The use of e-cigarettes is associated with an increased risk of heart attack, heart disease and stroke, according to research that is scheduled to be presented Feb. 6 at the American Stroke Association's International Stroke Conference in Honolulu.
Concern around the health effects of e-cigarette use has grown in recent years, fueled by a surge in their popularity and a belief that they're safe alternatives to normal cigarettes.
E-cigarette use among high school students increased by 900 percent between 2011 and 2015. In 2018, more than 3.6 million young people in the U.S., including 1 in 5 high school students, were users of e-cigarettes, according to the Centers for Disease Control and Prevention.
"There's a certain notion that e-cigarettes are harmless," says Dr. Paul Ndunda, the study's author and an assistant professor at the School of Medicine at the University of Kansas in Wichita. "But this study and previous other studies show that while they're less harmful than normal cigarettes, their use still comes with risks."
The researchers used data collected by the 2016 Behavioral Risk Factor Surveillance System, a phone survey sponsored by several federal agencies, including the CDC. The survey includes people in all 50 states, asking about risky health-related behaviors, like smoking, and whether respondents have been diagnosed with any health problems.
Of the more than 400,000 respondents in 2016, 66,795 reported having used e-cigarettes at least once, and compared with nonusers, e-cigarette users had a 71 percent higher risk of stroke, 59 percent higher risk of heart attack and 40 percent higher risk of heart disease.
Ndunda says that the nature of the analysis prevented the research team from accurately calculating the absolute risk of heart attack and stroke from the database.
The findings haven't been published in a peer-reviewed scientific journal, but Ndunda says the researchers plan on submitting their results soon.
"These results are important as they qualitatively and quantitatively agree with previous studies," says Stanton Glantz, a tobacco and e-cigarette researcher at University of California, San Francisco, who wasn't involved in this work but did publish another study that linked e-cigarette use to higher risk of heart attack. "The fact that the stroke and heart attack risk factors are not that different is also the same pattern you see with cigarette smoking, which adds extra weight to this study too."
However, many e-cigarette users also smoke conventional cigarettes.
In fact, Ndunda found e-cigarette users are twice as likely to also smoke conventional cigarettes, compared with people who don't use e-cigarettes.
To see the health effects 0f e-cigarette use alone, Ndunda and his colleague Dr. Tabitha Muutu compared people who had only used e-cigarettes — not conventional cigarettes — to nonsmokers.
"Even in that group there was a 29 percent higher risk of stroke and a 25 percent higher risk of heart attack," Ndunda says. Taken together, these two analyses point to an additive effect of e-cigarette and conventional cigarette use.
"So if you're a dual user, which many e-cig users are, you're actually worse off," says Glantz, who found a similar additive effect in his study.
Scientists aren't quite sure how e-cigarettes lead to this higher risk.
E-cigarette smoking may contribute to the gradual buildup of fatty deposits in arteries, Glantz says. But he thinks researchers may be detecting a link between increased risk for heart attacks and strokes and e-cigarette use because of a more immediate effect on the cardiovascular system.
You could have this pre-existing buildup, Glantz says, "and then you use the e-cigarette and that triggers a bunch of inflammatory processes, the release of oxidizing agents and things which then interfere with normal functioning of the blood and blood vessels, and that triggers a heart attack or stroke."
"This study certainly has limitations," Ndunda says. For one, this study couldn't distinguish between occasional e-cigarette use and those who vape more frequently. "It likely matters how much you're using, and we couldn't evaluate that here," Ndunda says.
E-cigarettes can deliver a range of nicotine concentrations and a wide variety of chemical flavorings, adding further complications to the analysis. The study's design also means it can only show an association between e-cigarette use and risk, not cause and effect.
Ndunda added that a study that identifies e-cigarette users early and then tracks their health over time would yield a clearer picture of the consequences of vaping.
Dr. Chitra Dinakar, a clinical professor of pulmonary and critical care medicine at the Stanford University School of Medicine who has studied the health effects of e-cigarettes, says this work, which only surveys adults 18 and older, "does not reflect the risk of stroke in younger users." Still, she says, "this is an important topic that merits ongoing scrutiny."
Jonathan Lambert is an intern on NPR's Science Desk. You can follow him on Twitter: @evolambert.