Following are questions and answers about the federal government's terrorism warnings:

Q Do intelligence officials know what kind of weapon is more likely to be used if there is an attack?

A Federal officials say they believe it is more likely that an attack would be in the form of a chemical or radiological weapon. Chemicals such as sarin and mustard gas are easier to turn into weapons than biological agents, and nuclear weapons are extremely difficult to construct and deliver to a target.

Is a radiological weapon the same as a nuclear weapon?

No. A radiological weapon is a conventional explosive such as dynamite packaged with radioactive material that scatters when the bomb goes off. Called "a dirty bomb," it kills or injures through the initial blast of the conventional explosive and, in the long term, by airborne radiation and contamination that can spread over a wide area, according to the Council on Foreign Relations.

There are so many possibilities that it seems impossible to prepare for every one. What should residents do?

Residents should have a battery-powered radio to listen to advisories from the federal government. Disaster management experts also say people should assemble emergency supplies for their house and car; make a plan so family members know where to meet (select two places, in the event the primary choice is affected in some way); select a contact outside the area whom family members can call to pass along messages; and mentally rehearse emergency plans. Officials also urge people not to panic. They say they know they are sending mixed messages, but both are important.

What should be in a disaster supply kit?

Medicine and other first-aid supplies, flashlights, plenty of batteries, a battery-powered radio or TV, bottled water, nonperishable food and a non-electric can opener, sleeping bags, clothing, birth certificates, passports, driver's licenses and other important documents, and cash. An extra pair of glasses. For parents of infants, diapers. Paper and pencil. Anything you can't live without.

Is there any way to protect yourself against a dirty bomb explosion?

The key words: shielding, distance and time. The best place to go in the event that a dirty bomb explodes is a location surrounded by dense materials. Good places are the basement of a large building, a subway tunnel or an underground home cellar. The denser the material around you, the better off you are. Brick is better than wood; a few boxes of heavy books around is better than nothing. How long people should stay hunkered down depends upon the size of an explosion and the wind currents. The government will test the air and issue advisories. If people can safely evacuate an area, they should.

Thousands of people are stocking up on potassium iodide tablets. Will those help?

Potassium iodide is a relatively simple salt that keeps the thyroid gland from absorbing radioactive iodine and helps prevent radiation-induced thyroid cancer. But it does nothing to protect against any other radioactive isotope, or to help fend off the heat and blast effects of an explosion of a bomb with radiological material. One 130-milligram tablet a day will do the job for an adult, less for a child.

If there is a chemical weapon attack, where is the best place to be in my house?

Experts advise finding a room in the house or other building, preferably without windows, where you can block yourself off. Inside should be supplies including a radio and batteries, a first-aid kit, blankets, food and bottled water. Books, games and other things to occupy children would help. During an attack, people instructed to retreat to a safe room by the government should turn off ventilation and, using plastic sheeting and duct tape, seal the doors, vents, windows and any other opening. Ten square feet of floor space per person will provide sufficient air to prevent carbon-dioxide buildup for up to five hours.

How much food and water should be stored?

The Federal Emergency Management Administration says everyone should have food and water to last each person for three days. If activity is limited, healthy people can survive on half their usual food intake for several days, FEMA says. Children and pregnant women should not be rationed.

Should children be told the truth about what is happening?

Yes, says Irwin Redlener, president of the Children's Health Fund and a member of the American Academy of Pediatrics Task Force on Terrorism. Honestly respond to children's questions with detail appropriate to a child's age. Be aware of less-direct evidence of anxiety, such as sleeplessness, nightmares or problems in school, and recognize that some children may need to see a health-care professional. Give reassurance and discuss the family's plan while telling children that officials are doing everything they can to keep them safe. Keep up family routines; they maintain stability. Physical proximity -- hugs -- can help establish psychological security.

Federal authorities are recommending that people store water. Are public water supplies vulnerable?

Christine Todd Whitman, administrator of the Environmental Protection Agency, has said that the possibility of contamination of a water system is small. It would take large amounts of chemical or biological agents to threaten the safety of a city water system, and beefed-up security at water reservoirs and other facilities since Sept. 11, 2001, would make it very difficult for anyone to introduce the quantities needed.

Will cell phones stop working?

Cell phones should continue working under these scenarios, though sheer volume could overload capacity, as was the case on Sept. 11, 2001.

What is the best way to evacuate from downtown Washington?

Transportation officials say the fastest way to move the most people is by rail.

Authorities also are devising road evacuation routes. But won't the roads be even more jammed than usual?

Yes. That's why David Paulison, the U.S. fire administrator, said that anybody who doesn't have to evacuate should stay put. D.C. engineers estimate that it would take the approximately 800,000 vehicles traveling in the city daily two or three hours to leave, depending on conditions.

Are there rules about where you can go if you are trying to leave by car?

Yes. Pennsylvania Avenue will serve as a dividing line for the evacuation of the downtown area; no vehicles will be allowed to cross it, D.C. officials said. Those people north of it should head to Maryland; those south should head toward Virginia.

-- Valerie Strauss

Todd Freundlich of Rockville, left, with help from Orion Cottrell of the Army-Navy Ranger Surplus store in Bethesda, picks out a first-aid kit along with duct tape and a lantern.Sean Nichols of Silver Spring leaves the surplus store with supplies he said he bought to give his wife a little peace of mind.