It took two years on the job and a chemical in something as ordinary as butter flavoring to turn a strapping factory worker into someone who sleeps tethered to an oxygen tank.

Francisco Herrera, 32, suffers from an aggressive disease that has destroyed 70 percent of his lungs and could kill him if he doesn't get a transplant. A physician diagnosed bronchiolitis obliterans after the flavoring plant worker became ill in 2003, concluding that the disease was caused by exposure to diacetyl.

This is the same chemical that federal scientists had determined the year before was toxic when vaporized and inhaled by workers in plants that produce microwave popcorn. Herrera's case is part of growing evidence, scientists say, of health hazards from diacetyl elsewhere in the food industry.

"Everyone knew that the diacetyl was harmful," said Herrera, a father of two who contends in a lawsuit that his employer never warned him of inhalation hazards. "But why didn't anyone tell the workers handling it?"

Diacetyl, which is found naturally in many foods, is artificially produced and widely used as a less expensive way to enhance flavor or impart the taste of butter. Thousands of workers in plants across the country make flavorings containing diacetyl and other chemicals or use flavorings to make products such as pastries, frozen foods and candies.

Consumers who prepare or eat them are not at risk, doctors say, because they are unlikely to experience the chemical concentration found in a workplace.

The National Institute of Occupational Safety and Health linked exposure to diacetyl and butter flavoring to lung disease that sickened nearly 200 workers at popcorn plants and killed at least three. Now investigators at NIOSH say the disease has been identified in more than two dozen workers from other parts of the food industry.

"Now we've got cases of bronchiolitis obliterans among workers in other plants that use flavorings and in plants that make the flavorings," said Dr. Kathleen Kreiss, chief of the field studies branch of NIOSH's division of respiratory disease studies.

"We need to get into some of these plants because we don't have confidence that the flavoring industry has taken steps to actually prevent this disease, and we need to determine how widespread the exposure may be."

Scientists at NIOSH and the Occupational Safety and Health Administration want to intensify investigations into illness caused by flavorings and issue federal regulations to protect workers. But top officials say they don't plan to act because in their view what is being done now is enough.

That response among agency heads is one example of how government has largely allowed the flavoring industry to police itself. The Food and Drug Administration has let flavoring producers and sellers decide which chemicals are safe, and California's occupational safety agency, Cal/OSHA, has delegated health examinations of flavoring workers to an industry-paid doctor.

Government physicians question the propriety of relinquishing the health evaluations to doctors paid by the plants.

"It needs to be made clear to everyone involved that accepted public health practices do not allow us to rely solely on medical conclusions obtained by industry or its paid consultants," said Dr. Robert Harrison, chief of occupational surveillance and epidemiology for California's Department of Health Services. "This practice would be unfair to the worker and contrary to the process under which we work."

The difficulty of assessing workplace illness is further complicated by employees who fear reprisal for complaining about hazards to anyone and by physicians who lack the training to recognize bronchiolitis obliterans and other occupational threats.

The safety of diacetyl, as well as many of the 2,000 chemicals blended to make flavorings, has never been tested by the government. The FDA classified them among substances "Generally Regarded As Safe." It took the word of a panel of scientists hired by the Flavor and Extract Manufacturing Association. Diacetyl was declared safe decades ago because the industry said it was safe, according to a spokesman for the FDA.

About 70 U.S. companies are involved in the making and sales of flavorings, according to the association, which is the largest trade group for the $3 billion-a-year industry. Of more than 8,000 employees, only about 3,000 are engaged in the actual production of flavorings. In the much larger food processing industry, tens of thousands of workers are estimated to work with flavorings.

Their wellbeing falls to physicians, scientists and industrial hygienists trained in occupational medicine, which is the study of workplace hazards - chemical and otherwise. They are the ones who have linked lung disease to exposure to flavorings.

And it is these specialists, both government and civilian, who are pressing OSHA and NIOSH to widen the investigation into worker safety in the flavoring industry.

Outbreak of rare illness