E-cigarettes compromise the immune system in the lungs and generate some of the same potentially dangerous chemicals found in traditional nicotine cigarettes, researchers at the Johns Hopkins Bloomberg School of Public Health report.

E-cigarettes are an emerging public health concern as they gain popularity among current and former smokers as well as those who have never smoked, including teenagers. The perception that e-cigarettes pose little health risk is so entrenched that some smokers are switching from cigarettes to e-cigarettes.

For their study, researchers divided mice into two groups: one was exposed to e-cigarette vapor in amounts that approximated actual human e-cigarette inhalation for two weeks, while the other group was just exposed to air. The mice exposed to the vapor were significantly more likely to develop compromised immune responses to when exposed to nasal drops of the flu virus and the bacterium responsible for pneumonia and sinusitis, which in some cases killed the mice, the researchers found.

Their findings were published today in the journal PLOS ONE.

"Our findings suggest that e-cigarettes are not neutral in terms of the effects on the lungs," said senior author Shyam Biswal, a professor in the Department of Environmental Health Sciences at the Bloomberg School. "We have observed that they increase the susceptibility to respiratory infections in the mouse models. This warrants further study in susceptible individuals, such as COPD patients who have switched from cigarettes to e-cigarettes, or to new users of e-cigarettes who may have never used cigarettes."

Since their introduction to the U.S. market in 2007, e-cigarettes have prompted debate as to their risk in general and relative to cigarettes. E-cigarettes, which at their simplest consist of a battery, an atomizer and a cartridge, produce a vapor that is inhaled and then exhaled by the user. E-cigarettes contain less nicotine than cigarettes, but actual nicotine intake by e-cigarette users can approximate that of cigarette smokers.

Previous analyses of e-cigarette vapor have identified chemicals that could be toxic or carcinogenic, including particulates, formaldehyde, and volatile organic compounds, but at lower levels than cigarette smoke. Another thing working in the favor of e-cigarettes in the risk continuum is that they don't combust the way cigarettes do, limiting some of the chemicals released in cigarette smoke.

The U.S. Food and Drug Administration last spring announced that it was going to begin regulating e-cigarettes. E-cigarette sales are projected to overtake cigarette sales in the next decade, and teen use of e-cigarettes outpaces cigarette use, according to a recent survey released by the National Institute on Drug Abuse. And, according to the U.S. Centers for Disease Control and Prevention, more than one-quarter million teenagers who reported never having smoked a cigarette reported using e-cigarettes in 2013.

The researchers believe this study, thought to be the first to examine animal response to e-cigarette inhalation, will serve as a model for future studies on the effects of e-cigarettes.

As part of their study, the researchers also determined that e-cigarette vapor contains "free radicals," known toxins found in cigarette smoke and air pollution. Free radicals are highly reactive agents that can damage DNA or other molecules within cells, resulting in cell death. Cigarette smoke contains 1014 free radicals per puff. Though e-cigarette vapor contains far fewer free radicals than cigarette smoke—1 percent as much—their presence in e-cigarettes still suggests potential health risks that merit further study, the researchers say.

"We were surprised by how high that number was, considering that e-cigarettes do not produce combustion products," says Thomas Sussan, PhD, lead author and an assistant scientist at the Bloomberg School. "Granted, it's 100 times lower than cigarette smoke, but it's still a high number of free radicals that can potentially damage cells."