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

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 Company of Indianapolis announced it had removed the butter flavor diacetyl from all its microwave popcorn varieties.

ConAgra Foods of Omaha, General Mills of Golden Valley, Minn., and the American Pop Corn Company of Sioux City, Iowa, all promised in September to change their microwave popcorn recipes.