Electronic cigarettes are 95 percent less harmful than conventional cigarettes and could help smokers quit tobacco, a UK government study recently concluded, wading into a long-running debate.

While much research has been done on conventional cigarettes, data on e-cigs is much more sparse. This is because e-cigs were only invented in 2003 by a Chinese pharmacist, while tobacco has been around for centuries.

Because of e-cigarettes’ relative newness, their potential risks or benefits are not well-understood. There aren’t decades-long epidemiological data for researchers to analyze. Even the U.S. Food and Drug Administration says it’s unclear whether e-cig users are inhaling potentially harmful chemicals. The California Department of Health earlier this year declared them a public health threat.

The general consensus is that e-cigs are less harmful than conventional cigarettes, mainly because there is no smoke or burning involved. “They’re simpler than cigarettes,” Jonathan Foulds, PhD, professor of public health sciences and psychiatry at Penn State University’s College of Medicine and Cancer Institute, tells Yahoo Health. “You burn it, cigarettes creates 7,000 different chemicals into your body. But with e-cigarettes, there’s no combustion. The good news is that you’re inhaling a vapor that’s got four to five things as opposed to 7,000 things.”

But not all e-cigs are created equal. There are hundreds of brands with simple products resembling conventional tobacco cigarettes to more high-end, sophisticated vape pens. The FDA has found inconsistencies with some e-cigs containing nicotine when they’re marketed as free of the substance, not to mention varying levels of quality. At this point, e-cigarettes are not regulated by the FDA, although the agency seeks to change that soon.

Not all e-cigarettes contain nicotine. And some of the products let users control the amount and potency of the liquid solution. What’s inside the liquid solution vary by brand and flavors, such as butter popcorn and cookies and cream, which carry different chemical agents. Other e-cigs are more straightforward, with about five ingredients.

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“The question is: What’s the effect on the body of the four to five things?” says Foulds.

The problem remains: lack of data. “There’ve been few studies that investigated the extent of biological effects,” says Jake McDonald, PhD, director of Environmental Respiratory Health Program and the Chemistry and Inhalation Exposure Program at Lovelace Respiratory Research Institute in New Mexico.

So here’s what we know so far about how e-cigarettes may affect your body:

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Mouth

Users have complained of cotton mouth, scratchy throat, and coughing. It’s unclear why this happens to some people other than due to hydration issues.

A review of existing medical research on e-cigarettes finds that absorption of nicotine is most likely to happen in the lining inside the mouth or the upper airways.

Related: Smoking in US Declines to All-Time Low

Lungs

One of the main ingredients used in e-cig liquid solutions is propylene glycol (which is used as fog in concerts and theater) and glycerin (which is proven safe in foods).

“We’re putting things in e-cigs proven safe in foods, but it’s not the same as putting it into a vapor and inhaling it,” Foulds explains. “A Mars bar is safe to eat, but I wouldn’t want to inhale it. If something is safe as a food, it’s not highly harmful, but we don’t know what happens when you inhale it.”

There are concerns that nanoparticles from the vapor can be embedded in the lungs, causing inflammation and leaving the lungs vulnerable to infection. A study published in the journal PLOS this year found that the vapors damage the epithelial cells in the airways, leaving them vulnerable to infection.

“E-liquids without nicotine and with nicotine inhibit the lungs’ innate immunity, which helps it defend itself against infections,” study author, Qun Wu, MD, Ph.D, told Yahoo Health earlier this year.

The American Lung Association has called for more governmental oversight because “the reality is that without FDA regulation and review, we don’t know what is in e-cigarettes.”

Heart

The few studies that looked at e-cigarettes’ cardiovascular effects find that the ones containing nicotine raise heart rate and blood pressure. This is from the nicotine kick, which acts as a stimulant and prompts an adrenaline rush in the body. But studies have also found that e-cigarettes didn’t cause the type of disruptions in the body seen with tobacco smoking.

One small European study compared the heart functions of 20 young tobacco smokers versus 22 e-cigarette users seven minutes after using their products. The study conducted in Greece found none of the heart problems associated with tobacco cigarettes among e-cigarette users. Its lead researcher, Dr. Konstantinos Farsalinos, said in a statement: “This is an indication that although nicotine was present in the liquid used (11mg/ml), it is absorbed at a lower rate compared to regular cigarette smoking.”

The American Heart Association says there needs to be more research into this topic and called for rigorous examination of the “long-term impact of this new technology on public health, cardiovascular disease and stroke.”

Related: Are Former Smokers ‘Safe’ After 15 Years?

Brain

When nicotine enters the brain, it releases a feeling of pleasure as dopamine levels increase.

Nicotine, while not considered a carcinogen, is still addictive and may “prime the brain to become addicted to other substances,” according to the National Institute on Drug Abuse.

One of the biggest concerns by anti-tobacco advocates is that e-cigarettes will get users hooked on nicotine and will serve as a gateway to tobacco use.

Your Baby

Nicotine can be detrimental to your baby if you’re pregnant. It affects the development of the baby’s lungs and brains, and can cause problems like preterm birth, low birth weight, and stillbirth in higher rates, research shows.

“For smokers of e-cigarettes, we know it’s going to increase premature birth [and] increase intensive care state,” Foulds says. “These are things that affect babies for life. Some believe e-cigs are completely safe — it’s not quite true.”

Read This Next: Are E-Cigarettes Heroes Or Harmful? A Look At The Science

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