How to Survive a Nuclear Attack: What to Do, Minute by Minute

Learning how to survive a nuclear attack comes down to physics. Most people who are not close to ground zero can survive one. The blast and heat kill within a short radius. Beyond that, the main danger is fallout, a radioactive dust that loses most of its strength within a day or two. If you know what to do in the right order, you can cut your radiation dose by 90% or more. What a very large dose does to the body is covered in radiation sickness symptoms.

This guide follows the clock: the flash, the first 15 minutes, the first 24 hours, the first week. Every step comes from official US guidance (Ready.gov, the CDC and FEMA’s Planning Guidance for Response to a Nuclear Detonation). Where we use numbers from other sources, or do our own arithmetic, we say so.

How to survive a nuclear attack: the short version

If you read nothing else, do this, in this order.

1. If you see a flash, do not look at it. Drop and take cover behind anything solid. Outdoors with nothing to hide behind, lie face down to shield exposed skin.
2. Wait for the shockwave to pass, then move. Beyond a few miles it arrives seconds to tens of seconds after the flash.
3. Get inside the nearest sturdy building. Brick, concrete and stone are best. Go below ground or to the middle of the building, away from windows and outside walls.
4. Stay inside for at least 24 hours. Official wording is 12 to 24 hours as a minimum, and Ready.gov says sheltering usually lasts at least 24.
5. Stay tuned. An AM/FM battery or hand-crank radio is best. Do not leave until officials tell you where it is safe to go.
6. Decontaminate after you come in. Remove outer clothing, bag it, wash with soap and water.

The rest of this article explains why each step works, and what to do when your situation is not ideal.

What a nuclear explosion actually does

You need a rough map of the hazards. A nuclear explosion produces a flash, a fireball, a blast wave, a thermal pulse, initial radiation and, if the fireball touches the ground, fallout. Each one has a different range.

FEMA’s planning guide uses a nominal 10 kiloton (kt) ground burst in a city as its reference case. A kiloton is the energy of 1,000 tons of TNT. Strategic warheads are much larger, so we show a larger yield too.

The 10 kt and 100 kt figures are FEMA’s. FEMA notes that for every tenfold increase in yield, the ranges typically only double. The 300 kt column is our own estimate: we scaled the FEMA numbers by the usual cube-root rule for blast, so treat it as rough. Height of burst changes everything. An air burst pushes blast and thermal effects further out and produces far less local fallout than a ground burst.

Three more FEMA numbers matter for planning:

• The fireball of a 10 kt detonation is about a quarter of a mile across.
• Flash blindness can occur over 10 miles away in daylight, farther at night. In daylight it lasts seconds; at night, with pupils wide open, it can last 5 to 10 minutes.
• Dangerous fallout (above 10 roentgen per hour) can reach tens of miles downwind. CDC puts the dangerous radiation zone at 10 to 20 miles downwind in its planning description.

Note: Nobody has measured what a modern nuclear weapon would do to a modern US city. FEMA says plainly that there is uncertainty and that real zones would be mapped by radiation readings after the event. Use the table to understand scale, not to calculate your odds.

Second 0 to 1 minute: the flash

The first sign is usually light. It is brighter than anything you have seen and it comes before the sound.

Do not look at it. Flash blindness does not need you to stare at the fireball. Reflected light is enough. Staring can cause permanent retinal burns at considerable distances, according to REMM, the HHS radiation medical resource.

What buys you time is simple physics. Light arrives instantly. The blast wave starts faster than the speed of sound and slows as it spreads, so the farther you are, the longer the gap. FEMA says that beyond a few miles there are several seconds between flash and shockwave, and farther out the wave can take tens of seconds. Sound covers about a mile in 5 seconds, which is a useful sanity check on those numbers.

Use the gap like this:

1. Turn away from the light and get behind something solid: a wall, a vehicle, a curb, the corner of a building.
2. Get down. Ready.gov says to lie face down if you are outside, so your exposed skin is covered and the blast pushes you less.
3. Stay down until the shockwave has passed and debris has stopped falling. Cover your head.
4. Then get inside. That is the step that matters.

