New US research has revealed another potential health risk of e-cigarettes, finding that the chemicals used to flavor e-cigarette liquids are toxic to white blood cells.

Also known as e-cigs, e-cigarettes work by heating a liquid to turn it into vapor which users then inhale, or 'vape.' These liquids are often flavored with chemicals that are considered safe to eat, therefore some believe e-cigarettes are a healthier alternative to traditional cigarettes, with many smokers using them when they attempt to stop smoking.

However, despite many recent studies the health effects of inhaling these chemicals are not yet well understood.

The new study, led by researchers at the University of Rochester Medical Centre, wanted to test the assumption that nicotine-free flavored e-liquids are safer than smoking tobacco cigarettes, looking at what effect e-cigs might have on the immune system.

To do this the researchers directly exposed a type of white blood cell called monocytes, which help the body fight infection, to e-liquids.

They found that e-cigarette flavoring chemicals and liquids can cause significant inflammation to monocytes, with many of the flavoring chemicals also causing significant cell death. Some flavors were found to be more harmful than others, with cinnamon, vanilla, and buttery flavors among the worst.

The researchers also found that mixing e-cigarette flavors has a much worse effect than exposure to just one flavor and caused the most toxicity to white blood cells.

The study's first author, Dr Thivanka Muthumalage, commented on the findings, saying that although these flavoring compounds may be safe for ingestion, the results show they are not safe for inhalation and add to a growing body of evidence suggesting that e-cigarettes are harmful to health.

Previous research has also found that the flavors used in e-cigarettes cause inflammatory and oxidative stress responses in lung cells.

Senior author Dr Irfan Rahman expressed concern: "Our scientific findings show that e-liquid flavors can, and should, be regulated and that e-juice bottles must have a descriptive listing of all ingredients. We urge regulatory agencies to act to protect public health," he said, also warning that, "alluring flavor names, such as candy, cake, cinnamon roll and mystery mix, attract young vapers."

The team are now planning further research and are calling for further long-term human studies to understand better the harmful effects of e-cigarettes.

The findings can be found published online in the journal Frontiers in Physiology.