QWhen might you most likely want to shelter in place?

AWhen 24 inches of snow falls in your neighborhood, making roads perilous and the fireplace inviting. And that is the best reason to lay in a three-day cache of food, water and medicine.

But those supplies -- in addition to a flashlight, a portable radio and extra batteries -- would also be useful in the event of the terrorist attack that federal authorities keep telling us to prepare for.

What is the difference between "sheltering in place" and a "safe room"?

Sheltering in place means bringing people and pets indoors, closing doors and windows and turning off the heating, cooling and ventilating systems. This offers protection from outside contaminants, says John H. Sorensen, an emergency preparedness expert at the Oak Ridge National Laboratory.

A safe room -- which Sorensen calls "expedient protection" -- ideally is a windowless space in the center of the home that provides "an extra barrier between a room you tape and seal and the rest of the house," he says. "It is only a short-term thing, one to two hours [other experts say five hours if each person has 10 square feet of floor space]. And you need to get out and ventilate that room after whatever you are protecting yourself from has passed."

Safe rooms work best with prior warning, such as an alarm or news flash about a chemical spill. Terrorists would likely strike without warning, he says.

What kind of attacks would require staying put?

Chemical, biological and radiological.

How might a chemical attack occur?

Through sabotage of an industrial facility or the release of chemicals transported by train or tanker truck, says Amy E. Smithson, a chemical and biological weapons expert at the Henry L. Stimson Center.

People nearest an incident may recognize an attack right away if they "smell something funny or see birds drop out of the sky," says Smithson. They should shut all windows and doors, turn off all ventilation systems and try to seal the house. (Remember that pre-cut plastic sheeting -- the thicker, the better -- and duct tape.)

If they believe they have been exposed to chemicals, they should strip, making sure to cut open such garments as T-shirts and dresses with scissors rather than pull them over the head.

Contaminated clothes should be stored in a plastic bag. Exposed individuals should wash themselves thoroughly from head to toe with soap and warm water, and remain inside until told by authorities the threat has dispersed, Smithson says.

What about bioterrorism?

That scenario could entail an aerosol release at a crowded event or at a transit point like an airport or Metro station, or through a ventilation system, says Smithson. In most circumstances, people wouldn't know they are inhaling these microscopic pathogens; hospitals would begin noting large numbers of people falling ill.

By the time the disease is identified, the pathogen would already have done its damage. But some diseases are so contagious, says Smithson, that "you may need to hunker down and stay away from other people" for two weeks or longer.

The smallpox incubation period, for example, is up to 17 days, and the disease itself can last up to a month. Pneumonic plague incubates for two or three days and lasts another one to six. This requires a far larger supply of food, water and medicine to avoid leaving home.

What about radiation exposure from a nuclear device?

While government officials do worry about a "Hiroshima-type" nuclear assault, an easier and likelier terrorist weapon is a "dirty bomb," says Jon Wolfsthal, a proliferation expert at the Carnegie Endowment for International Peace. Such a bomb combines standard explosives with such radiological substances as cobalt-60, strontium-90 or radioactive iodine, all used for industrial and medical purposes.

The worst injuries may be caused by the explosion and flying debris, but the radiation "can be really bad and you can get very high doses even after just a few minutes of exposure," Wolfsthal says.

"The rule of thumb is if you can hear the blast, you're probably too close and you want to leave the area quickly. If it's 10 blocks away and you are not sure, the best thing you can do is stay put. We are not talking about microscopic atoms or viruses, but dust particles and droplets."

If you choose to stay in place, shut the windows, turn off the ventilation system, remove all clothing and shower vigorously. Seek shelter in an interior, windowless room or a basement and get information on wind direction from radio or television.

"Weather usually moves from west to east," Wolfsthal notes, so a person in Rockville who learns of an attack on Capitol Hill is upwind, and therefore all right.

"And I wouldn't drink tap water," he adds.

There are two nuclear power plants within 75 miles of Washington: Calvert Cliffs in Lusby, Md., and North Anna in Spotsylvania County, Va. What should you do if radioactive material is released?

A siren or alert system would sound and, if you live within 10 miles of either plant, you should turn on a TV or radio for instructions regarding shelter and/or evacuation, said Patricia Milligan, a physicist with the Nuclear Regulatory Commission.

Those close to either nuclear plant should also take potassium iodide to guard against the effect of released radioactive iodine. Maryland officials handed out 36,000 such pills last August to schools, businesses and residents around Calvert Cliffs.

If you live within 50 miles, the primary concern is eating locally produced food. After the Chernobyl nuclear plant blew up in 1986, many Ukrainians were exposed to radioactive fallout through milk -- grass irradiated by the plant explosion was eaten by cows. So the protective action would be confiscating potentially contaminated food, and livestock would be fed stored grain.

Milligan considers it "highly unlikely" that people in the Washington area would suffer adverse health effects: "The hazards to public safety would be minimal."

What if you have pets?

Bring them inside if you shelter in place. Take them with you if you evacuate, she says. But only service animals, such as guide dogs, may be allowed in public shelters.

Rosemary Sump of Chevy Chase stocks up on emergency supplies after the government raised its alert level to Code Orange.