I didn't know about that!
This is what I found on Wikipedia,
"The United States National Institute for Occupational Safety and Health has suggested that diacetyl, when used in artificial butter flavoring (as used in microwave popcorn), may be hazardous when heated and inhaled over a long period. Workers in several factories that manufacture artificial butter flavoring have been diagnosed with bronchiolitis obliterans, a rare and serious disease of the lungs. After the workers filed a lawsuit against the manufacturers, the Environmental Protection Agency began an investigation into the chemical properties of microwave popcorn butter flavoring. On July 19, 2005, jurors awarded a popcorn plant worker in Missouri $2.7 million for his claim of diacetyl-induced respiratory problems."
http://en.wikipedia.org/wiki/Diacetyl