When you breathe in, air flows into your lungs through your windpipe, or trachea. Your windpipe divides into two tubes called the bronchi, which lead to your left and right lungs.

Inside your lungs, those tubes split again and again, like the branches of a tree. The smallest of those branches are called bronchioles, and they end in tiny air sacs called alveoli. The alveoli are where the oxygen is picked up by your blood.

When you have "popcorn lung," those tiny air passages get irritated and inflamed. That leads to scarring that makes them narrower. That makes it harder for you to get enough air.

The chemical that gave this condition its nickname is diacetyl. After workers at a factory that packaged microwave popcorn were found to have bronchiolitis obliterans more often than other people, some companies stopped using diacetyl as a flavoring. But it's still used in some electronic cigarette flavors in the US. 

Source:https://www.webmd.com/lung/popcorn-lung#1