WEDNESDAY, Jan. 21, 2015 (HealthDay News) -- E-cigarette vapor can contain cancer-causing formaldehyde at levels up to 15 times higher than regular cigarettes, a new study finds.

Researchers found that e-cigarettes operated at high voltages produce vapor with large amounts of formaldehyde-containing chemical compounds.

This could pose a risk to users who increase the voltage on their e-cigarette to increase the delivery of vaporized nicotine, said study co-author James Pankow, a professor of chemistry and civil and environmental engineering at Portland State University in Oregon.

"We've found there is a hidden form of formaldehyde in e-cigarette vapor that has not typically been measured. It's a chemical that contains formaldehyde in it, and that formaldehyde can be released after inhalation," Pankow said. "People shouldn't assume these e-cigarettes are completely safe."

The findings appear in a letter published Jan. 22 in the New England Journal of Medicine.

Health experts have long known that formaldehyde and other toxic chemicals are present in cigarette smoke. Initially, e-cigarettes were hoped to be without such dangers because they lack fire to cause combustion and release toxic chemicals, a Portland State news release said.

But newer versions of e-cigarettes can operate at very high temperatures, and that heat dramatically amps up the creation of formaldehyde-containing compounds, the study found.

"The new adjustable 'tank system' e-cigarettes allow users to really turn up the heat and deliver high amounts of vapor, or e-cigarette smoke," lead researcher David Peyton, a Portland State chemistry professor, said in the news release. Users open up the devices, put their own fluid in and adjust the operating temperature as they like, allowing them to greatly alter the vapor generated by the e-cigarette.

When used at low voltage, e-cigarettes did not create any formaldehyde-releasing agents, the researchers found.

However, high-voltage use released enough formaldehyde-containing compounds to increase a person's lifetime risk of cancer five to 15 times higher than the risk caused by long-term smoking, the study said.

Formaldehyde is a known human carcinogen, according to the U.S. National Cancer Institute. It is a colorless, strong-smelling gas, commonly used in glues for products such as particle board, and in mortuaries as an embalming fluid.