The Center for Disease Control has identified seven cases of the potentially deadly lung disease known as "popcorn workers lung" at four California flavor factories.

Caused by the food flavorings used in microwave popcorn, the CDC maintains that there is no known risk to consumers who eat microwave popcorn products.

The disease, bronchiolitis obliterans, is a rare and life-threatening form of fixed obstructive lung disease, says the CDC. Obstructive lung disease makes it difficult for air to flow out of the lungs. In fixed obstructive lung disease, this difficulty is not reversible.

"Food flavorings are designated 'generally recognized as safe' when approved by the U.S. Food and Drug Administration; flavorings are not known to put consumers at risk for lung disease," states the CDC.

The CDC describes the seven cases, which occurred between 2002 and 2006, in its Morbidity and Mortality Weekly Report.

Six of the seven affected workers handled chemicals including diacetyl when mixing flavorings. The seventh worker packaged powdered flavorings.

Insufficient ventilation at the factories and inadequate paper dust masks worn by six of the seven workers may have been an issue, according to the CDC's report.

Studies have shown that diacetyl is a lung hazard in animals. No limits have been set for safe occupational exposure to diacetyl and many other flavoring chemicals, notes the CDC.

The seven affected workers were all lifelong nonsmokers. One had had asthma as a child.

Their symptoms included cough, wheezing, or shortness of breath on exertion. Those symptoms began between one month and five years of working at the flavor factories.

The risk of occupational lung disease has previously been identified in the microwave popcorn industry, which has made improvements to address the problem, says the CDC.

The same occupational risk hasn't been established in other food-flavoring production workplaces, according to the CDC.