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

Following are questions and answers about the latest terrorism warnings:

Q Government officials have elevated the terrorist threat level but haven't told us what kind of attack to expect. Why not?

A Federal officials know that al Qaeda has been experimenting with biological, chemical and radiological weapons but won't predict what type of weapon might be used in an attack. It is easier for terrorists to make a weapon out of lethal chemicals than out of biological agents, experts say. But officials at the U.S. Centers for Disease Control and Prevention say biological weapons, which can disperse anthrax or smallpox, pose the greatest hazard to public health.

Should parents rush to schools to pick up their children in the event of an attack?

No. Officials say schools may be the safest place for children in some circumstances. The Red Cross encourages parents to learn about the emergency plan at their child's school and about the kind of authorization required to take a child out of school.

What should we tell our children about all of this?

Discuss the family emergency plan with children and respond honestly to questions, but use details appropriate to a child's age, said Irwin Redlener, president of the Children's Health Fund and a member of the American Academy of Pediatrics Task Force on Terrorism. Tell children that officials are working hard to keep them safe. Sleeplessness, nightmares and problems in school could be signs of severe anxiety. Consult a health care professional if needed. Hug your kids.

Should we admit to our children that we are nervous?

Marilyn Benoit, president of the American Academy of Child & Adolescent Psychiatry, said it would be unnatural for parents to pretend they are not concerned, but she stressed that the way parents handle the anxiety will determine how well their children do. Don't let children see panic. Benoit recommends activities that bring families together, such as cooking or painting a room.

How should we prepare for an attack that we can't predict?

There are some basics. Make sure you have a battery-powered radio or television to listen to government advisories. Assemble emergency supplies for home and car. Make a family emergency plan. Coordinate with neighbors. Mentally rehearse what you would do in an emergency.

What should be included in the family emergency plan?

The Red Cross says families should select two places to meet in the event of a disaster: right outside the home in case of a sudden emergency, such as a fire, or outside the neighborhood if they can't return home. Ask an out-of-state friend to be a contact for your family members because it is often easier to call long distance after a disaster. All members of the family should call that number and report their whereabouts. Plan to care for the infirmed and for pets.

What should be in a disaster supply kit?

Medicine and other first-aid supplies, flashlights, plenty of batteries, a battery-powered radio or television, bottled water, nonperishable food and a manual can opener, sleeping bags, clothing, sanitation supplies, birth certificates, passports, driver's licenses and other important documents in a waterproof container, a credit card and cash. An extra pair of glasses. For parents of infants, diapers. Paper and pencil. Food for your pet. The Red Cross suggests storing it all in sturdy, easy-to-carry backpacks or duffle bags.

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

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. Potassium iodide is a relatively simple salt that keeps the thyroid gland from absorbing radioactive iodine and helps prevent radiation-induced thyroid cancer. It won't protect against any other radioactive isotope, though.

If a chemical or biological weapon attack is announced, what should we do?

If authorities order an evacuation, move swiftly by rail, car or foot to leave the area. If they tell you to go inside and seal yourself off, move quickly; in such an instance, immediately go inside or stay inside, close and lock all windows and doors, close the damper of fireplaces, and shut off heating, cooling and ventilating systems. If a safe room with few or no windows has been selected, go there. Have inside a battery-operated radio, food, water, a first-aid kit. Install plastic sheeting over windows and vents with long strips of duct tape to enhance air tightness.

The edges of any doors should be covered with long strips of duct tape. If duct tape won't block the space at door's bottom, stuff a wet towel there. Also tape over electrical outlets.

Officials say that in small rooms, the oxygen could be used up quickly. Is it better to select a larger room with windows or a smaller room with no windows?

Ten square feet of floor space per person will provide sufficient air to prevent the buildup of carbon dioxide for up to five hours. There's no telling how long an emergency would last. Use your judgment in picking a room that can be enclosed and sustain your family for some time.

It seems everyone is running out to buy plastic sheeting and duct tape to seal off windows in the event of an attack. Will this prove effective?

It won't protect against radiation from a dirty bomb. Experts say that for chemical attacks, it can help prevent the spread of chemical vapors and aerosols. The critical part is sealing off all routes of air infiltration, according to a study conducted by Oak Ridge National Laboratory in 2001 for the Federal Emergency Management Agency.

How much food and water should be stored?

The Federal Emergency Management Agency says everyone should have food and water to last for three days. For water, that amounts to one gallon per person per day, according to the Red Cross. 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.

Will phones keep working?

If the electricity is out, wireless systems in your home whose base stations require power will not work. Cell phones should continue working, though sheer volume could overload capacity, and authorities could send out an alert telling people to stay off cellular networks so they can be used for emergencies.

-- Valerie Strauss