How to Survive a Nuclear Attack: Shelter, Fallout and the First 72 Hours

Before an attack: stay or go?

If the threat is days away rather than minutes, the first decision is whether to prepare where you are or to leave. The honest answer for most families is to stay. Evacuation makes sense only when all of the following are true: you live in a large city or near a military target, you have somewhere specific to go where people expect you, the roads are clear and the authorities have not yet restricted movement, and you have the fuel to get there without stopping. If any of those is missing, you risk being caught between home and destination with whatever you can carry, among a great many other people making the same mistake.

If you do go, go early and go once. Being two days early costs little; being two hours late can put your family on a blocked road when the fallout arrives. If you stay, use the time you have to do the four things below, in this order: assign tasks, store water, stock food and supplies, build the shelter.

What fallout is and why time is on your side

A nuclear explosion near the ground pulls tons of earth and debris into the fireball, makes it radioactive and lifts it miles into the air. The heaviest particles come down within minutes close to the detonation; the lighter dust drifts downwind for hours and can travel hundreds of miles. It settles like dust or light snow, on roofs, roads and ground, and rain can concentrate it into hot spots you cannot see.

The dust itself is not the main danger; the radiation it gives off is, and that radiation passes through walls. But it fades fast. The rule of thumb is that for every sevenfold increase in time since the detonation, the radiation level drops tenfold. Whatever the level is one hour after the blast, it is about a tenth of that after seven hours, a hundredth after two days, and a thousandth after two weeks. The first hours are by far the most dangerous, which is why getting inside quickly matters more than anything else you can do.

Shelter: distance and mass

Two things reduce your dose: distance from the fallout outside, and mass between you and it. A basement gives you both, because the ground outside is above your head and the floors above you are between you and the roof. The center of the middle floors of a tall concrete building works for the same reason. Emergency planners rate shelters by protection factor: a single-story wood house gives perhaps 2 or 3, a typical basement around 10, a good basement corner with added mass 40 or more, and the core of a large building or a deep basement 100 and up. A protection factor of 100 means a dose outside that would kill in an hour becomes one you can live with for days.

Any heavy material adds protection. To cut radiation to a tenth takes roughly 3 inches of steel, 11 inches of concrete, 16 inches of packed earth, 2 feet of water or about 3 feet of wood. You will not have steel, but you have books, bricks, full water containers, bags of earth or sand, furniture and food. Thickness is what counts.

An expedient basement shelter in a few hours

• Choose the basement corner where the ground outside is highest against the wall. Push the heaviest table you have into it, or take interior doors off their hinges and lay them across sturdy supports to make one.
• Pile mass on top of and around it: books, bricks, cordwood, sandbags, full containers, file cabinets, boxes and pillowcases filled with earth. Reinforce the table so it cannot collapse.
• Leave a small crawl-in entrance with mass you can pull in behind you. Keep two air gaps of a few inches, one high at one end and one low at the other; a piece of cardboard fanned by hand moves air if it stalls.
• Cover basement windows from the inside with wood and anything heavy, and from the outside with earth or sandbags if you can. The air coming into a reasonably sealed basement does not need filtering; air does not become radioactive, and what you are keeping out is dust.
• Add mass on the floor directly above your corner and against the outside of the wall if there is time.

If you have no basement, use the same method in the most central ground-floor room with the fewest outside walls, and use more mass. Better still is a nearby building with a basement or several floors: a school, church, office block or underground car park. Decide now which one, and how you would get in.

How long to stay in

Ready.gov's instruction is to stay inside for at least 24 hours unless local authorities say otherwise; FEMA's current public messaging says to plan on 12 to 24 hours and then follow updated guidance. Those figures are for the general population across a wide area. If you are directly downwind of a detonation, you may need several days of full-time sheltering before brief trips out become reasonable, and a week or two before you spend most of the day outside. Listen for official readings for your area; without a meter of your own, the radio is your only way to know.

One judgment call: if your first shelter is poor (a car, a single-story wooden house) and a much better one is within about fifteen minutes' travel, planners now advise moving to it right away, before the fallout thickens, rather than waiting. If the better shelter is further than that, stay where you are for the first day.

Water first

Lack of clean water will hurt your family faster than lack of food. If you have warning, fill every container you own before the supply fails: bottles, pots, the bathtub, the washing machine, new garbage cans lined with fresh bags, children's pools. Your water heater holds 30 to 50 gallons more. Aim for a gallon per person per day for drinking and washing, for two weeks at least.

