They are usually marketed as a safe alternative to conventional cigarettes, but a new study suggests that e-cigarettes may still pose serious harm to health.

Researchers at the New York University School of Medicine in New York City found that mice exposed to electronic cigaratte (e-cigarette) vapor experienced DNA damage in the lungs, bladder, and heart, which could increase the risk of cancer and heart disease.

Such damage was also found in cultured human lung and bladder cells that had been exposed to e-cigarette vapor for the equivalent of 10 years.

Study co-author Moon-shong Tang, of the Department of Environmental Medicine at the New York University School of Medicine, and his colleagues recently reported their findings in the Proceedings of the National Academy of Sciences.

E-cigarettes—also known as e-cigs—have soared in popularity in recent years, particularly among teenagers and young adults. According to the Centers for Disease Control and Prevention (CDC), more than 2 million middle and high school students in the United States report having used e-cigarettes in the past 30 days.

E-cigarettes are not approved by the Food and Drug Administration (FDA) as a quit-smoking aid, after the US Preventive Services Task Force concluded that there is "insufficient evidence to recommend for or against [their] use for smoking cessation."

However, research has found that more than 80% of smokers believe that e-cigarettes could help them to quit, while more than 97% believe that the battery-powered devices can help to reduce the use of conventional tobacco.

The debate over e-cigarette safety

When it comes to our health, the CDC believe that e-cigarettes are not as harmful as conventional ones. Still, because e-cigarettes are relatively new to the market, precisely how harmful they are remains unclear.

That said, we are learning more and more about the potential dangers of e-cigarette use, or "vaping."

In 2017, a study linked the flavorings in e-cigarette liquid to impaired heart muscle function. A more recent report claims that there is "substantial evidence" that e-cigarette use increases heart rate, and that some chemicals in the vapor can harm DNA.

The study by Tang and team provides further evidence of the harms of e-cigarettes, after finding that exposure to e-cigarette vapor can damage DNA in a way that could cause cancer and heart disease.

E-cigarette vapor caused DNA damage

The researchers came to their findings by exposing 10 male mice to e-cigarette vapor—containing 10 milligrams of nicotine, which is comparable with what humans inhale—for 3 hours per day, 5 days per week, for 12 weeks.

When compared with a control group of mice that inhaled filtered air, they found that the mice exposed to e-cigarette vapor showed DNA damage in their heart, lungs, and bladder.

What's more, the scientists found that exposure to e-cigarette vapor halted DNA repair processes in the lung tissue of mice. They found the same outcomes when testing long-term exposure to e-cigarette vapor on human lung and bladder cells.

In conclusion to their findings, Tang and colleagues write:

"Based on these results, we propose that ECS [e-cigarette smoke] is carcinogenic and that e-cig smokers have a higher risk than non-smokers to develop lung and bladder cancer and heart diseases."

Critics of the research, however, claim that this study does not prove that e-cigarettes are harmful to health.

Talking to The Guardian, Peter Hajek—the director of the Tobacco Dependence Research Unit at Queen Mary University of London in the United Kingdom—says that the "study shows nothing at all about the dangers of vaping."

He continues, saying, "It doesn't show that vaping causes cancer. This is one in a long line of false alarms, which may be putting people off the switch from smoking to vaping which would undoubtedly be of great benefit to them."

It seems that the jury is out on whether or not vaping can cause cancer and heart disease, but one thing is clear: more studies are needed to find out.