Although cigarette smoke causes significant DNA damage in lung cells, the vapor from electronic cigarettes doesn't, says a study on human lung cells conducted by British American Tobacco.

Scientists at the tobacco company, which also manufactures electronic cigarettes, said that the vapor had no effect on lung cells' DNA even at doses 28 times that of cigarette smoke.

The most damaging type of DNA damage is double-strand break, which means that both strands of the DNA molecule have been broken. This is a possible precursor to cancer and is potentially deadly to the cell.

"We have been able to show that there is significant DNA damage in human lung cells exposed to smoke, but that this is not case with e-cigarette vapor," said researcher Dr. James Murphy. "These findings add to evidence on the likely reduced risks of vaping, compared to smoking."

The study is published in Toxicology Letters.

Other studies, however, have found that the vapor from e-cigarettes does indeed damage lungs.

A Greek study published earlier this year found that e-cigarettes have an immediate effect on pulmonary function. Researchers studied young cigarette and e-cigarette smokers; half had mild controlled asthma and the others were healthy. After smoking e-cigarettes, measurements of airway obstruction and inflammation were worse in both groups, but were more severe in asthmatics.

"These results show that as it happens with cigarette smoking, e-cigarette smoking has more deleterious short-term effects on asthmatics compared with healthy smokers," said Dr. Andreas Lappas of the Hellenic Cancer Society, Athens, Greece.

Additional studies found electronic cigarettes to be just as dangerous as traditional cigarettes. A Harvard study found that of 51 e-cigarettes tested, at least one toxin was found in 47 of them, and 75 percent contained diacetyl, a chemical linked to a severe respiratory disease called bronchiolitis obliterans or "popcorn lung."

Even more frightening, the amounts of diacetyl found in 39 of the e-cigarettes contained amounts higher than the laboratory was capable of measuring.

A study from the Veterans Affairs San Diego Health System found that e-cigarettes damaged DNA in ways that could cause cancer and cell death. "Based on the evidence to date," said lead researcher Dr. Jessica Wang-Rodriquez, "I believe they are no better than smoking regular cigarettes."