A malfunctioning electrical transformer exploded yesterday at the Commerce Department, spewing a cancer-causing liquid that required the decontamination of nearly 50 people. The accident sent thousands of federal workers home and snarled traffic for much of the day.

The explosion occurred at 6:15 a.m. in the basement of the 1.6 million-square-foot structure and was heard by two maintenance engineers working on the building elevators, according to D.C. Fire Department Capt. Craig Dwyer. The workers doused the fire with a dry chemical and then pulled the fire alarm. The fire was contained to the basement and was extinguished within 15 minutes.

When emergency crews arrived, they thought it was a "simple fire," Dwyer said. But after a worker cleaning up the scene had difficulty breathing and building workers told fire officials that polychlorinated biphenyls, or PCBs, were in the electrical equipment, officials realized it was more.

"That's when it went from a structural fire to a hazardous material fire," Fire Chief Donald Edwards said. The fire department's hazardous materials unit was told that PCBs had been emitted, according to Edwards.

Symptoms of exposure to PCBs, which were used commonly as an insulating oil until they were banned in new equipment more than a decade ago, include upper respiratory problems, eyelid swelling and skin irritation.

Emergency crews from Arlington and Prince George's counties were summoned, along with Montgomery County's hazardous materials unit, which performed the initial air testing. The unit has a special kit to do the testing, Edwards said.

The building was evacuated immediately, and those believed to have been exposed to the chemical were allowed to shower and were given new clothes in the courtyard of the center of the Commerce Building, officials said.

Forty-eight people, including 10 firefighters, were taken to George Washington University Hospital for decontamination. The hospital, which has the largest decontamination unit in the District, closed its emergency room to other cases to handle the crisis.

Anthony Macintyre, an emergency physician at the hospital, said it was hard to tell how many people actually suffered PCB contamination, but he believed two were exposed, one of whom was held for observation and further testing. "There is no test for PCB exposure," Macintyre said at a news conference. "You can only test things such as liver function."

Macintyre said the victims probably suffered more damage from smoke inhalation than from PCB exposure. The man held for observation was suffering from dizziness, shortness of breath and headaches. Doctors were going to test his hemoglobin for indications of exposure. The man was in stable condition, Macintyre said. Acute PCB contamination can cause dizziness and liver damage, but it would take a "very high dosage," he said.

By midafternoon, those taken to the hospital had been released. One of the men checked for exposure had his blood drawn, performed dexterity tests and had his eyes examined.

"Ten years, and this is the first time I've been exposed," said D.C. firefighter Rudolph Blythe. "You don't know if it's harmed you. It could take five or 10 years. I'm going to think about that."

The fire forced delays in the releases of a monthly report on income and consumer spending for August, which had been set for 8 a.m., and of an August report on construction spending, which had been set for 9:30 a.m. Because of the building's closure, the reports were instead given to reporters on the street and issued via the department's Web site shortly after the normal deadline.

The Commerce building will remain closed at least through the weekend, until the walls and air can be tested for contamination, according to Deputy Commerce Secretary Robert Mallett. Environmental teams from the General Services Administration, which is responsible for building maintenance, will do the testing to determine the level of contamination in the rest of the building. The building also houses the National Aquarium, and officials said the fish would be tested for contamination.

Generally, the equipment owner is responsible for the cleanup, fire officials said. Pepco owns the transformer, fire officials said. Nancy Moses, spokeswoman for Pepco, said the transformer equipment belonged to Commerce and, therefore, the cleanup will be a federal responsibility.

Meanwhile, D.C. police spent much of the day untangling traffic near the building. Fourteenth Street NW between Constitution Avenue and Pennsylvania Avenue was closed to morning rush-hour traffic but was later reopened, according to Sgt. Joe Gentile, D.C. police spokesman.

The police department was called again about 10:30 a.m. to shut down 14th Street between Constitution Avenue and Pennsylvania Avenue and E Street, Gentile said. The street reopened at 2:45 p.m.

The Associated Press contributed to this report.