Dr. Cecile Rose, a pulmonary specialist at Denver's National Jewish Medical and Research Center has issued a warning letter stating butter flavored microwave popcorn may lead to lung disease. It appears the possible culprit is a butter flavor ingredient known as diacetyl.
According to the Occupational Safety and Health Administration (OSHA) website, "One of four rats died after a 6-hour exposure to vapors from heated (55°C) artificial butter flavoring containing 285 ppm diacetyl (~64% of total VOCs)."
Several people who work in popcorn plants have developed a rare lung disease called bronchiolitis obliterans. This disease is referred to as "popcorn packer's lung" and has been linked to the inhalation of butter flavoring vapors. In some instances, the disease has become so severe that some individuals are now awaiting a lung transplant.
Diacetyl is produced by yeast during fermentation and occurs naturally in beer, butter, coffee, vinegar and other food products. It is used as an artificial flavoring in butter, butterscotch, processed cheese, cottage cheese, sour cream, salad dressing, non- and low-fat dairy products, syrups, icings, marinades, baked goods, some fast foods, snack foods, and microwave popcorn.