Popcorn lung, also known as bronchiolitis obliterans or obliterative bronchiolitis, is a serious respiratory condition that results in damage to the lungs and can cause difficulty breathing and coughing. The main chemical responsible for the name "popcorn lung" is diacetyl, which is used as a flavoring in microwave popcorn and other food products. However, popcorn lung can also be caused by other lung conditions.

When a person inhales, air enters the lungs through the trachea (windpipe) and is divided into two bronchi, which lead to the right and left lungs. These tubes then branch off into smaller and smaller tubes, eventually ending in tiny air sacs called alveoli. It is in these alveoli that the blood picks up oxygen. In individuals with popcorn lung, the alveoli become inflamed and scarred, causing the passages within to narrow. This narrowing makes it harder for the person to get enough air and can lead to symptoms such as shortness of breath, wheezing, a dry cough, and fatigue.

Despite rumors to the contrary, there is no evidence that vaping causes popcorn lung. In fact, all e-liquids in the UK do not contain diacetyl, as it is a banned substance and each liquid that is laboratory tested will fail if diacetyl is detected. Cigarette smoke, on the other hand, contains much higher levels of diacetyl, yet smoking is not linked to popcorn lung.

In 2016, some articles claimed that a study had found a connection between e-cigarettes and popcorn lung. However, the study in question, a 2015 Harvard study, did not conclude that vaping caused popcorn lung. It merely noted the presence of diacetyl in some e-liquids (overseas), which was already known by e-cigarette vendors, and recommended further research. There have been no confirmed cases of popcorn lung associated with vaping. It is clear that vaping is a safer alternative to smoking.