You will not have an exact countdown. Close to ground zero, there is no window at all. That is why the severe damage zone is the one place where official guidance says very few people survive.

Myth: “Duck and cover is useless.” It is not useless. It protects you from glass and debris in the outer damage zones. It is just incomplete: cover for the shockwave, then shelter for the fallout.

Minutes 1 to 15: get inside, and pick the right inside

Ready.gov states that you will have 10 minutes or more to find adequate shelter before fallout arrives. FEMA says that in many areas fallout arrives an hour or more after the detonation. Do not bank on the long end. Move as soon as the blast has passed.

Fallout is dirt and debris sucked into the fireball, made radioactive, and then dropped back down over minutes to hours as the cloud cools. FEMA describes the particles near the blast as large, roughly sand-sized, and visible as they fall. The radiation hazard is mostly gamma rays from particles lying on the ground and roofs. That is why distance and mass work so well against it: gamma rays are weakened by every wall, floor and meter of earth between you and the dust.

Where to go

FEMA’s rule: get inside a basement or the middle of a large, dense building. The ranking looks like this:

A protection factor (PF) of 10 means you receive one tenth of the dose of a person standing in the open. We cover the diagram and how to improve a weak spot in our apartment fallout shelter guide. If you do not have a basement, that article is the one to read.

Inside the building:

1. Move away from windows, doors, corners and outside walls.
2. Go down, or go to the middle. Avoid the top floor and the roof.
3. Put as many walls as possible between you and the outdoors.
4. Turn off fans, air conditioners and forced-air heating that pull in outside air. Close windows, doors and fireplace dampers (Ready.gov).
5. Bring pets inside. Ready.gov says pets and service animals should be inside too.

Sealing the room with tape is not necessary. FEMA says structures do not need to be airtight to protect against fallout radiation. The danger is gamma rays from the dust outside, not gas leaking through cracks.

If you are in a poor shelter

FEMA gives a rule for planners that is worth knowing. If you are in a very poor shelter (PF about 2, like a wood house without a basement) and an adequate shelter is within 15 minutes of travel, the best time to stay in the first one is no longer than about 30 minutes after the detonation. If your shelter is a bit better (PF about 4), stay one to two hours before moving. FEMA also says official early-evacuation orders may be issued for poorly shielded structures.

Without an official order and without a better building close by, staying put is usually right. The same guide says that unless fire or other immediate threats force you out, no evacuation should be attempted until basic information on fallout and dose rates exists.

Safety: Leave a burning or collapsing building. FEMA states that even inside the dangerous radiation zone, people should leave a burning building. The risk of delayed radiation effects is lower than the risk of immediate death from fire.

Hours 1 to 24: the shelter phase

Now the decay curve is working for you. Fallout loses a large share of its radiation within hours because most of its isotopes are short-lived. FEMA says it gives off over half of its energy in the first hour. The full decay math is in our article on how long nuclear fallout lasts, and the fallout decay calculator lets you test your own numbers.

Why 24 hours? FEMA’s guidance is that the dangerous radiation zone can go from tens of miles to a mile or two in a single day as the radiation decays. It also says that in the first few hours, lethal levels may exist even tens of miles from ground zero, where nothing looks wrong.

What to do in the shelter:

1. Get information. Run the radio on batteries or a crank. Use text alerts and the FEMA app if the networks still work. Expect instructions to be updated as officials learn more. A siren alone tells you little, and our guide to what a nuclear siren sounds like explains the signals.
2. Do not go out to check. You cannot see or smell radiation. Fallout may be invisible on rough surfaces, so the absence of visible dust tells you nothing, according to FEMA.
3. Drink and eat from what you have indoors. Ready.gov says food kept indoors is safe, and sealed containers from outside are fine once wiped with a damp cloth. Do not eat garden produce or uncovered outdoor food until officials say so.
4. Limit travel inside the building. If you are in a large building, stay in the best-protected area, not near the top floor.
5. Treat injuries. Call 9-1-1 for a real medical emergency. Stop severe bleeding first; our tourniquet guide covers it.

