As the number of young people using electronic nicotine delivery systems grows, so does evidence that e-cigarettes may be harmful to human health. More studies are needed to determine the long-term impact these devices may have on the heart and lungs, according to a new science report.

The scientific statement from the American Heart Association, published Monday in the journal Circulation, highlights the latest usage data and scientific evidence showing health effects of e-cigarette use, also called vaping. It also recommends research priorities to better understand how these products may affect people's health over time.

"E-cigarettes deliver numerous substances into the body that are potentially harmful, including chemicals and other compounds that are likely not known to or understood by the user," Dr. Jason J. Rose, chair of the statement writing committee, said in a news release. He is an associate professor of medicine and the associate dean of innovation and physician science development at the University of Maryland School of Medicine in Baltimore.

E-cigarettes and other vaping systems have been available in the U.S. for about 15 years. Use has been rapidly growing among youth and young adults, more than doubling from 2017 to 2019 among middle and high school students.

Vaping mimics cigarette smoking using battery-operated systems that heat liquid to create an aerosol inhaled into the lungs. These systems most often deliver nicotine, which has been shown to have negative health effects as well as being highly addictive.

But the devices also can be used to deliver tetrahydrocannabinol, or THC, the psychoactive element in marijuana, and other substances, such as methamphetamine, methadone or vitamins. The liquids include humectants that act as solvents to create a water aerosol or vapor, flavoring agents, cooling agents such as menthol and sweeteners, and metals from the heating coil, along with other chemicals.

"Young people often become attracted to the flavors available in these products and can develop nicotine dependence from e-cigarette use," Rose said.

But, he said, the long-term risks of e-cigarettes are unknown and might not be visible for decades. "What is equally concerning is that studies show that some youth who use e-cigarettes go on to use other tobacco products, and there is also a correlation between e-cigarette use and substance use disorders."

Vaping has been associated with an increased risk for respiratory diseases and a condition specifically related to e-cigarette use, called EVALI – short for e-cigarette or vaping product use-associated lung injury. The Centers for Disease Control and Prevention recognized EVALI as a condition in August 2019. By mid-February 2020, roughly 2,800 e-cigarette users had been hospitalized and 68 people had died.

Research has linked nicotine-containing e-cigarettes with acute changes in several measures of blood flow, including increases in blood pressure and heart rate, Rose said. Other e-cigarette ingredients, particularly flavoring agents, independently carry risks associated with heart and lung diseases in animals, and negative effects have been shown in studies of individuals exposed to chemicals in commercially available products, he said.

E-cigarette companies position their products as one way to help people quit smoking traditional cigarettes, but there is no strong evidence to support this beyond any short-term benefit, experts say.

E-cigarette products are not approved by the Food and Drug Administration for tobacco cessation. The AHA recommends cessation counseling, along with personalized nicotine replacement therapy with FDA-approved doses and formulations to help people stop smoking. It also recommends medication to help control cravings.

There are limited studies looking at the effect of e-cigarette use on heart attacks and strokes. And much of the research that has been done has focused on people who were former or current traditional cigarette users. However, a recent analysis cited in the report found a significant association between former or current e-cigarette users and the development of several types of respiratory diseases within two years of use.

The writing committee recommended that future research focus on the long-term effects of e-cigarette use on the heart, blood vessels and lungs. The statement also calls for studies to include people with pre-existing cardiopulmonary disease, such as coronary artery disease or chronic obstructive pulmonary disease, to compare the effects of e-cigarette use with traditional smoking and how people who use both types of nicotine delivery may be affected.

"It is necessary for us to expand this type of research since the adoption of e-cigarettes has grown exponentially, especially in young people, many of whom may have never used combustible cigarettes," Rose said.