Response officials 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; nuclear weapons are extremely difficult to construct and deliver to a target.

No. According to the Council on Foreign Relations, 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.

Residents should have a battery-powered radio to listen to advisories from the 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.

The key words: shielding, distance and time. The best place to go in the event of an explosion 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 tha.n 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.

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.

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.

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.