Water and supplies matter because you may not leave when you want to. A day is the minimum. FEMA’s own guidance says optimal shelter stay times can range from a few hours to several days depending on local dose rates, so having more than a day of water and food is sensible.

Potassium iodide

Do not take iodine pills on your own initiative. Potassium iodide (KI) only blocks the thyroid from absorbing radioactive iodine. It does not protect against external radiation. Take it only when public health or emergency officials tell you to. Details and the FDA dose chart are in our potassium iodide guide.

Decontamination: the 90% step

If you were outside while fallout fell, you have dust on your clothes, hair and skin. That dust keeps irradiating you.

Ready.gov says removing the outer layer of contaminated clothing can eliminate up to 90% of radioactive material. FEMA says effective decontamination of people from fallout can be done by brushing, removing or changing the outer layer of clothing, including footwear, and wiping off exposed skin.

Do it in this order:

1. Remove outer clothing and shoes slowly, rolling them inward, so you do not shake dust into the air. Keep your hands away from eyes, nose and mouth.
2. Put them in a bag and leave it outside or far from where people sit.
3. Shower or wash with soap and water. If you cannot, wipe skin and hair with a wet cloth or wipe.
4. Use warm water and plenty of soap and shampoo, gently. The CDC says not to scrub or scratch the skin, and to keep cuts covered so particles stay out of wounds.
5. Do not use conditioner. The CDC says it makes radioactive material stick to hair.
6. Brush pets, then wash them if you have soap and water.

Skip hand sanitizer; Ready.gov states it does not protect against radioactive material.

FEMA adds one reassurance. Fallout particles are large enough that inhaling them is a small hazard compared with the external dose from particles on the ground. A cloth mask over the nose and mouth is a reasonable extra measure.

Day 2 to week 1: when to leave

After the first day, radiation has dropped sharply. The strategy changes from hiding to a planned exit.

• Wait for instructions. FEMA’s approach is staged, facilitated evacuation for fallout-affected areas. Evacuations are supposed to occur after routes are identified and cleared.
• Move sideways, not straight downwind. FEMA says lateral evacuation (at right angles to the fallout path) is often the best way to minimize dose.
• People with good shelter and supplies are a low priority to move. Uninjured people in adequate shelters with access to safe food and water do not need to rush out.
• Do not return home until local authorities say it is safe. Ready.gov is clear on this.

If your home is outside the dangerous radiation zone, officials may simply lift the shelter order. If you are inside it, expect a phased plan.

Longer term, a ground burst leaves a wide area of low-level contamination. FEMA calls this the hot zone: 0.01 to 10 R/h at first, which is not immediately dangerous to life, but can extend hundreds of miles downwind and may require food and water advisories. Your local authorities decide what is eaten, drunk and farmed. Our guide to the safest places in the US in a nuclear war looks at likely targets and downwind patterns.

How to survive a nuclear attack in your house

Most readers live in a house or an apartment, not a bunker. Here is what the official diagram says about ordinary buildings:

1. House with a basement. Go to the basement and stay away from windows. Piling dense material (bookcases full of books, filled water containers, stacked furniture) between you and the nearest exterior wall adds shielding, because FEMA lists concrete, brick, stone and earth as good shielding and many layers of lesser material as a workable substitute.
2. Wood-frame house without a basement. You are in a PF 2 to 3 building. Move to the most interior room with the most walls between you and the outside, and plan to move to a better building if one is a few minutes away and officials say to. A neighbor’s brick basement or a nearby school, supermarket or office building is better.
3. Apartment. Middle floors of a large concrete building are strong shelters. Avoid the top floor. See the apartment guide.

