When it comes to e-cigarette safety, flavorings added to the liquid nicotine burned in the devices may influence how much the fumes lead to health problems, a recent study suggests.

Researchers tested vapor released by a variety of devices filled with some popular e-cigarette flavors – tobacco, pina colada, menthol, coffee and strawberry – to compare levels of chemicals known to cause inflammation and cell damage. They also looked at how much these chemicals appeared in tobacco smoke and in smoke-free air.

As expected, cigarette smoke was more toxic than fumes from e-cigarettes, said senior study author Maciej Goniewicz of Roswell Park Cancer Institute in Buffalo, New York.

E-cigarettes, however, had more toxic chemicals than smoke-free air, especially when the devices had higher power or voltage and when used with flavored liquid nicotine.

“We found that strawberry-flavored product was the most toxic among all liquids we tested,” Goniewicz added by email.

The researchers exposed human lung cells in a dish to smoke, e-cigarette vapors and air. The cells reacted most strongly to smoke, by reducing their metabolic activity and increasing output of inflammation-related chemicals. Similar, but not as strong, reactions were seen with e-cigarette vapor, compared to no change with plain air.

Past research has found that higher-voltage e-cigarettes produce a bigger nicotine jolt, but may also increase levels of dangerous chemicals like formaldehyde.

“Our findings raise some concerns about the safety of additives used in e-cigarettes,” Goniewicz said. “If an e-cigarette user experiences any side effects, for example cough or chest pain, he or she should consider changing the flavorings in the product and operating their device in lower-power settings.”

Big U.S. tobacco companies are all developing e-cigarettes.

An international review of published research by the Cochrane Database of Systematic Reviews last month concluded that the devices could help smokers quit but said much of the existing evidence on e-cigarette safety was thin.