Fairfax County fire officials last night ordered a U.S. Commerce Department building in the Annandale area closed indefinitely because of contamination with the carcinogenic chemical PCB. The contamination apparently was caused by a small fire at the building Tuesday night.

The extent of the contamination at the National Technical Information Service building was unknown, but the level of PCB in a utility room where the fire broke out was measured at 1,900 times the level considered permissible by the Environmental Protection Agency, a Fairfax fire and rescue spokesman said.

If the contamination of the two-story brick building, which measures about 300 feet by 100 feet, is found to be extensive, its destruction might be indicated if efforts to decontaminate it through scrubbing proves to be ineffectual, said Lt. Michael Reilly, the fire and rescue spokesman.

The building, in the 5200 block of Port Royal Road, was evacuated after firefighters who went to extinguish the blaze in a utility room about 11 p.m. Tuesday found a sign warning of the presence of PCB (polychlorinated biphenyl) in the electrical transformers that were on fire.

The chemical once was widely used as insulating material in electrical equipment such as transformers. The manufacture of PCB was banned in 1977 because it was found to cause cancer and birth defects in laboratory animals.

The fire was in transformers of about "1972 vintage" that supplied electricity to computers, Reilly said.

He said that about 50 employees and firefighters were decontaminated -- a process involving "vigorous scrubbing and a hot shower for at least 15 minutes" -- and the employees were sent home.

Members of the county's hazardous materials unit, wearing protective suits, used swabs to take samples from the walls and equipment in the utility room, and analysis at a laboratory showed the presence of 950,000 parts of PCBs per million, Reilly said. He said the limit allowed by the EPA is 500 parts per million.

The Commerce Department employees assigned to the building will work at another facility the agency owns in Fairfax County until the one on Port Royal Road is safe or other arrangements are made, an official said last night.

Tom Collamore, assistant Commerce secretary for administration, said the National Technical Information Service is a document clearinghouse for the federal government and sells publications, especially scientific journals, that are produced by other federal agencies.