The manufacturer of Orville Redenbacher and Act II microwave popcorn plans to remove a flavouring chemical linked to a rare lung ailment.

ConAgra Foods Inc., the largest microwave popcorn maker in the United States, says it will be remove from its products the chemical diacetyl, which helps give microwave popcorn a buttery flavour, over the next year.

A ConAgra spokeswoman says the company decided to remove the flavouring from its popcorn because of the risk the chemical presents to workers who handle large quantities.

Diacetyl has been linked to an ailment called bronchiolitis obliterans, or "popcorn workers lung." It's a life-threatening form of obstructive lung disease whose only cure is a lung transplant. Sufferers can breathe in deeply but have difficulty exhaling.

Numerous studies in recent year have shown links between the disease and diacetyl. One flavouring manufacturer worker has died of the disease. Flavouring manufacturers in the U.S. have paid out more than US$100 million to settle lawsuits by people sick with popcorn workers lung over the past five years.

Last week, another U.S. popcorn manufacturer, Weaver Popcorn Co., said it would replace the butter flavouring ingredient.

This week, Denver doctors said they may have recorded the first case of the ailment in a consumer. The afflicted patient reportedly popped several bags of extra butter flavoured popcorn every day for years.

The unidentifed, 53-year-old, non-smoker patient developed progressively worsening symptoms of coughing and shortness of breath. Tests found his ability to exhale was deteriorating, although his condition seemed to stabilize after he quit making microwave popcorn.

"We cannot be sure that this patient's exposure to butter flavored microwave popcorn from daily heavy preparation has caused his lung disease,'' cautioned Dr. Cecile Rose, the pulmonary specialist treating the patient. "However, we have no other plausible explanation.''

Rose has written to federal agencies to alert them of the case and of the potential public health implications.

The Food and Drug Administration is reportedly evaluating Rose's letter and "carefully considering the safety and regulatory issues it raises.''

The occupational safety arm of the Centers for Disease Control and Prevention, the National Institute for Occupational Safety and Health, said it is working on a response to the letter.

The Flavor and Extract Manufacturers Association of the U.S. (FEMA) says it is concerned about the report about Rose's patient. They note that diacetyl has been used as a flavour ingredient for many years and is approved by the FDA.

"Diacetyl is naturally occurring in a wide variety of foods including butter, milk, cheese, fruits, wine and beer and provides a 'buttery' flavor to butter itself and other foods," the group said in a statement Tuesday.

"However, out of an abundance of caution, FEMA recommends that its members who manufacture butter flavors containing diacetyl for use in microwave popcorn consider reducing the diacetyl content of these flavors to the extent possible."