Diacetyl, or 2,3-butanedione, is a chemical used to flavor and color food and other products. It's often used in dairy flavorings like butter or cheese and in brown flavors like caramel or butterscotch. It's known in particular for its use in the butter flavoring of microwave popcorn, and because of the severe lung disease that some workers experience from inhaling diacetyl on the job.
In May 2000, eight people who had worked at a plant in Missouri that manufactured microwave popcorn developed bronchiolitis obliterans, a rare interstitial lung disease. An interstitial lung disease is one in which the lungs become scarred, or fibrosed. In bronchiolitis obliterans, the lung's smallest airways, the bronchioles, become damaged and inflamed ( in this case, by the particles of diacetyl). Because this condition was first associated with diacetyl's use in popcorn flavoring, it has become known as "popcorn lung" or "popcorn worker's disease."
Diacetyl affects mainly the lungs, but it can also affect your eyes, nose, throat and skin. Symptoms can start gradually and worsen over time, or happen suddenly and be very severe. Exposure to diacetyl can cause your eyes to sting, and your nose and throat to burn. If the chemical comes in contact with your eyes, it can cause a chemical burn needing immediate medical attention. Diacetyl can also irritate your skin and cause a rash. Typical respiratory symptoms include a persistent cough and breathlessness when exerting yourself, and wheezing even when you don't have a cold. The symptoms generally don't resolve, even when you're no longer exposed to the vapors or spray. The damage to your lungs can be permanent, and in extreme situations, may require a lung transplant or even lead to death.