A chemical used to give butter flavor to popcorn can damage the lungs and airways of mice, U.S. government experts reported on Thursday.

Tests on mice show that diacetyl, a component of artificial butter flavoring, can cause a condition known as lymphocytic bronchiolitis, said the team at the National Institute of Environmental Health Sciences, part of the National Institutes of Health.

The condition can lead to obliterative bronchiolitis -- or "popcorn lung" -- a rare and debilitating disease seen in workers at microwave popcorn packaging plants and at least one consumer.

At least two microwave popcorn makers -- ConAgra Foods Inc and Weaver Popcorn Co Inc -- have said recently they would stop using diacetyl.

Laboratory mice made to inhale diacetyl vapors for three months developed lymphocytic bronchiolitis, the NIEHS team said.

"This is one of the first studies to evaluate the respiratory toxicity of diacetyl at levels relevant to human health," Daniel Morgan at NIEHS, whose team led the study, said in a statement.

Writing in the journal Toxicological Sciences, the researchers said findings suggest that workplace exposure to diacetyl contributes to the development of obliterative bronchiolitis.

The hard-to-treat condition causes vague symptoms such as cough and shortness of breath, and steadily worsens, according to the U.S. Centers for Disease Control and Prevention.

Congress has been working on a bill to order quick action by the Occupational Safety and Health Administration to limit exposure to diacetyl. The House of Representatives passed a bill last year but the Senate has not acted.

The Food and Drug Administration said last September it was investigating a report of a man who came down with the life-threatening disease after eating several bags of butter-flavored microwave popcorn each day.