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Compared with traditional cigarettes, electronic cigarettes can seem simple and harmless. After all, they trade the eye-stinging smoke of burning tobacco leaves for something that looks like steam and might smell like air freshener.

But that aroma masks a complex chemical mix that definitely worries experts who study the ingredients and effects of e-cigarettes, or vapes as they’re also known.

“It is not water vapor, and they are not harmless,” said Dr. Suchitra Krishnan-Sarin, the Albert E. Kent Professor of Psychiatry at the Yale University School of Medicine in New Haven, Connecticut.

Understanding exactly what each puff is doing to the body and brain when people use these products can be a challenge, said Dr. Mary Rezk-Hanna, an associate professor at the University of California, Los Angeles Joe C. Wen School of Nursing.

“There are hundreds of liquids out there,” along with concoctions people mix themselves, plus a constant stream of new, untested devices appearing on the market, said Rezk-Hanna. She and Krishnan-Sarin both helped write a 2023 ·¬ÇÑÊÓƵ on e-cigarettes and vaping.

But in the wake of recent Food and Drug Administration action to authorize the sale of some flavored e-cigarettes, the researchers explained what’s in the devices and what they could be doing to people’s health.

Why worry about e-cigarettes and vaping?

In general, e-cigarettes heat solvents to create an aerosol. The aerosol lacks many of the thousands of components of tobacco smoke. But people who use e-cigarettes are still inhaling plenty of substances to worry about, including:

• Heavy metals. Heating coils in e-cigarettes have been found to release chromium, lead and nickel, Rezk-Hanna said. “Exposure to these metals has implications for cardiovascular and respiratory health, and some of them also raise concern for cancer risk.”
• Particulates. “These are tiny little particles that can reach deep into the lungs,” said Rezk-Hanna. Those particles can be “harmful,” she said. They’ve been linked to high blood pressure, heart disease and heart attacks.
  Metals and particulate matter can do “almost immediate damage” to the lungs, Krishnan-Sarin said.
• Propylene glycol and vegetable glycerin. These clear liquids create the aerosol and carry other chemicals into the body.

In other contexts, some of these substances may have practical applications. Propylene glycol, for example, is used in foods, medications and cosmetics – as well as antifreeze. Vegetable glycerin, or glycerol, is used in everything from chewing gum to toothpaste, skin moisturizers and laxative suppositories.

But just because a substance may be OK in some cases doesn’t mean it is a good idea to inhale it, Krishnan-Sarin said. Inhalation affects the body differently than other uses, such as digestion. “When you inhale something, it’s going straight into your throat, your lungs, and through your lungs into your blood.”

And when heated, propylene glycol and glycerin can yield “many toxic compounds,” Rezk-Hanna said. Those include chemicals called aldehydes, which are thought to be the primary contributors to cardiovascular disease and chronic obstructive pulmonary disease in people who smoke traditional cigarettes.

Examples include:

• Formaldehyde. A colorless and flammable chemical with a strong odor, formaldehyde is widely used in home-building products, adhesives and other consumer products. It has been linked to high blood pressure and, at high levels, may cause cancer.
• Acetaldehyde. Acetaldehyde is found in wood smoke and automobile exhaust. It’s sometimes used in herbicides and insecticides – and in small amounts as a food additive. It irritates the eyes, skin and respiratory tract and is considered a probable human carcinogen. Both it and formaldehyde form from propylene glycol.
• Acrolein. Formed from glycerin, acrolein is used to kill unwanted algae, weeds and mollusks in water. It is associated with a higher risk of cardiovascular disease.

The physical effects of vape flavor chemicals

Some e-cigarette components raise concerns that go beyond direct harm.

Flavoring chemicals, for example, can be unhealthy by themselves. “Many flavoring chemicals have toxic respiratory effects when inhaled,” Rezk-Hanna said. “They may also have adverse cardiovascular effects.”

Two that have been studied the most are:

• Diacetyl, a buttery flavoring that may be found in many vape liquids. It is associated with the respiratory disease known as “popcorn lung.”
• Cinnamaldehyde, a cinnamon flavoring that, when inhaled, has been shown to impair respiratory immune cells.

Menthol and other “cooling” agents are another concern. in Circulation: Arrhythmia and Electrophysiology in June linked coolants to irregular heartbeats in mice and changes to human heart cells in the lab. (Research in mice does not necessarily translate to humans.)

Rezk-Hanna said that by masking harshness in the throat, flavorings may suggest to people that what they’re smoking is less harmful than a traditional cigarette.

That’s why, beyond direct effects, flavors can compound other damage, said Krishnan-Sarin, by making it easier to smoke more. And “if you vape more or you smoke more, then you’re going to get more harmful effects.”

It’s a similar story with one of the best-known ingredients in e-cigarettes, cigarettes, pouches and other tobacco products: nicotine.

Nicotine’s two-pronged attack on your health

Nicotine has “a very specific effect of increasing heart rate,” Krishnan-Sarin said. It stimulates the sympathetic nervous system, often known as the system that controls the body’s “fight-or-flight” response. Long-term overstimulation of that system can lead to irregular heartbeats and other cardiovascular problems.

But direct effects are only half the problem with nicotine.

Nicotine is also highly addictive. Like flavorings, Krishnan-Sarin said, it drives people to inhale more and more.

That easy addictiveness makes vapes a real threat to young people who use them, Rezk-Hanna said. It creates the cravings that keep people using vape products “even when they want to stop.”

Nicotine also affects the reward pathways in young, still-developing brains, she said, and that may increase susceptibility to using other products in the future.

The challenges to e-cigarette research

Discussions about dangerous ingredients in e-cigarettes can be complicated by the fact that some people use the devices in their efforts to quit using cigarettes. E-cigarettes are not approved by the Food and Drug Administration as an effective way to quit. (The ·¬ÇÑÊÓƵ recommends these proven approaches.)

And research about the effects of these ingredients takes time, Krishnan-Sarin said. Some harms might take years to show up. In the same way that the now well-established dangers of cigarette use didn’t emerge until decades after mass-market cigarettes appeared, “I think a lot of these chronic effects of e-cigarettes are not something that are going to emerge very quickly,” she said.

On top of that, she said, “companies are constantly innovating to change the devices and put new ingredients into these products to maintain their appeal. And we as scientists are constantly struggling to keep up” with these changes.

Rezk-Hanna agreed. “There are so many areas where we are just scratching the surface,” she said. But to her, the key to understanding the potential harm from e-cigarette and vape components is simple.

“There is no established safe level of exposure to the chemicals found in any of these products,” Rezk-Hanna said.

E-cigarettes are the most commonly used tobacco product among middle and high school students. The ·¬ÇÑÊÓƵ supports public policies at the federal, state and local levels to prohibit the sale of all flavored tobacco products, including menthol products. Take the next step: Visit for ways to get involved.