Conagra has removed diactyl, a chemical used for butter flavoring in microwave popcorn, from its Act II and Orville Redenbacher brands of microwave popcorn.  Scientests linked the chemical to a severe lung disease, bronchiolitis obliterans, in 2001.  The chemical caused three deaths and caused hundreds of cases of severe lung disease in popcorn plant workers who were exposed to the chemical in a dust form. 

While disease related to the chemical has been limited to popcorn plant workers, according to an article in the Milwaukee Journal Online, doctors found health problems similar to those in the plant workers in a man who ate 2 bags of microwave popcorn everyday.