WASHINGTON — The Environmental Protection Agency has been withholding the results of a 2003 study that may indicate potential health risks from inhaling artificial butter flavor vapors from microwave popcorn, a group of scientists and former Occupational Safety and Health Administration officials say.

Some workers at microwave popcorn factories have been diagnosed with a rare and fatal lung disease — dubbed "popcorn workers' lung" — and several of them are on lung transplant lists.

The disease, bronchiolitis obliterans, is a serious and irreversible inflammation of the airways leading to the lungs. Studies in toxicology journals have linked diacetyl — a common food flavoring and main ingredient of artificial butter flavoring — to disease in laboratory animals.

It is not clear how many workers have been affected, but thousands of workers have been exposed to artificial butter vapors containing diacetyl and at least one employee has died.

"This is a tragic example of the failure of the public health regulatory system," said David Michaels, associate chairman of the department of environmental and occupational health at George Washington University.

Michaels said it is unclear whether everyday consumption of microwave popcorn or foods that contain artificial butter flavoring could produce health risks. He said because the EPA has not released its study findings, it is unknown what additional risks there might be.

"The public needs to be reassured that it is safe," Michaels said.

Former officials from OSHA and NIOSH and another group of occupational health experts have signed a letter to Secretary of Labor Elaine Chao asking for OSHA to issue a federal standard for the food industry that regulates what levels of diacetyl plant workers can be exposed to. The United Food and Commercial Workers International Union and the International Brotherhood of Teamsters also signed the petition.

The Food and Drug Administration only tests artificial flavorings based on ingestion safety, not inhalation safety. In 2003, the EPA completed research on chemical emissions from popping and opening microwave popcorn, and diacetyl was one of several compounds it tested for.

But the EPA will not release the study's findings until an internal review is completed, which may be submitted for publication in the fall, said EPA spokeswoman Suzanne Ackerman.

Case studies have, however, already shown linkages between exposure to diacetyl and disease. In 2000, the National Institute for Occupational Safety and Health, the occupational safety division of the Centers for Disease Control and Prevention, began to investigate the chemical and, in 2004, released a report of workers developing lung disease at a Missouri microwave popcorn factory.

OSHA has since distributed workplace safety guidelines specific to microwave popcorn factories — such as having workers wear respirators and proper ventilation — and discussed the matter with popcorn manufacturers. Dr. Allen Parmet, who diagnosed the first cases of popcorn workers' lung, said bronchiolitis obliterans is simply the "tip of the iceberg." Parmet is an occupational safety doctor who has visited many work sites where artificial butter flavoring is used.

"At any place where the butter flavors are vaporized, we are seeing an excess of lung disease" that is "way above anything you would expect in the general population," said Parmet, adding that he has seen conditions in factory workers resembling mild asthma or mild emphysema.

"As a practicing occupational doctor, I don't understand why nothing has been done, because we know how to stop it," Parmet said. "There is no reason for another person to get sick."

Popcorn workers' lung was first identified in workers at a Jasper, Mo., popcorn plant run by the Gilster-Mary Lee Corp. in 1999. Parmet notified the Missouri Department of Health and the CDC, and in 2000, NIOSH investigated why eight employees had developed the condition.

The scientists concluded that workers' respiratory ailments were linked to the prolonged inhalation of diacetyl in butter flavoring.

The employees included those who worked in quality control, opening bags of buttered microwave popcorn. A study found that the plant workers had 3.3 times the national rate of pulmonary disease for smokers and 10.8 times the national rate for non-smokers.

E-mail: kkiang@coxnews.com.