England Journal of Medicine raises a new worry about electronic cigarettes — exposure to formaldehyde.

Under certain conditions, taking 10 puffs from an e-cigarette would expose a user to about 2.5 times as much formaldehyde as he or she would get from smoking a single tobacco cigarette, according to the study.

Formaldehyde is the pungent chemical that was used to preserve the frog you dissected in your high school biology class. It’s used as an industrial disinfectant and as an ingredient in permanent-press fabrics, plywood, glues and other household products, according to the National Cancer Institute. It is also formed when the propylene glycol and glycerol in e-cigarette liquids and oxygen are heated together.

The World Health Organization’s International Agency for Research on Cancer says formaldehyde can cause leukemia and nasopharyngeal cancer. The U.S. Environmental Protection Agency considers the chemical a “probable human carcinogen.”

In experiments at Portland State University, researchers used a tank system type of electronic cigarette to produce nicotine vapor. The e-liquid vapor was captured in a tube and analyzed using a technique called nuclear magnetic resonance spectroscopy. Each sample consisted of 10 puffs of vapor (at 3 to 4 seconds per puff) collected over 5 minutes.

The researchers calculated that the lifetime cancer risk incurred by inhaling formaldehyde would be 5 to 15 times higher for long-term e-cigarette users than for long-term tobacco smokers.

Proponents of electronic cigarettes were quick to criticize the study.