Water in a closed container is not harmed by fallout; radiation passing through it does not make it radioactive, and dust can be rinsed off the outside. The risk is germs, not fallout. To make questionable water safe, bring it to a rolling boil for one minute (three minutes above 6,500 feet), or add unscented household bleach: 8 drops per gallon of 6 percent bleach, or 6 drops per gallon of 8.25 percent bleach, stir, and wait 30 minutes. Double the dose if the water is cloudy, after letting it settle or straining it through a cloth. Pouring treated water between containers improves the taste.

Food and supplies

Packaged food that was indoors, or in a sealed container, is safe to eat after you wipe the outside. Do not eat anything that was uncovered outdoors after fallout arrived. In the shelter itself keep food that needs no cooking: canned goods and a manual opener, peanut butter, crackers, dried fruit, nuts, bars, dried milk. Do not cook inside a sealed shelter; the fire and fume risk is real. Store the bulk of your food just outside the shelter entrance where you can pull it in.

If stores are still open and safe, buy for two weeks at minimum and ideally much longer; after a nuclear attack, normal supply will not return quickly. The shopping list is the home supply list on our emergency preparedness guide, plus: extra batteries for radios and lights, LED headlamps, a five-gallon bucket with a seat and bag liners for a toilet, garbage bags, unscented bleach, plastic sheeting, duct tape, dust masks (N95 or better), cheap hooded rain ponchos, baby wipes, prescription refills and a stocked first-aid kit. Print the one-page checklist and add these to it.

Potassium iodide: what it does and does not do

Potassium iodide (KI) tablets fill the thyroid with stable iodine so that it cannot take up radioactive iodine, one product of a nuclear explosion. That is all they do. They do not protect any other organ, do not protect against any other kind of radiation, and do not make it safe to go outside. Shelter and time do far more than KI ever will.

The CDC's guidance: take KI only when public health or emergency officials say to, as close to the time of exposure as possible. Adults take 130 mg once every 24 hours; children aged 3 to 18 take 65 mg (adult dose if over 150 pounds); infants from one month to three years 32 mg; newborns a single 16 mg dose. Adults over 40 generally should not take it, because their risk of thyroid cancer from radioactive iodine is lowest and their risk of a reaction highest; pregnant and breastfeeding women take one dose only. More than one dose a day does not add protection. Do not substitute table salt, iodized or otherwise, and do not paint iodine solutions on the skin; the CDC advises against it and older guides that recommended it are out of date. KI is sold without a prescription; if you want it, buy FDA-approved tablets ahead of time and ask your doctor now whether anyone in the household should not take it.

Keeping fallout off you and out of the shelter

• Anyone caught outside after fallout arrives removes their outer clothes at the door and seals them in a bag. Shoes too. That alone removes most of the material.
• Wash with soap and warm water, including hair; no hair conditioner, which binds particles; no scrubbing, which breaks the skin. If there is no water, wipe exposed skin with a damp cloth.
• Exposure does not make a person radioactive, and radiation sickness cannot be passed on. Someone who is ill needs care, not isolation.
• Keep pets inside. Brush and wash a pet that was out.
• Seal the last door you use with duct tape round the edges, and keep hooded ponchos and masks by the entrance for anyone who must go out later.

Information, light and sanitation

• Radio. Battery or hand-crank AM/FM and NOAA Weather Radio will work and is how officials will tell you when it is safe to come out. Keep spares and plenty of batteries. A detonation at altitude can produce an electromagnetic pulse that damages electronics over a wide area; a spare radio kept unplugged, wrapped in paper or bubble wrap inside a metal container, is cheap insurance.
• Light. LED flashlights and headlamps, not candles. Bring books and games for children; you may be in there a while.
• Toilet. A five-gallon bucket with a seat and a bag liner, sealed after each use and kept by the outgoing air gap, with a lined garbage can just outside the entrance for the bags when it is safe to step out.
• Tools. Keep a crowbar, jack and basic tools in the shelter in case you have to dig out, plus plastic sheeting and tape for repairs.

A dirty bomb is not a nuclear weapon

A dirty bomb is conventional explosive packed with radioactive material. It contaminates the area around the blast and downwind of it, but it does not produce a nuclear explosion or widespread fallout. If you see or hear an explosion in a city and cannot be sure what it was, move away from it, keeping the wind to your side rather than at your back, cover your nose and mouth, and when you reach shelter remove your outer clothing and wash. Then follow official instructions. Emergency services are well equipped to handle a dirty bomb; it is the nuclear weapon case that leaves families on their own.