(CNN) -- Are you considering kicking your smoking habit for the New Year by trying e-cigarettes?
Though many former smokers swear by it, some experts are wary.
E-cigarettes work by heating a pure liquid called "e-juice" -- composed of flavorings, propylene glycol, glycerin and often nicotine -- until it vaporizes. The resulting vapor is much less offensive to many -- both smokers and non-smokers -- and some studies have shown it helps smokers quit. In fact many e-cigarette users don't call themselves smokers, preferring to use the term "vapers" instead.
But are e-cigarettes really an effective way to quit smoking? And does "vaping" introduce health issues all its own? Science and public policy have bounced back and forth for over a decade, as different studies produce different -- and sometimes contradictory -- results. Let's take a look at how this controversy over the years:
2003 Invention of e-cigarettes
Three pack-a-day smoker Hon Lik, a 52-year-old Beijing pharmacist, creates the first successful electronic cigarette after his father, another heavy smoker, dies of lung cancer. By 2007, e-cigarettes are marketed in Europe and the U.S. by the manufacturer Ruyan as a way to safely stop smoking tobacco.
Hon is not the first person on record to have the idea for an electronic non-tobacco option. Herbert A. Gilbert filed for a patent back in 1963, in an era when tobacco smoking was widely accepted and the health risks were less apparent.
2008 WHO slams e-cigarette marketing
In September 2008, the World Health Organization announces marketers should immediately remove any claims that e-cigs are a "safe and effective smoking cessation aid" because there is "no scientific evidence to confirm the product's safety and efficacy."
Soon after, a study funded by e-cigarette manufacturer Ruyan declares the product to be 100 to 1000 times less dangerous than smoking tobacco, adding that when using their device, nicotine is "apparently not absorbed from the lung, but from the upper airways."
2009-2010 The battle heats up
In May, 2009, the Food and Drug Administration releases the results of a test of two U.S. e-cig brands, NJOY and Smoking Everywhere, that finds "very low" amounts of nicotine in cartridges labeled as nicotine free. Then in July, an FDA press release discourages the use of e-cigarettes, saying they contain carcinogens and an ingredient used in antifreeze, diethylene glycol.
Another concern of the FDA's: E-cigarettes are often marketed and sold to youngsters who, intrigued by the many flavors such as chocolate, bubblegum and mint, might easily adopt a smoking habit as a result of trying the devices.
Vape supporters counter that diethylene glycol was found at a very low, non-toxic level of 1%, and that the carcinogens are at the same levels as other FDA-approved nicotine cessation products, like patches and gum.
By the end of the year, Amazon and Paypal restrict the sale of e-cigs on their websites.
2011 Interest in vaping for smoking cessation is high
Science begins to ramp up studies on the topic. Several studies find interest in e-cigarettes is high among the American public: Google searches for e-cigs are higher in the U.S. than any other nation.
A questionnaire of 3500 e-cigarette users finds most vape because they think it is less toxic and cheaper than tobacco, and will help them quit or cut down on tobacco smoking. Most ex-smokers (79%) in the study are afraid they will relapse if they stop using e-cigarettes. The study doesn't examine the safety of the product.
Another, much smaller email study of 216 e-cigarette users finds that 31% were tobacco free at six months, while 66% were able to cut back on the number of conventional cigarettes they smoked. A still smaller study of 40 smokers also finds adding e-cigarettes helped smokers reduce the number of traditional cigarettes they smoked each day.
2012 E-cigarette use doubles in adolescents
The Centers for Disease Control announce that e-cigarette use among U.S. middle and high school students doubles between 2011 and 2012, mirroring a similar increase in adult use. Most alarming for policy makers: CDC concerns that vaping among adolescents may serve as a gateway to tobacco use.
To measure nicotine delivery, United Kingdom researchers test 16 e-cigarettes with an automatic smoking machine and find wide variations in nicotine levels per puff, ranging from 0.5 to 15.4 mg. In contrast, the typical level from a tobacco puff only ranges from 1.54 to 2.60. The wide variation between e-cigarette brands leads researchers to question how well they can function as a nicotine replacement device.
