U.S. Capitol Police responded about 9:40 a.m. yesterday to a call about an injured person and noxious fumes in the Dirksen Senate Office Building's north cafeteria. A hazardous-materials team, an advanced life-support unit, doctors, four teams of paramedics and D.C. Fire Chief Donald Edwards soon followed. First Street NE was closed to traffic between Constitution Avenue and C Street.

About 50 people from the cafeteria were evacuated, and nine cafeteria workers were sent to the hospital, officials said. Congressional staff from some of the building's basement and first-floor offices also converged on the street.

Could a bag of onions be the root of all this commotion?

Food workers and servers who were preparing breakfast reported the fumes.

"Everyone was getting sick -- headaches, stomachaches, vomiting, diarrhea," said a cafeteria cashier who asked not to be identified.

The fallen cafeteria employees, all women, were taken to George Washington University Hospital, where they were treated and released, said Amy Pianalto, a hospital spokeswoman. The affected part of the building was cleared for reentry at 11:20 a.m., and the cafeteria was reopened an hour later.

One of the workers suffered head and neck injuries when she collapsed, and the eight others reported nausea, said Battalion Chief Stephen M. Reid, a spokesman for the fire department.

"I threw up. I had a headache. I felt very lightheaded, nauseated," said Anna Lee, one of the workers, who went home after leaving the hospital. Lee said she had been frying eggs on the grill when she smelled what seemed like gas, "like if you turn the stove on."

The fire department said the source of the noxious odors was a bag of onions.

"The haz-mat unit went down, and all the readings were negative," Reid said. "What they found was a bag of onions . . . and they just gave off a strong odor."

That explanation, however, was questioned by the office of the architect of the Capitol, the cafeteria workers themselves, and onion experts.

"We're still trying to investigate the source of the odor," said Herb Franklin, administrative assistant to Alan M. Hantman, the architect of the Capitol. Franklin said he couldn't offer any other explanation for the odors. Upgrade work on the building systems is taking place far from the cafeteria area, he said.

Irwin L. Goldman, an associate professor of horticulture at the University of Wisconsin at Madison who studies onions, said "there are toxic effects of onion and garlic and other members of that family, but you have to ingest them" to get sick.

"The sulfur compounds that are unique to onions and garlic, they do stay in the air," Goldman added. "It certainly is possible people would be able to smell them from cut onions. But whether they would be able to make people nauseous, I couldn't guess."

Lee, the cafeteria cook, said she is certain that the source of the odor that nauseated her is still to be determined.

"It wasn't a food smell," she said. "Onions do not smell like that."

Police said there were few other explanations.

"There was no gas supply there, and there was no indication of any combustible gases in the air," said Deputy Chief James P. Rohan of the Capitol Police, adding that the kitchen uses electric and steam grills.

Cafeteria workers who saw the plastic bag said it contained 15 to 20 peeled and sliced onions, which were to be placed on the cafeteria salad bar.