Thousands of office workers and shoppers who fled a geyser of natural gas laced with a small amount of oil and the toxic chemical PCB were advised today to scrub themselves, put what they were wearing Tuesday in double plastic bags and await further instructions.

Crews from Pacific Gas & Electric Co. were mopping up an eight-block area from which 30,000 workers and shoppers were evacuated Tuesday after a gas main rupture.

Many people who had parked their cars on streets and in garages in the financial district were stranded until this morning, when most of the area was reopened. Some stayed overnight in downtown hotels, where they disposed of their clothes and telephoned family or friends to bring them some clean attire.

PG&E, which serves 94,000 square miles of northern and central California, is taking the names and telephone numbers of those exposed to the polychlorinated biphensyls that were mixed with the natural gas.

The utility said it will contact those who were exposed to the PCBs when it has determined whether their clothes should be disposed of and, if so, how.

The gas leak occured when a crane operator employed by Santa Fe Pomeroy Construction Co. of Petaluma ruptured a 16-inch gas main at Sacramento and Battery Streets Tuesday afternoon.

Workers from several high-rises, including the Embarcadero Center, descended jammed stairwells, holding their noses and clamping handkerchiefs to their mouths to minimize "the skunk smell," an odor introduced into natural gas to alert people when it leaks.

People fleeing the area were asked not to smoke, and police motorcycles were banned from the area because of concern that sparks from the bikes would ignite the gas.

"We were worried that the air-conditioning systems in Embarcadero Center were sucking in natural gas," said Fire Chief Andy Casper. If pockets of the gas settled in the buildings, "a strike of a match, which momentarily reaches 2,000 degrees Fahrenheit, could have ignited the gas, which can burn at 900 to 1,000 degrees."

Gas valves were not turned off until 2 1/2 hours after the rupture, and not until 4 1/2 hours later, long after firefighters and other workers had rushed to the scene, did PG&E disclose that the oily substance that accompanied the gas, spotting clothing and making sidewalks slick, contained a low level of PCBs. PCB is a nearly indestructible toxic chemical used for industrial insulation.

After analyzing the substance, PG&E reported the level of PCBs in the gas at 25 parts per million, a figure confirmed by the California Occupational Safety and Health Administration.

The Environmental Protection Agency has set 50 parts per million as "the standard for regulation," according to aide JoAnn Semones. Anything below 50 parts per million "does not represent a safe exposure level," she said, adding that there "is not really" any safe level.

Used in the past in gas valves and compressors for insulation and to minimize corrosion, PCB apparently leaked into the gas pipes from which it escaped, PG&E spokesman Paul Girard said.

Two years ago Congress banned the manufacture of PCB, after it was learned that the chemical can cause liver and skin damage, sterility and brain damage, and may be linked to skin cancer.

Today's cleanup effort including spreading a form of "kitty litter," or combination of sand and absorbent, on streets and sidewalks overnight.