2013 Do e-cigs really help smokers quit?
Several 2013 publications show minimal evidence that e-cigarettes help smokers quit. A cross-sectional study of 1,836 tobacco smokers finds a significant association with e-cig use and "unsuccessful quitter" status, but none with 'quitter" status.
Another study of callers to state tobacco quitlines finds e-cigarette users significantly less likely to be tobacco-free seven months after they first try vaping compared with participants who never try e-cigarettes.
A New Zealand Health Research Council study of 657 smokers finds e-cigarettes modestly effective in helping smokers quit. Interestingly enough, it didn't seem to matter if they contained nicotine or not. But the results were similar to already FDA approved nicotine patches.
Some researchers worry e-cigarette use might distract smokers from proven safe and effective methods for kicking their tobacco habit. "It's filter tipped, low tar, déjà vu all over again," say Dr. Frank Leone and Dr. Ivor Douglas in their essay "The Emergence of E-Cigarettes: A Triumph of Wishful Thinking over Science."
2014 Poison center calls skyrocket
The Centers for Disease Control release data showing the number of calls about nicotine e-juice to poison centers rose from a scant 1 call/month in 2010 to 215 calls/month in 2014. More than half of the calls involved children under the age of 5 ingesting, inhaling or getting the substance in their eyes or on the skin.
A study of over 75,000 Korean adolescents finds e-cigarette use to be strongly associated with current and heavy traditional cigarette smoking. Data from the CDC finds the use of e-cigarettes among U.S. high school students grew from 4.5% in 2013 to 13% in 2014. In that same time period, use grew among middle schoolers from 1% to 4%.
A study comparing the e-cigarette inhaler to the barely-used FDA approved nicotine inhaler for smoking cessation finds the e-cig version a clear winner with users, providing more satisfaction and a better image.
In regard to safety, a study finds e-cigarettes do contain tobacco-specific nitrosamines and heavy metals like cadmium, nickel and lead, but the levels are 9 to 450 times lower than traditional cigarettes. The effect on lung function of glycol derivatives found in e-cigarettes was also much less than conventional cigarettes.
Still researchers aren't convinced. "Although these data suggest that e-cigarettes may be a safer alternative to conventional cigarettes," say reviewers Bradley Drummond and Donna Upson, "there are no data regarding the long-term cancer risk associated with low-level exposure to the detected carcinogens. Similar to cancer risk, there are no published data describing the long-term lung function or cardiovascular effects of e-cigarettes."
2015 Is vapor safe?
A New England Journal of Medicine study sets off alarm bells by reporting that some e-cigarettes release formaldehyde, a probable carcinogen, when heated by high voltage batteries.
A Cleveland Clinic clinical review restates ongoing concerns about propylene glycol, or PG, and the various food flavorings that are part of e-juice. While considered safe to eat in food in small quantities, researchers say PGs have not been studied when "inhaled deeply and repeatedly."
Another study looks at 51 of the nearly 7,000 e-cigarette flavors currently marketed to check for a flavoring chemical called diacetyl that has been shown to be associated with a disease called popcorn lung. The study finds diacetyl at higher than laboratory normal levels in 39 of the tested flavors. Researchers call for "urgent action" to evaluate the "potentially widespread exposure."
In December, a San Diego Veterans Affairs lab investigation finds two e-cigarette products damaged cells in ways that could lead to cancer, even when nicotine-free. Creating an extract from e-cig vapor, the researchers exposed cells in petri dishes. The exposed cells showed breaks in DNA strands, which can lead to cancer, and a greater tendency towards cell death.
However, a 2015 report by Public Health England encourages the medical licensing of e-cigarettes in the UK as nicotine replacement therapy, stating that the use of vaping is "95% safer than smoking" and "can help people to quit smoking and reduce their cigarette consumption" even "among those not intending to quit and rejecting other support."
And research from France's Health Barometer, which conducted telephone interviews with over 15,000 people, finds that e-cigs "could have helped several hundreds of thousands" quit smoking, at least temporarily.
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