Lawrence Livermore National Laboratory researchers modeled more than 10,000 buildings and reported that offices, churches, supermarkets and schools often protect better than surrounding housing, as summarized in an LLNL article. Know your nearest big concrete or brick building before you need it.

Myths that get people hurt

“I will drive out of the city.”

FEMA says cars are not adequate shelters. The flash can blind drivers over 10 miles away and cause crashes, and blocked roads will trap people in the open while fallout arrives. Evacuating before knowing where fallout is headed can put you on the plume. The official advice is to get inside first.

“Nuclear war means no survivors, so why bother.”

The planning guide says the opposite. Based on modeling from DOE national laboratories, deaths and severe injuries from fallout can be almost eliminated if people get inside before an incident happens, and drastically reduced if they get inside soon after. Close to ground zero, survival is rare. Beyond the severe damage zone, shelter choice and the next 24 hours decide much of the outcome.

“You need to live underground for weeks.”

For most people far from ground zero, the guidance is 12 to 24 hours as a baseline, then follow instructions. Some people near heavy fallout will need longer, and officials will say so.

Frequently asked questions

How far away is safe from a nuclear bomb?

There is no single number. For a 10 kt ground burst, FEMA puts severe damage out to about half a mile and light damage out to about 3 miles. Dangerous fallout can reach tens of miles downwind. Larger weapons reach farther. Wherever you are, the same rule applies: get inside a sturdy building and stay there. For local examples, see what would happen if a nuke hit Los Angeles or Chicago.

How long do you have to get to shelter?

Ready.gov says you will have 10 minutes or more before fallout arrives. FEMA says in many areas it arrives an hour or more after the detonation. Use the time to go to the best nearby building, not to drive. Seconds count mostly for the blast wave, which arrives before the fallout.

Can you survive a nuclear blast in a basement?

Often, outside the severe damage zone. A basement shields against blast debris, heat and fallout. FEMA lists houses with basements, large multi-story buildings and parking garages as adequate shelters that can cut fallout doses by a factor of 10 or more. Near ground zero, no ordinary basement is guaranteed to survive.

Should I seal windows with plastic and tape?

No. Fallout is dust, not gas, and its radiation passes through thin barriers anyway. FEMA says structures do not need to be airtight. Ready.gov does say to close windows and doors and turn off fans and air systems that pull in outside air. Mass and distance are what lower the dose.

Do I need a Geiger counter?

It helps officials, not most households. Our guide on whether you need a Geiger counter covers the exceptions. Radiation readings define the hazard zones, and FEMA stresses relying on measurements. You can follow official instructions without one. Do not leave shelter because a cheap meter shows a low number unless authorities say it is safe.

What do I do if I am outside when fallout falls?

Get inside as fast as you can. Then remove outer clothing, which can take off up to 90% of the contamination per Ready.gov, and wash. A brief time outdoors in the early period is less harmful than staying out, but the dose grows with every minute of exposure.

Lab verdict

You cannot control whether an attack happens or where. You can control four things: knowing the flash rule, knowing your nearest good shelter, having a radio and a day or more of water and food, and staying put until you are told otherwise.

Most of the harm outside the blast zones comes from fallout, and fallout is the part that obeys simple rules. Mass between you and the dust works. Time works, because the radiation decays fast. Wasting those minutes on a car, on a Geiger counter debate or on a panic trip is the usual way to lose both.

Prepare the boring parts now. Pick your shelter spot at home and at work. Keep a battery radio and a change of clothes. Know which large concrete or brick building is closest. Then read the details for your living situation in our apartment guide and the decay timeline.

Sources

• Ready.gov: Nuclear Explosion
• FEMA: Planning Guidance for Response to a Nuclear Detonation, Third Edition (2022)
• CDC: How to Self-Decontaminate after a Radiation Emergency
• CDC: Nuclear detonations, preparing and planning
• REMM (HHS): Nuclear Detonation overview
• LLNL Science & Technology Review: Sheltering Science Saves Lives
• Glasstone and Dolan, The Effects of Nuclear Weapons (1977), via OSTI