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E-cigarettes may compromise the immune system in the lungs and generate some of the same potentially dangerous chemicals found in traditional nicotine cigarettes, a new study in mice said Wednesday.

"We have observed that they increase the susceptibility to respiratory infections in the mouse models," senior author Shyam Biswal, professor of the Johns Hopkins University, said in a statement.

"This warrants further study in susceptible individuals, such as COPD (chronic obstructive pulmonary disease) patients who have switched from cigarettes to e-cigarettes or to new users of e- cigarettes who may have never used cigarettes."

For the study, Biswal and colleagues divided the mice into two groups: one was exposed to e-cigarette vapor in an inhalation chamber in amounts that approximated actual human e-cigarette inhalation for two weeks, while the other group was just exposed to air.

The researchers then divided each group into three subgroups. One received nasal drops containing Streptococcus pneumoniae, a bacteria responsible for pneumonia and sinusitis, among other illnesses, in humans. A second received nasal drops of the virus Influenza A, and the third subgroup did not receive either virus or bacteria.

The mice exposed to e-cigarette vapor were significantly more likely to develop compromised immune responses to both the virus and the bacteria, which in some cases killed the mice, the researchers found.

They also determined that e-cigarette vapor contains "free radicals," known toxins in cigarette smoke and air pollution that can damage or even kill cells.

Though e-cigarette vapor contains only one percent as much free radicals as cigarette smoke, their presence in the battery-powered devices still suggests potential health risks that merit further study, the researchers said.

Cigarette smoke contains 1,014 free radicals per puff, according to the study published the U.S. journal PLOS ONE.

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. They look like cigarettes and deliver an aerosol of nicotine and other chemicals.

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.

The U.S. Food and Drug Administration last spring proposed rules that could eventually ban the sale of e-cigarettes to individuals under the age of 18. Endite