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Heavy exposure to electronic cigarette vapor damages DNA in cell cultures, causing genetic instability that could lead to cancer, according to a study by VA San Diego Healthcare System and University of California, San Diego, researchers.

Moreover, even nicotine-free vapor induces this damage, indicating that other substances in e-cigarettes can damage cells, the study stated.

The study won't come close to scientifically settling whether e-cigarettes represent a great new danger, a harmless diversion or something in between. It does provide more grounds for suspicion that e-cigarettes are not entirely benign, and carry health risks of an unknown magnitude.

Worldwide attention has been focused on e-cigarettes as a possible means of weaning smokers off tobacco, or alternatively as a new public health menace. But since e-cigarettes became popular scarcely a decade ago, there hasn't been time to collect long-term evidence, such as the population studies that linked smoking to lung cancer.

The study was published Monday in the journal Oral Oncology. Weg M. Ongkeko was the senior author on the research team, and Vicky Yu was first author.

The new research doesn't prove that the damage takes place in people, because it was performed only in cell cultures, said Laura Crotty Alexander, one of the research team study authors. But it strongly suggests such an effect takes place. Further work is needed to confirm this damage, and at what levels of exposure to e-cigarette vapor the damage kicks in.

She has previously conducted e-cigarette research indicating that the vapor makes the "superbug" MRSA harder to kill.

An even bigger question, whether e-cigarettes are as bad for one's health as regular cigarettes, also isn't answered in the study, Crotty Alexander said. That question bedevils public health advocates who are wrestling with the issue of how to deal with e-cigarettes.

In a statement that made headlines around the world, Jessica Wang-Rodriguez, another study author, said e-cigarettes "are no better than smoking regular cigarettes."

That statement was made in a press release picked up by reporters who didn't cite from the study itself. These press release rewrites were strongly criticized by other researchers.

"To declare that smoking is no more hazardous than using e-cigarettes, a non-tobacco-containing product is a false and irresponsible claim," e-cigarette researcher Michael Siegel of Boston University told the Daily Caller. Siegel supports using e-cigarettes to get smokers to quit.

Crotty Alexander said the evidence simply isn't definitive, in any direction.

"The problem is that we really cannot say that the e-cigarettes are safer in humans," Crotty Alexander said. "I feel uncomfortable saying that e-cigarettes are equally bad or worse than conventional combustible cigarettes, but that is some people's opinion."

The paper itself notes that cigarette smoke extract kills cells at a lower concentration than does e-cigarette vapor, Crotty Alexander said. And it kills more rapidly.

"Because of the high toxicity of cigarette smoke extract, cigarette-treated samples of each cell line could only be treated for 24 h(ours)," the study stated.

Cells were exposed to extracts containing 1 percent e-cigarette vapors in a number of tests, one of which is called a "neutral comet assay" that measures DNA damage.

The e-cigarette extract was tested for eight weeks on a cell line representing normal epithelial tissue, and for one week on two cell lines representing cancers. The extract-containing liquid was replaced every three days.

Results showed a statistically significant increase of up to 1.5-fold in DNA strand breaks, as compared to an untreated control cell culture.

Whether e-cigarettes are harmful and should be avoided can't be answered with a simple yes or no. The evidence on harm is hotly disputed in the scientific literature. And that evidence is minuscule, compared to the copious evidence about the harm from tobacco.

If vaping is harmful, but less so than smoking cigarettes, then smokers who switch to e-cigarettes are making a healthy choice. But non-smokers who take up the habit would be harming their health.

While cigarettes were invented in the 19th century, e-cigarettes became popular only about 10 years ago. Prototypes were developed in the 1960s, but Hon Lik, a Chinese pharmacist, is credited with developing the first modern e-cigarette, in 2003.

So long-term data on the risks or benefits of vaping simply doesn't exist. And the implications of using e-cigarettes depend on who is vaping, and one's personal values.

E-cigarettes come in innumerable configurations, including single-use models and more expensive reusable kinds with rechargeable batteries and refillable reservoirs. They can be bought in liquor stores or specialty vape shops dedicated to vast array of hardware used by aficionados. This hardware inspires a devotion not unlike hot rod or computer fans.

They all contain a reservoir for the flavored juices, an atomizer that vaporizes the liquid, a battery that powers the atomizer, and a mouthpiece for inhaling the vapors.

Some of the criticism takes aim at the vast and lightly regulated market for the liquids, which are made with nicotine in varying levels and no nicotine at all. They contain various flavoring agents which are not well understood.

"The specific substances in e-cig liquids are still under investigation, as many formulations are proprietary information. However, our findings are consistent with previous assessments of e-cig effects on pulmonary tissue and cell lines, which implicated flavoring compounds as primary toxicants within e-cigs," the study stated.

People who have quit smoking within the last year are four times more likely to use e-cigarettes daily than current smokers to use e-cigarettes daily, according to a study led by Rutgers University researchers released in November. The researchers say this is evidence that e-cigarettes help smokers quit.

And a study by Public Health England released in August found that e-cigarettes are 95 percent less harmful than smoking and can help smokers quit.

However, a study released in August 2014 found that the number of middle- and high school student nonsmokers who used an electronic cigarette in the previous year tripled in two years. The number rose from 79,000 in 2011 to 263,000 in 2013, according to a report from the Centers for Disease Control and Prevention.

The danger, say e-cigarette foes, is that adolescent nonsmokers who take up vaping will find it easier to start smoking-the supposed "gateway" effect marijuana is blamed for leading those who inhale to try narcotics.

Sometimes, the same study can be characterized in opposite ways about e-cigarette risks or benefits. That's the case with the National Institute of Health's 2015 Monitoring the Future survey, released Dec. 16.

Michael Siegel, the Boston University researcher, said the MTF study refuted the CDC's claims that vaping leads to smoking.

"First, the MTF survey revealed that despite the huge number of youth using electronic cigarettes and the dramatic increase in use of e-cigarettes by youth over the past four years, the rate of youth smoking has declined dramatically during the same time period. Moreover, the decline in smoking continued from 2014 to 2015," Siegel wrote on his blog.

Siegel was responding to a statement from the Campaign for Tobacco-Free Kids that said the CDC study indicated e-cigarettes threaten progress against tobacco products.

"For the second year in a row, the survey finds that significantly more teens reported using e-cigarettes than regular cigarettes in the past 30 days," the campaign said. It also said e-cigarette makers irresponsibly market to children, by providing "kid-friendly flavors, such as gummy bear, cotton candy and watermelon."

Vaping supporters responded on Twitter to the VA-led study, saying that it's actually a benefit that e-cigarette liquids preferentially kills cancer cells.

They also pointed to the sentence in the study about the difficulty of comparing the effect of vaping liquids to that of cigarette smoke because of the latter's high toxicity.

Another limitation of the study, which Wang-Rodriguez said she plans to address in future research, is that it tested lab-grown cells, which may not give the same results as cells living inside a person.

Crotty Alexander said a new study on whole animals is under way. This should give results more representative of what would be seen in people.

While human testing would be ideal, it runs into moral problems, Wang-Rodriguez said. Exposing people to suspected carcinogens would be ethically wrong. That problem wouldn't apply if current e-cigarette users were recruited. But even a direct study in humans would be more limited than animal studies.

"It's harder to take a tissue sample of their lung," she said. "But I think a population or epidemiologic study is worth doing."

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