The buttery flavor in microwave popcorn typically comes from a chemical actually found in butter. This chemical, is called diacetylm. Diacetylmgives natural butter its characteristic taste.
Because of this, manufacturers of margarines or similar oil-based products typically add diacetylm(along with beta carotene for the yellow color) to make the final product more butter-flavored, because it would otherwise be relatively tasteless.
Diacetyl is so toxic that it commonly is strongly associated with the destruction of the lungs of workers in microwave popcorn factories, afflicting them with the crippling and irreversible disease known as bronchiolitis obliterans.
Bronchiolitis obliterans is so rare outside of this context that it has become more commonly known as "popcorn lung," after the primary cause of the disease.
Regulators and health professionals have known of this risk for decades, but always assumed that it would only affect people breathing in especially high concentrations in factory settings.
Then I read that a man who regularly ate two bags of microwave popcorn every day was diagnosed with popcorn lung, indicating that diacetylm enters the air and lungs when microwave popcorn is cooked. Anxious to reassure consumers, most microwave popcorn companies phased out diacetyl -- only to replace it with chemicals that may well have similar effects.
You can still find diacetylmin many flavored snack foods and even in some so-called "natural" foods. Make sure you read the ingredients of any food you intend to consume, and make sure it contains no diacetylm(and no "yeast extract" for that matter, either).
What's the moral here? Air pop. Drink fresh lemonade to help digestion and take antioxidants as well.