The aerosol produced by e-cigarette use tends to garner little notice, if any at all. And even when it is noticeable, the airborne evidence of vaping, which typically involves inhaling and exhaling the aerosol from a battery-powered e-cigarette, tends to dissipate quickly. By contrast, smoke from a traditional combusted tobacco cigarette often lingers.

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E-cigarettes' inconspicuous nature, in addition to their promotion as potential quit aids to help stop smoking, may contribute to the notion that vaping has no secondhand effects, experts say. But as further study continues to shine light on the negative health impacts of these unregulated products, it's becoming clear that besides not being entirely safe for the user, vaping may harm nonusers as well.

Most e-cigarettes contain nicotine, and even some e-cigarettes advertised as nicotine-free contain the highly addictive drug, according to the Centers for Disease Control and Prevention. Aerosol contains many other harmful or potentially harmful substances.

According to the CDC, substances found in vaping aerosol include:

Ultrafine particles that can be inhaled deep into the lungs.

Flavoring like diacetyl, a chemical linked to a serious, irreversible lung disease called obliterative bronchiolitis.

So-called volatile organic compounds, or gases emitted into the air that may have adverse health effects.

Cancer-causing chemicals.

Heavy metals, including nickel, tin and lead.

The American Cancer Society notes that the cancer-causing substance formaldehyde, for one, may form "if e-liquid overheats or not enough liquid is reaching the heating element (known as a "dry-puff")."

"People don't, I think, always realize that the secondhand smoke emissions from vaping or e-cigarette use can be harmful," says Dr. S. Christy Sadreameli, a pediatric pulmonologist at Johns Hopkins Hospital in Baltimore, and a volunteer spokesperson for the American Lung Association. The ultrafine particles and substances in aerosols "could be inhaled by children who are nearby, and so we worry about that," she says.

While the debate continues about how the risks of e-cigarettes stack up against traditional cigarettes, "we don't think they're harm-free, and we don't think the secondhand emissions are safe for children," Sadreameli emphasizes.

Given that vaping is a modern trend compared with traditional smoking, research is limited on the dangers of firsthand use and the effects on those who may be around others who vape. Still, experts emphasize, there's ample reason to avoid using e-cigarettes when kids may be present.

"Exposure to any nicotine or tobacco product is dangerous for infants and children," says Dr. Jonathan Winickoff, the director of pediatric research at the Tobacco Research and Treatment Center at Massachusetts General Hospital. "The developing brain is exquisitely sensitive to nicotine." Nicotine can impact the developing structure of the brain – altering reward pathways that navigate positive reinforcement in the brain – so that a child becomes more vulnerable, or is at increased risk, for becoming addicted to products that contain nicotine and other drugs in the future.

Secondhand smoke from traditional cigarettes is known to cause tens of thousands of deaths annually in the U.S., and is implicated in some infant deaths, or sudden infant death syndrome. Likewise, chemicals in e-cigarette aerosol may also contribute to secondhand health problems.

"People who are doing the vaping themselves, can have lung irritation and can have worsening of asthma," says Dr. Thomas Houston, a family physician and former chair of the American Academy of Family Physicians Commission on Health of the Public and Science and Smoking Cessation Advisory Committee. "So whether this will translate to children having exacerbations of their asthma if they're in the same room with adults who are vaping, we just don't know yet."

But some reports show bystanders can experience excess cough and lung irritation from being around secondhand vaping, he notes. And a study published in the journal Chest in January 2019 found that teens with asthma who were exposed to secondhand aerosol, without vaping themselves, were more likely to have an asthma attack.

Besides the possibility of worsening asthma, ultrafine particles breathed deep into young lungs may impact lung development. "Lung tissue can be affected by nicotine exposure and exposure to e-cigarette aerosol," Winickoff says.

Animal research shows that exposure to e-cigarette vapor leads to a decrease in the growth of tiny air sacs called alveoli in developing lungs. Based on such findings, while not definitively known since e-cigarettes haven't been on the market for long, exposure to nicotine early on in childhood may mean decreases in lung function, he says. What's more, where secondhand smoke can contribute to higher levels of attention deficit hyperactivity disorder, secondhand vaping could possibly lead to problems with attention and hyperactivity as well, he says.

While experts emphasize more study is needed to understand the effects of vaping aerosol, they also reiterate there's enough potential for harm that parents should never vape around kids – or in spaces like the home or car, where they'll be.

Smoke-Free Doesn’t Mean Vape-Free

Survey data finds more than half of e-cigarette users are dual users, meaning they smoke and vape. Experts say some dual users vape with the intent to stop smoking altogether, using both e-cigarettes and traditional cigarettes during the transition. Others vape as a supplement to smoking, such as where smoking laws ban traditional cigarette use or where it's less socially acceptable to smoke. Although the survey data doesn't drill down to distinguish between the reasons for dual use, or show which is scenario is most common.

Even as some parents continue to smoke around kids, despite well-established science showing the harmful – and even deadly – effects of secondhand smoke, many more report that they vape around kids at home and in their cars.

"We saw three-fold higher rates of parents using electronic cigarettes inside their homes than smoking," says Winickoff, who was the senior author on research evaluating this dichotomy, which was published in the journal Pediatrics in April. Of the parents interviewed who were dual users, nearly 64% had a smoke-free home policy, compared with only 26%, who had a vape-free home policy. In addition, dual users and e-cigarette-only users together were more likely to have a smoke-free car policy, than a vape-free car policy. Still, only 35% had a smoke-free car policy and just 22% had a vape-free car policy.

The findings seem to underscore an errant perception of e-cigarettes as safe – though experts emphasize they’re not. And that misperception is increasing kids' exposure to vaping not only firsthand – as more and more adolescents take up the habit, alarming public health officials – but secondhand, and even thirdhand.

Experts emphasize kids' environments should be 100% vape-free. This is something pediatric health providers should be emphasizing with parents as well, researchers involved in the Pediatrics study add – and a message that's often missed. "To protect children from secondhand and thirdhand exposure, parents should adopt strictly enforced policies that prohibit e-cigarette use at all times in homes and cars," they urge.

Increasingly research shows how nicotine and other harmful substances settle on surfaces, like furniture, window dressing and flooring, as well as clothing. These lingering substances may cause harm thirdhand, even after a person is done smoking. But cigarettes aren't the only culprit: Aerosols from e-cigarettes can leave a chemical residue as well. Past research finds, for example, that "e-cigarettes leave deposits of nicotine on surfaces when used inside," researchers note in the Pediatrics study.

Apart from possible secondhand and thirdhand effects, e-juice flavors can be enticing to kids. Small children have put the small pods filled with vape juice in their mouths, which has led to nicotine poisoning in some cases and even child deaths, when the pods were swallowed. So experts say it's critical these products are always kept locked away, out of reach of children.

The best – and often most difficult – solution is to quit smoking and vaping altogether, experts say.

"If you are interested in quitting tobacco use, first try using the FDA-approved smoking cessation medicines including two forms of nicotine replacement therapy: the patch plus either gum or lozenge for breakthrough cravings," Winickoff says. "Then talk to your doctor about other safe medicines to help you quit."

Aside from a parent's own health, quitting is best because kids often follow the example of their parents. "Kids whose parents use cigarettes are up to four times more likely to use – to become smokers themselves – during the teenage years," he says. But for parents who are able to quit, that "social modeling" could decrease the likelihood their child will ever start smoking or vaping.