WASHINGTON-Hard to anticipate, often disguised and frequently deadly all describe mail bombs.

Fortunately, mail bombs also are relatively rare and manageable risks, say security experts. But a rash of mail bombs, seven on Jan. 2 alone, has heightened awareness of this invidious exposure and placed new urgency on recognizing and responding to mail bombs.

Mail bombs are not daily occurrences in the United States. According to a spokeswoman for the U.S. Postal Inspection Service, only 13 mail bombs of all types were investigated during the fiscal year that ended last Sept. 30. The inspection service says the average annual number of investigations in recent years has been 16, minuscule considering the postal service processes about 170 billion pieces of mail a year. But during the current fiscal year, 12 mail bombs have been found.

"Given the vast amount of mail that goes through the U.S., you'd have to put it on a numerical basis as kind of rare," said Mayer Nudell, an independent security consultant based in Falls Church, Va., and co-author with Norman Antokov of "The Handbook for Effective Emergency and Crisis Management."

But, added Mr. Nudell, "consequences could be very high."

Fortunately, none of the January mail bombs exploded before being detected and disarmed. Four of the bombs, disguised as Christmas cards, arrived at the Washington bureau of the London-based Arabic-language newspaper Al Hayat in two different mail deliveries during the same day. The discovery of the bombs by an Al Hayat reporter led to the evacuation of the National Press Building, which houses Business Insurance's Washington bureau.

A fifth mail bomb addressed to Al Hayat was intercepted at a Washington post office. Three other mail bombs bore the address of "parole officer" at the federal penitentiary in Leavenworth, Kan., where they were intercepted Jan. 2 and Jan. 3.

Although Al Hayat published a report last week claiming the bombs had originated in Libya, the Libyan government has denied any involvement in the mail bomb campaign, and no group has taken responsibility for the bombs found in Washington and Leavenworth, all of which bore postmarks from Alexandria, Egypt.

Businesses already are responding to the perceived increased exposure.

"We have seen an interest in learning how to screen for packages," said Bill Daly, a managing director of Kroll Associates, a New York-based investigative consulting firm partly owned by American International Group Inc.

This year's flurry of mail bombs and the 1996 arrest of a suspect alleged to have carried out the Unabomber's 17-year mail bomb campaign against businesses and universities have heightened awareness of the exposure, he said.

Mr. Nudell noted that the sheer volume of mail makes it impossible for the postal service to screen everything. As a result, "you should take some measures to protect against the possibility that it could happen to your organization," he said.

One step is to centralize mail and package delivery at any given site, he said.

"That gives you the ability to concentrate your screening measures." Centralization also holds down costs, he said. For example, an employer only has to pay for and maintain a single X-ray machine rather than have them scattered throughout a facility.

Training a core of people who handle mail and packages to recognize suspicious signs also is critical, said Mr. Nudell. The training should include the procedures that need to be followed once the people encounter a suspicious package.

Kroll's Mr. Daly emphasized that the training should not necessarily be confined to mail room personnel. Other people, including secretaries and high-profile employees such as public relations spokespeople who appear in the media and who might provide a mail bomber with an enticing target, also often need to be trained, he said.

The training should be bolstered by such actions as posting lists of potential indications of mail bombs in the mail room or delivery room, said Mr. Daly.

The U.S. Postal Inspection Service has posted a list of some characteristics of mail bombs on its Web site. These include:

Mail bombs may have excessive postage. Generally, a bomber does not want to mail a parcel over the counter and have to deal face-to-face with a window clerk.

The return address may be fictitious or non-existent.

The postmark may show a different location than the return address.

Mail bombs may bear restricted endorsements, such as "personal" or "private." This is particularly important when the addressee does not usually receive personal mail at the office.

Mail bombs may display distorted handwriting, or the name and address may be prepared with homemade labels or cut-and-paste lettering.

Parcel bombs may be unprofessionally wrapped with several combinations of tape used to secure the package, and may be endorsed "Fragile-Handle with care" or "Rush-Do not delay."

Letter bombs may feel rigid or appear uneven or lopsided.

Package bombs may have an irregular shape, soft spots or bulges.

Mail bombs may have protruding wires, aluminum foil or oil stains, and may emit a peculiar odor.

It was this last characteristic-protruding wires-that alerted the reporter at Al Hayat that the "Christmas cards" arriving at his office contained explosives.

A suspicious letter or package demands careful treatment, according to the postal inspectors. It should not be opened but should be isolated. It should not be put in an enclosed space, such as a drawer, or in water. If there are windows in the immediate area, they should be opened to vent any potentially explosive gases. And, of course, local police need to be called immediately.

The decision on "how to evacuate and whom to evacuate" rests with management, said Mr. Daly. "It really needs to be handled with the utmost discretion."

Mail bombs are anti-personnel devices rather than anti-property devices, he pointed out. "Generally, they are designed to injure or harm the person who opens it," not to destroy a structure. "It's made so it only goes off when you open the package," he said.

"You're always balancing what you believe the actual level of risk is against the lives of the people there, but you don't want to unduly alarm or hamper those who are not at risk," Mr. Nudell pointed out.

"Sometimes when you do an evacuation, people can get hurt as you try to help them," he said.

Even worse, there have been cases, though none yet in the United States, where bombs have been reported at one location by terrorists who knew people would be evacuated to a second location where the actual explosives were planted. The result can be fatal, he said.

Mr. Nudell said the evacuation of the National Press Building after the discovery of the mail bombs went a little more smoothly than might have been expected in a Washington winter, as occupants savored near-spring-like temperatures while they waited to return to the building.

"We were lucky; the bad guys picked good weather in which to have their letter bombs arrive," he said.