Diacetyl, the substance used to impart a buttery taste to the movie-time snack, made headlines last summer when a Colorado man contracted "popcorn lung"—a.k.a. bronchiolitis obliterans—a severe respiratory disease linked to breathing large amounts of the stuff. (The condition has been a problem for workers in popcorn manufacturing plants.) Although dozens of foods from Twinkies to red wine contain diacetyl, it is harmless when eaten. The rub is that when heated to high temperatures, like those used to cook microwave popcorn, diacetyl vaporizes and becomes toxic.
As with most toxins, the devil's in the dosage. Reportedly, the man ate an average of two bags of extra-butter-flavored microwave popcorn daily for more than ten years and (here's the kicker) hovered over the steaming-hot pouch to fully enjoy its buttery bouquet. Cecile Rose, MD, director of the occupational medicine program at the National Jewish Medical and Research Center in Denver, Colorado, treated the man. "It is just a single case," she says, "but I wouldn't dismiss this concern."
Manufacturers have taken that concern to heart: The country's largest makers of microwave popcorn—ConAgra, General Mills, American Pop Corn Company, and Pop Weaver— no longer use diacetyl in their products, which include Act II, Orville Redenbacher, Pop Secret, Pop Weaver, and Jolly Time.
As for the remaining chemicals in microwave popcorn, you can play it safe by following a few steps outlined by David Michaels, PhD, director of the Project on Scientific Knowledge and Public Policy at the George Washington University School of Public Health and Health Services, an expert on microwave popcorn fumes: Wait until the bag is cool to the touch before opening (the vapor will condense), and open the bag under a stovetop exhaust fan.
The best fix, says Michaels, is to make your popcorn the old-fashioned way: on the stovetop.