“Popcorn lung” is an irreversible scarring of the small airways in the lung. Popcorn lung is caused by chemicals some manufactures add to their microwave popcorn to give it a buttery taste. The chemical is diacetyl, which is a natural by-product found in cheese, butter, yogurt and wine. Damage from this chemical happens when large amounts are inhaled. Although the majority of victims of popcorn lung work in popcorn or flavoring manufacturing facilities, several consumers claim to have popcorn lung.
Several manufacturers have reduced or replaced the chemical, but there is no way of knowing whether your favorite microwave popcorn contains diacetyl. If you want to lower potential risk of inhaling it, allow the bag to cool before you open it, and use your kitchen exhaust. Source: Nutrition Action Healthletter, March 2013