In an unprecedented security measure, emergency masks will be provided to all U.S. Olympians in Greece, U.S. Olympic Committee officials said this week.

Trainers for each team will carry the masks in their medical bags for more than 500 U.S. athletes, who are being briefed on their use and availability as they disembark in Athens. The devices, known as emergency escape masks, fit over the head and seal at the neck and are designed to provide protection against tear gas and other agents.

They are different, however, than conventional wartime gas masks, which can handle a wider range of atmospheric toxins.

The USOC purchased more than 1,000 masks for the organization's 800-person contingent in a measure designed to complement the record $1.5 billion in security being provided by the Greek government and Athens Organizing Committee. USOC officials did not provide the product name; however, masks similar to those described by USOC officials retail online for under $150 apiece.

USOC head athletic trainer Ed Ryan advised the USOC to purchase the masks after an incident involving tear gas at the 2003 Pan American Games in Santo Domingo, Dominican Republic, created breathing problems for the U.S. women's volleyball team. A disturbance outside the partly open-air venue led to the dispersion of the gas, which filtered into the venue.

USOC officials are offering demonstrations of the devices during security briefings at the American College of Greece in Athens. The sessions are designed to provide information rather than create panic, officials said. USOC Director of Security Larry Buendorf is advising athletes that the masks are not intended to protect against exposure to toxic chemical agents.

"They are being issued specifically in the event of the disbursement of tear gas or some other agent like that," USOC Chief Communications Officer Darryl Seibel said. "They are in no way intended to be used if something more severe is introduced into the environment."

Other aspects of security specific to the U.S. team were still in the planning stages, USOC officials said.

"This has been more complicated than any Games we have participated in recent memory," USOC CEO Jim Scherr said this week. "There's no question the measures for the team are unprecedented. . . . It's just a different world we're living in."

Buendorf told The Post in April that air marshals likely would be aboard flights that brought U.S. Olympians into Athens. He also said then that the U.S. team would be provided additional protection from Greek and international authorities, and that other nations deemed high-risk with respect to terrorist threats -- such as Israel, Britain and Spain -- would receive similar attention.

"At the Games themselves, I think there will be a more visible presence of security . . . than probably at any Games in recent memory," Scherr said.

USOC officials emphasize that they have been advised of no specific threats to the Olympics or the U.S. interests in Athens during the Games.

"My greatest concern is for my family that is going to be there," District rower Aquil Abdullah said this week, before departing Thursday. "They have definitely given us information about who's going to be looking out for our safety over there. I think it's going to be pretty difficult even for friends and family to actually get in touch with us."

Athletes from the sports of rowing and boxing arrived in Greece yesterday, the third day of U.S. team processing. During introductory security briefings, athletes are being urged to travel in groups when they go about the city and wear red, white and blue and U.S. team gear with some discretion when they are outside of venues and the Olympic Village.

But though the USOC wants its Olympians to tone down nationalistic displays that might draw unnecessary attention or be construed as offensive, it has stopped short of telling them to avoid showing their nation's colors or wearing their uniforms with pride inside venues -- which officials consider to be an important distinction.

"We've been told to just keep a low profile," Abdullah said. "Although I do have concerns, it will make me more aware, but it won't change the way I live my life."