Electronic cigarettes are often thought to be safer than cigarettes because they don’t produce smoke or contain the tar and chemical of tobacco. But a new study is once again raising questions about the safety of vaping e-cigarettes.

Researchers at Penn State College of Medicine says they have found that electronic cigarettes produce free radicals, which are molecules that cause cell damage and can lead to cancer.

In cigarette smoke, free radicals are the molecules that cause the most oxidative damage. They are a leading culprit in smoking-related cancer, cardiovascular disease, skin aging, and chronic obstructive pulmonary disease, or COPD.

The researchers found that e-cigarettes produce levels of free radicals that are about 1,000 to 100 times lower than in regular cigarettes. But the levels are still in the range of what might be seen in the air of a heavily air-polluted area.

Previous studies have found aldehydes in e-cigarette vapour, which are compounds that can cause oxidative stress and cell damage. A study published earlier this year in the New England Journal of Medicine, for example, found e-cigs produce high levels of formaldehyde, which is a known carcinogen.

"This is the first study that demonstrates the fact that we have these highly reactive agents in e-cigarette aerosols," said Prof. John P. Richie Jr., who teaches public health sciences and pharmacology at Penn State.

Richie says there's a perception that e-cigarettes are healthier than regular cigarettes, but his team’s findings suggest the devices may not be free from harm.

"While e-cigarette vapour does not contain many of the toxic substances that are known to be present in cigarette smoke, it's still important for us to figure out and to minimize the potential dangers that are associated with e-cigarettes," he said in a statement.

He said his team is currently conducting studies to measure the amount of free radicals in e-cigarette aerosols and to identify their chemical structures.

“That will help us interpret the data better to know how dangerous they are," he said.

The full results are published in the journal Chemical Research in Toxicology.