OMAHA, Neb. - The nation's four biggest makers of microwave popcorn have removed from nearly all their products a flavoring chemical that has been linked to a lung ailment in popcorn-plant workers.

The companies say all their microwave-popcorn recipes should be changed by January.

But it might take several months for the reformulated popcorn to replace all the older varieties on store shelves.

In August, the Weaver Popcorn Co. of Indianapolis announced it had removed the butter flavor diacetyl from all its microwave-popcorn varieties.

ConAgra Foods Inc., of Omaha, General Mills Inc., of Golden Valley, Minn., and the American Pop Corn Co., of Sioux City, Iowa, all promised in September to change their microwave-popcorn recipes. Those three companies sell Orville Redenbacher, Act II, Pop Secret and Jolly Time microwave popcorn.

"We want to assure our consumers they can continue to enjoy their favorite popcorn with complete confidence," said Con-Agra's Stan Jacot, who oversees popcorn marketing for the company.

The chemical diacetyl has been linked to cases of bronchiolitis obliterans, a rare life-threatening disease often called popcorn lung.

Diacetyl occurs naturally in foods such as butter, cheese and fruits, and the FDA has approved its use as a flavor ingredient.

The Occupational Safety and Health Administration has stepped up its inspections of microwave-popcorn plants that use the flavoring and a program to minimize or eliminate the workers' exposure to chemical hazards.

Early this year, a pulmonary specialist at Denver's National Jewish Medical and Research Center wrote to federal agencies to say doctors at the center believe that they have the first case of a consumer who developed lung disease from the fumes of microwaving popcorn several times a day for years.

But generally popcorn lung has been associated with people who worked in microwave-popcorn plants mixing large vats of flavors. Hundreds of workers have said they have severe lung disease or other respiratory illnesses from inhaling diacetyl vapors.

More than 500 lawsuits are pending against the companies that produce or use the butter flavoring. About $50 million has been awarded in verdicts that were later settled for confidential amounts. Another 100 cases have been settled that reportedly involve tens of millions of dollars.

The Washington, D.C.-based Flavor and Extract Manufacturers Association has said consumers shouldn't worry about eating microwave popcorn as long as they follow directions, which typically include a warning to open bags of popcorn away from the face.

The food companies have not pinpointed exactly when the new popcorn will be available at all stores because the timetable depends on how much popcorn a store sells and how much of the product stores keep in their own warehouses. *