Many smokers harbor the illusion that smoking e-cigarettes is less harmful than regular ones, but a recent study by researchers at Harvard University‘s T.H. Chan School of Public Health has raised new concerns about this form of ‘safe-smoking’ — proving that there’s actually no such thing.

According to the study, ‘diacetyl’, a chemical flavoring in e-cigarettes, can lead to the development of an irreversible, life-threatening lung disease called ‘popcorn lung‘. The name comes from where the illness was first detected — among workers at microwave-popcorn factories, who got it after inhaling fumes of artificial butter flavoring in the product.

The disease causes the airways in the lungs to become narrower and weaker, causing scarring and respiratory problems that become worse as it progresses.

The researchers at Harvard found ‘diacetyl’ in 39 out of 51 flavors in e-cigarettes. Although the chemical is considered a ‘safe’ additive to food, it can cause respiratory damage when inhaled regularly, as with e-smoking.

The symptoms of ‘popcorn lung’ are very similar to other problems smokers suffer, such as loss of weight, night sweats, asthma, and ear-nose-throat inflammation, so it’s difficult to detect before too much damage has already been done. The safest bet, therefore, is to steer clear of the deadly e-cigarette.