E-Cigarettes: How They Work, Ingredients & Health Effects

E-cigarettes heat liquid into an inhalable aerosol without burning anything. That one design choice, no combustion, is why they entered public conversation as a harm reduction tool and why the debate about their safety is still unsettled.

Since Juul reached U.S. shelves in 2015, the category has expanded into dozens of device types and thousands of e-liquid formulations. The science on what that aerosol does to the body is growing fast, and not all of it is reassuring.

How E-Cigarettes Work

No combustion, no smoke, but still an inhaled chemical cocktail. A battery powers a metal coil called the atomizer, which heats e-liquid stored in a cartridge, pod, or tank until it aerosolizes. The user inhales that aerosol.

Because there’s no burning, e-cigarettes don’t produce tar or the thousands of carcinogens generated by combustible tobacco. That distinction matters when comparing them to cigarettes. It does not mean the aerosol is inert.

Device Types: A Direct Comparison

The type of device largely determines how much nicotine you get and how fast. The market has consolidated around four categories:

Type	Form Factor	Nicotine Form	Typical User
Ciga-like	Cigarette-sized	Freebase nicotine	Beginners
Vape pen	Pen-sized	Freebase or salt	Moderate users
Box mod	Large, customizable	Freebase (lower %)	Experienced vapers
Pod system	Compact	Nicotine salts (high %)	Heavy smokers, Juul users

Pod systems reshaped the market after 2016. Juul’s nicotine salt formula can deliver up to 59mg/mL of nicotine, several times higher than most earlier ciga-likes, which drove rapid adoption and contributed directly to the youth vaping surge between 2017 and 2019.

What’s in E-Liquid

E-liquid has four basic ingredients, and all four create health considerations worth knowing: nicotine, propylene glycol (PG), vegetable glycerin (VG), and flavorings.

Nicotine is the addictive component. Nicotine salt formulations allow very high concentrations to be inhaled without harshness, speeding dependence significantly faster than older freebase nicotine products.

PG and VG produce the visible aerosol. At elevated heating temperatures, both can break down into formaldehyde and acetaldehyde.

Flavorings are the most variable and least-regulated ingredient. A 2016 Harvard study found diacetyl in 39 of 51 flavored e-liquids tested, a compound linked to bronchiolitis obliterans, more commonly known as popcorn lung. Manufacturers are not required to disclose how flavor compounds behave at vaping temperatures.

Health Effects: Where the Evidence Actually Lands

Vaping sits somewhere between “much less harmful than smoking” and “not safe.” Both things are true simultaneously, depending on who is using it and why.

For People Who Already Smoke

Public Health England reviewed available research and concluded vaping is approximately 95% less harmful than smoking for adults who switch completely. Dr. Konstantinos Farsalinos, a Greek cardiologist who has published more than 50 peer-reviewed papers on e-cigarettes, has consistently argued that dismissing harm reduction potential puts confirmed smokers at greater risk. The caveat matters: the benefit only applies when cigarettes are dropped entirely, not when both products are used together.

The EVALI Outbreak

In 2019, a lung injury outbreak the CDC named EVALI hospitalized 2,807 people and killed 68 by February 2020. Most cases were traced to vitamin E acetate in illicit THC cartridges, not standard commercial nicotine products. The outbreak still exposed how little independent oversight existed over what goes into vape liquid sold at retail.

Cardiovascular and Respiratory Effects

Nicotine raises heart rate and blood pressure regardless of delivery method. Research published in the Journal of the American College of Cardiology documented elevated oxidative stress and endothelial dysfunction in e-cigarette users compared to non-users. What vaping does to your lungs over time is still being studied, but early data on airway inflammation is not encouraging.

Youth Vaping: The Clearest Problem

The harm reduction argument collapses entirely when the population is teenagers who would not have touched nicotine otherwise. At the 2019 peak, 5.4 million U.S. middle and high school students used e-cigarettes, according to CDC survey data. By 2023, that number had dropped to 2.13 million, still high enough to sustain classification as a public health epidemic.

Why teens vape is not complicated. Fruit and candy flavors, discreet pod designs, and influencer-driven social media exposure drove adoption among young people with no prior tobacco history. Dr. Stanton Glantz, who researched tobacco control at UCSF for decades before his death in 2022, documented evidence suggesting e-cigarette use increases the likelihood of later cigarette smoking in adolescents, a relationship the industry disputes but longitudinal studies increasingly support.

Regulation

The FDA classified e-cigarettes as tobacco products under its 2016 Deeming Rule, requiring manufacturers to seek premarket authorization before selling new devices. Enforcement has struggled to keep pace with the market: as of 2024, only a handful of products had received that approval, and Juul’s application was denied in 2022 before the FDA reopened the review.

Most flavored products currently on the market exist in legal gray zones. States including California, Massachusetts, and New York have moved ahead of federal action with outright flavor bans and stricter age enforcement.

Using E-Cigarettes to Quit Smoking

The cessation evidence is real but modest. A 2019 randomized trial published in the New England Journal of Medicine, led by Peter Hajek at Queen Mary University of London, found e-cigarettes produced a one-year quit rate of 18%, compared to 9.9% for traditional nicotine replacement therapy. Most participants in the e-cigarette group were still using their devices at the one-year mark rather than becoming nicotine-free.

For strategies focused on actually stopping vaping rather than switching products, the approach differs. The full quit-vaping guide covers behavioral approaches, NRT options, and what the first 72 hours of withdrawal actually look like.