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The Food and Drug Administration says it plans to regulate e-cigarettes, along with cigars and other tobacco products. Health advocates say it’s about time, but many “vapers” who use e-cigarettes say regulation will damage a product that’s a far safer substitute for cigarettes.

“The FDA has over stepped their boundaries,” supporters wrote in an online petition posted this week. “DoNot, allow the FDA to take control of a life saving product (sic).”

Inside the Vaper's Den: E-Cigarette Salvation and New Dangers

Even health experts agree that electronic cigarettes, which deliver nicotine in a heated mist of water, glycerin and propylene glycol, might be useful in helping people who want to quit smoking. So where’s the harm in them?

Mostly, it’s the unknown, the FDA says. “We can’t even tell you what the compounds are in the vapor,” FDA’s Mitch Zeller told reporters.

FDA regulation would require the companies to tell the agency, but not necessarily the public, what’s in their products. FDA is sensitive to protecting competitive secrets.

The FDA is also asking for research on potential harms from inhaling the heated mixture. It might not be as harmful as burning tobacco leaves, but it might not be completely benign, either, says Dr. John Spangler, who runs a smoking cessation clinic at Wake Forest Baptist Medical Center in North Carolina.

“It is true that electronic cigarettes are safer than regular cigarettes. On the other hand there are some effects in the lung of the vapors which mimic the same kind of changes that go along with asthma,” Spangler told NBC News.

“That happens within 5 minutes of using an electronic cigarette. We don’t know how long that will last or whether it will lead to permanent lung damage.”

Vaping enthusiasts will argue that using the products allows them to skip regular tobacco cigarettes, whose harms are well-documented. If that’s the case, many health advocates say they are all for them. But there’s not much research to say whether that’s true.

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Spangler’s been studying this and doesn’t have final data yet. But in general, e-cigarettes appear slightly less effective than nicotine gum or patches or drugs such as Chantix, he says.

“I do have about 20 percent of my smoking patients in my clinic who are using electronic cigarettes,” he said. “Of the patients who use them, about 10 percent of them actually quit smoking using electronic cigarettes.”

"Most consumers would be shocked to realize the products they buy have less oversight than a bag of dog food."

And experts argue that if e-cigarette makers wanted their products used as quit-smoking aids, they’d have submitted them to the FDA as such. Instead, manufacturers fought FDA’s attempts to regulate them in that way, and won in federal appeals court.

Either way, people who use them should want them regulated, says pulmonologist Dr. Nathan Cobb of Georgetown University School of Medicine.

“Electronic cigarettes may represent the next evolution of nicotine replacement, supplanting the gum, patch and the existing inhaler. However, most consumers would be shocked to realize the products they buy have less oversight than a bag of dog food, and are often manufactured and imported from countries that have histories of tainted pharmaceutical and food products,” Cobb added.

The fear is that smokers will use e-cigarettes as a bridge that gets them through the day in a society that increasingly restricts cigarettes. And the even bigger fear is that e-cigarettes will appeal to children, who will get addicted to nicotine.

“Kids should not be initiating even an e-cigarette that contains no burning tobacco leaves because of the effect that nicotine can have on the developing brain,” Zeller says. Several studies suggest nicotine might slow the growth of the adolescent brain.

The Centers for Disease Control and Prevention and FDA say 10 percent of high school students have tried vaping, with numbers rising steadily. More than 21 percent of adults have tried vaping at least once.

The e-cigarette industry itself welcomes the FDA’s proposal. “We have no problem divulging our ingredients,” says Miguel Martin, president of e-cigarette maker Logic. Martin and other makers also say they support restricting sales to people 18 or older.

"FDA has worked over three years to meet the regulatory challenges presented by this very new and technology-driven industry, while preserving and supporting the enormous potential for harm reduction it offers. We share that goal," added the Smoke Free Alternatives Trade Association.

“You should always be suspicious when the tobacco industry applauds something."

Public health advocates say even that worries them. “You should always be suspicious when the tobacco industry applauds something,” says Tom Glynn, senior director of cancer science and trends for the American Cancer Society.

Glynn and others remember that the tobacco industry first told Americans that cigarettes were healthful, and then battled the U.S. government in court for decades when the Surgeon General declared that smoking caused cancer. It took decades to prove that tobacco companies colluded to make cigarettes ever more addictive and to cover up research showing tobacco caused not only cancer, but heart disease, stroke, emphysema and other diseases.

They also point to industry attempts to market “light” cigarettes as less dangerous — research shows they are not — and companies have pushed menthol cigarettes despite evidence that menthol worsens the health effects.

“Like cigarette companies, e-cigarette makers claim they don't market to kids. But they're using the same themes and tactics tobacco companies have long used to market regular cigarettes to kids,” says Matthew Myers, president of the Campaign for Tobacco-Free Kids.

And nicotine itself may not be so harmless. The concentrated nicotine juice used in vaping can poison people, and Spangler says nicotine build-up in vaping salons could be dangerous, also.

“We don’t know what will happen to nicotine that settles into the environment,” he said.