Exposure to synthetic butter in food production and flavoring plants has been linked to hundreds of cases of workers whose lungs have been damaged or destroyed. Diacetyl is found naturally in milk, cheese, butter and other products.

Heated diacetyl becomes a vapor and, when inhaled over a long period of time, seems to lead the small airways in the lungs to become swollen and scarred. Sufferers can breathe in deeply, but they have difficulty exhaling. The severe form of the disease is called bronchiolitis obliterans or “popcorn workers’ lung,” which can be fatal.

Dr. Cecile Rose, director of the occupational disease clinical programs at National Jewish Medical and Research Center in Denver, said that she first saw the Colorado man in February after another doctor could not figure out what was causing his distress. Dr. Rose described the case in a recent letter to government agencies.

A furniture salesman, the man was becoming increasingly short of breath. He had never smoked and was overweight. His illness had been diagnosed as hypersensitivity pneumonitis, an inflammation of the lungs usually caused by chronic exposure to bacteria, mold or dust. Farmers and bird enthusiasts are frequent sufferers.

But nothing in the Colorado man’s history suggested that he was breathing in excessive amounts of mold or bird droppings, Dr. Rose said. She has consulted to flavorings manufacturers for years about “popcorn workers’ lung,” and said that something about the man’s tests appeared similar to those of the workers.

“I said to him, ‘This is a very weird question, but bear with me. But are you around a lot of popcorn?’ ” Dr. Rose asked. “His jaw dropped and he said, ‘How could you possibly know that about me? I am Mr. Popcorn. I love popcorn.’ ”

The man told Dr. Rose that he had eaten microwave popcorn at least twice a day for more than 10 years.