Formaldehyde, a substance known to cause cancer in human tissue, is found in cigarette smoke.
And new research shows formaldehyde also dwells in the vaporised liquid of the popular e-cigarettes.
According to a report published in the New England Journal of Medicine, the exposure to formaldehyde from e-cigarettes could be five to 15 times higher than from smoking cigarettes.
"It's way too early now from an epidemiological point of view to say how bad they are," co-author James F. Pankow, professor of chemistry and engineering at Portland State University in Oregon told NBC News.
"But the bottom line is, there are toxins and some are more than in regular cigarettes. And if you are vaping, you probably shouldn't be using it at a high-voltage setting."
The researchers have pointed out that there are more than 8,000 chemicals in tobacco smoke, so it is hard to say whether formaldehyde is the main cause of cigarette-related cancers.
This isn't the first study to raise questions around e-cigarettes.
A previous study by the Health Equalities Group and the Centre for Public Health at Liverpool John Moores University suggested that e-cigarettes are not just being used by those trying to quit smoking.
Vaping is becoming more common among young people who have never smoked traditional tobacco cigarettes.
“This is particularly concerning given that the safety of e-cigarettes has not yet been thoroughly scientifically evaluated. It is clear that urgent action is needed to educate and protect young people," Robin Ireland, Chief Executive of Health Equalities Group said at the time.
Commenting on the latest study, Pankow echoed Ireland's comments that more research into vaping needs to be done.
"A lot of people make the assumption that e-cigarettes are safe and they are perfectly fine after using for a year," he said.
"The hazards of e-cigarettes, if there are any, will be seen 10 to 15 years from now when they start to appear in chronic users."