Did you know you can get a condition called "popcorn lung" from breathing the artificial butter flavoring from microwave popcorn? The artificial butter flavoring is a naturally-occurring chemical called diacetyl. Diacetyl causes no problems in the butter, milk, cheese, beer, and wine where it's found, but when vaporized it can cause damage to the bronchioles in the lungs, eventually deteriorating them into the serious irreversible condition called bronchiolitis obliterans. If you nuke a bag of popcorn every now and then, it's not a health concern for you, but workers in the factories producing the butter-flavored popcorn are at risk for lung damage, as are consumers who pop a couple of bags of corn daily. I would guess theater concession stand employees would also fall into this category.
So, what should you do to avoid lung damage from popcorn? You can avoid popping the butter-flavored corn and then add liquid butter flavoring after the corn has popped or if you absolutely love that flavor of corn (like me), then just don't go crazy. Enjoy it a few times a week at most.
Image: Chemical structure of diacetyl. (NEUROtiker)
How Popcorn Pops | "Butter" Means Something Different in Chemistry