A State Department official who was held captive in Tehran during the Iranian hostage crisis said yesterday that Americans "must keep our cool" in the face of mounting terrorist attacks overseas and advised that not all foreign travel is dangerous.

L. Bruce Laingen, 63, who was charge d'affaires at the U.S. Embassy in Tehran when 52 Americans were taken hostage in 1979 by the revolutionary government of Ayatollah Ruhollah Khomeini, said yesterday that terrorism is "the scourge of our time and a continuing learning experience." Laingen told a luncheon meeting of the Bethesda-Chevy Chase Rotary Club, "There's no 100 percent protection, and it will continue."

He said that U.S. policy must center on the option of armed force but remain flexible enough to deal with a continued onslaught of terrorist attacks against Americans in a wide range of locations worldwide.

Laingen, who now serves as vice president of the National Defense University at Fort McNair, wore a yellow ribbon with a brass plaque embossed with "Free the Hostages" as a reminder of six Americans still held captive in Beirut.

Flexibility is needed, he said, because "every terrorist incident is going to be different in some way -- in geography . . . in numbers of terrorists, in numbers of victims . . . . We need a tool box of options," including the use of force, economic sanctions, diplomacy, third-party negotiators, the help of allies and strong intelligence.

A recent State Department survey of the economic cost of terrorism found that 1.8 million Americans out of 5.1 million expected to travel overseas this year have canceled their trips, Laingen said. The survey estimated that there would be a 25 percent drop in U.S. tourism in Western Europe, a 50 percent decline in Greece and in Egypt, a 40 percent drop in West Germany and a 25 percent decline in France, he said.

"I would be the last one to cancel travel plans. I wouldn't go to Tripoli or Beirut and I would be careful in Cairo," Laingen said during an interview after the speech. "What the terrorists seek is to plant worry, concern and fear in the minds of all of us -- they accomplish that to the extent that we cancel travel plans."

Laingen's comments came as Fairfax County School Superintendent Robert R. Spillane canceled all school-sponsored summer trips to Europe yesterday, citing the threat to students posed by terrorism. The decision, which had been expected, followed cancellations of similar trips by other school districts and colleges in the region and around the country.

Spillane said in a news conference that the 100 students who had signed up for four trips to six European countries would be exposed to "unnecessary risks I'm not willing to take."

Fatal terrorist attacks against Americans overseas have steadily increased since 1982, according to figures released last week by the State Department. According to those figures, six Americans have died in three overseas attacks this year, 25 Americans died in 1985 and 16 died in 1984.