Metro confronted the new world of potential terrorist attacks yesterday, revising instructions for how train and bus drivers should minimize public exposure to hazardous substances and adding daily security sweeps in subway and bus stations.

The changes came two weeks after Metro was criticized for its handling of an incident on the Green Line in which police thought a man had sprayed a hazardous liquid inside a rail car. Transit officials said the new policies are based partly on lessons learned from that situation, in which a possibly contaminated train continued along much of its route. The spray turned out to be cleaner.

Officials said yesterday that they needed to help bus and train drivers distinguish between how to handle obvious attacks and how to deal with the presence of unknown powders and liquids in the system. No one was seriously hurt in the Green Line incident, but it closed the Southern Avenue Station for about six hours.

Until now, Metro's hazardous materials policies focused mostly on accidents, such as how to handle a damaged tank truck leaking a chemical into a subway tunnel, said Transit Police Chief Barry McDevitt. The new policies focus more on attacks and suspicious substances, above and below ground.

"It's just the world we live in today and the significant increase in threats for anthrax that are unfortunately happening here," McDevitt said. "That raises the bar on how you react to it."

The policies announced yesterday include more formal daily inspections of stations, rail yards, bus garages and buildings. Metro officials do daily checks now, but they will soon use a checklist of specific items to inspect, said Metro spokesman Ray Feldmann. Metro headquarters downtown also will have tighter security.

In the last six weeks, security has become a major focus for Metro, which moves 610,000 rail passengers and 550,000 bus riders a day. In 1998, 40 percent of terrorist attacks worldwide targeted transportation, with a growing number against buses and rail systems, according to the Federal Transit Administration.

With that in mind, Metro has asked the federal government for $190 million in security equipment, including $81 million to make it the world's first subway system with sensors to detect a biological or chemical attack.

Riders are apparently more mindful as well. The system has seen a huge increase in reports about suspicious liquids and powders. There have been about 170 reports of suspicious substances or packages since Sept. 11, McDevitt said. The system used to receive no more than a handful of such complaints in a typical six-week period, and almost all of those concerned suspicious packages, McDevitt said.

Feldmann said Metro has not received specific threats of chemical or biological attacks. All the reports about suspicious substances have turned up such things as spilled soda or powdered dairy creamer. Still, he said, Metro must respond to all reports, especially with heightened concern about anthrax exposure in the region, while causing minimal disruption to Metro passengers.

"We can't shut down the railroad for hours every time someone sees some powdery substance," Feldmann said. "On the other hand, we can't treat it lackadaisically. We tried to put procedures in place to balance safety issues with our need to move people."

Under the new guidelines, if an unknown substance is found on a Metro train, the operator will stop at the next station, shut down the train's ventilation system, evacuate all passengers and take the train to the closest rail yard for inspection. Passengers will be allowed to continue on another train, Feldmann said.

If a toxic substance were found on an underground train, supervisors would stop the train, shut down the ventilation system, evacuate the next rail station and slowly bring the contaminated train into that station. Passengers would be evacuated and detained in case they needed decontamination.

If a contaminated train were above ground, the train would stop until firefighters inspected it, or proceed to the next station if that station also were above ground.

If an unknown powder or liquid is reported on a rail platform, train service will be discontinued to that station while the material is investigated. Other trains will be allowed to pass through the station during the investigation.

Bus passengers and drivers are to be evacuated as soon as any suspicious substance is reported.

Feldmann said Metro officials made the changes after meeting Oct. 12 with Washington area fire officials.

In the incident two weeks ago, Metro police suspected that a Cheverly man, who had evaded the fare gate, had sprayed a chemical from a plastic bottle during an altercation with a police officer in a rail car.

The car in question was evacuated and sealed off, but the train continued for five more stops before all the cars were evacuated.