What to do if a nuclear explosion happens

Short answer. Get inside, stay inside, stay tuned ¹². After a detonation you will have 10 minutes or more to reach adequate shelter before fallout arrives ¹. Go to a basement or the middle of a large brick or concrete building, away from windows and outer walls ¹². Stay there at least 24 hours unless officials say otherwise or the building is on fire or unsafe ¹³.

What happens in the first seconds?

A nuclear explosion first produces an intense flash of light. For people beyond a few miles, several seconds pass between the flash and the arrival of the blast wave, which travels about a mile every five seconds ². FEMA notes that this may give people who see the flash a few seconds to take cover and reduce injury from flying debris ².

If you are outside when you see the flash, Ready.gov says to lie face down to protect exposed skin from heat and flying debris ¹. If you are inside, move away from windows ¹: FEMA notes that the blast wave can break windows with enough force to injure people standing directly behind them ².

The flash can cause temporary flash blindness more than 10 miles from the detonation in daylight, and farther at night ². In daylight it lasts several seconds; at night, when pupils are dilated, it may last 5 to 10 minutes ². Looking directly at the fireball can burn the retina and leave permanent damage ²⁴. If you are driving, flash blindness is a real road hazard: FEMA expects traffic accidents and blocked roads far from the damage zones ².

How much time do I have to get inside?

For people who survive the blast, FEMA calls getting inside the most important protective action, because it reduces exposure to fallout ². Ready.gov states that you will have 10 minutes or more to find adequate shelter before fallout arrives ¹. FEMA's planning guidance says it is best to be in a shelter when fallout arrives, and that any shelter is better than staying outside for extended periods ².

Use those minutes to reach the best building nearby, not to drive home. FEMA states that cars do not provide adequate shelter ², and the CDC says cars do not provide good protection, while being inside any building is safer than being outside ³.

If you are in a weak shelter, such as a small wood-frame house without a basement, and a better building is close, FEMA gives planning figures: with adequate shelter within 15 minutes' travel, people in poor shelters (protection factor 2) should move within about 30 minutes of the detonation, and people in somewhat better shelters (protection factor 4) can wait an hour or two before moving ². Officials may give you specific instructions about this.

Where is the safest place in a building?

Fallout settles on roofs and on the ground around buildings, so the goal is to put as much distance and dense material as possible between you and the outside ²³.

• Underground first. Basements, subway tunnels and underground parking garages offer the best radiation protection ².
• Otherwise, the centre of a large building. Go to the middle of a large, heavy building, away from the roof and outside walls ²³.
• Brick or concrete beats wood. The safest buildings have brick or concrete walls ¹. Wood, drywall and thin sheet metal give little shielding on their own, although many layers of interior walls add up ².
• Away from windows, doors and corners ².

FEMA describes adequate shelter as one that reduces radiation exposure from fallout by a factor of 10 or more ². The table below summarizes how FEMA classifies common locations.

In FEMA's illustration of building protection factors, a factor of 10 means receiving one tenth of the dose of a person in the open, and a factor of 200 means one two-hundredth ². These figures describe typical buildings for planning; they are not a measurement of any specific building. Assessing a particular structure is a job for a qualified engineer.

How long should I stay inside?

The agencies agree on a minimum of about one day:

• Ready.gov: sheltering usually lasts at least 24 hours ¹.
• CDC: staying inside for at least 24 hours can protect you until it is safe to leave ³.
• FEMA's planning guidance: at least 12 to 24 hours, unless you receive other instructions ².
• Government of Canada: be ready to shelter in place for at least 24 hours ⁵.

The reason is decay. Fallout gives off over half of its energy in the first hour and then continues to decay rapidly ², so sheltering during the first few hours can save lives ⁴. Leave earlier only if you face an immediate threat such as fire, building collapse or a medical emergency ². FEMA explicitly accepts that leaving a burning building is the lower risk, even in a high-radiation zone ².

What should I do once I am inside?

Clean off. Remove your outer layer of clothing, very carefully so radioactive dust does not shake loose ¹. The CDC says this alone can remove up to 90% of radioactive material ³. Shower or wash with soap and water; if you cannot, wipe exposed skin and hair with a wipe or a clean wet cloth ¹. Hand sanitizer does not protect against radioactive material ¹.

Close the building. Close windows and doors to limit outside air, while keeping enough ventilation for indoor air quality ².

Stay tuned. Listen for official instructions. FEMA notes that AM/FM radio is best, with television, mobile phones or the internet as options if they work ². Power will likely be out near the detonation, while AM/FM radios should still receive stations outside the area ², so keep a battery-powered radio.

Stay where you are, even for family. The CDC advises that if loved ones are in schools, day cares, hospitals or care homes, you should stay where you are: going out to get them could expose you and them to dangerous radiation ³. Those facilities have plans in place to keep everyone safe inside ³.

Pets. Keep pets and service animals inside. If they were outside, gently brush their coat to remove particles and wash them with soap and water if available ¹.

What official agencies recommend

U.S. and Canadian agencies give the same core message; they differ mostly on the minimum duration and the level of detail.

• Ready.gov (FEMA): get inside, stay inside, stay tuned; 10 minutes or more before fallout arrives; basement or centre of a brick or concrete building; at least 24 hours; lie face down if caught outside ¹.
• FEMA planning guidance (2022): the best initial action for the public, regardless of zone, is to get inside, stay inside and stay tuned; the message should go initially to everyone within 50 miles unless fallout areas have been identified; shelter at least 12 to 24 hours ².
• CDC: basement or middle of the building; at least 24 hours; removing outer clothing removes up to 90% of contamination; do not go out to collect family ³.
• HHS REMM (medical guidance): fallout exposure is most dangerous in the first few hours; shelter at least 24 hours when possible; interior and below-ground locations give more protection ⁴.
• Government of Canada: nearest building, at its centre or in its basement, away from doors and windows; at least 24 hours; listen to official advice, which may include sheltering, evacuating or taking potassium iodide pills ⁵.

The main difference is the minimum: FEMA's planning document says 12 to 24 hours ², while Ready.gov, the CDC and Canada's public pages say at least 24 hours ¹³⁵. In practice, plan for at least 24 hours and follow local instructions.

Frequently asked questions

Should I get in my car and drive away?

No, not in the first hours. FEMA states that cars do not provide adequate shelter because they lack dense shielding ². Roads may also be blocked by accidents caused by flash blindness ². Get into the nearest solid building and wait for instructions.

What if I am in a wooden house without a basement?

It is still far better than being outside ². Stay as far as you can from the roof and outside walls, with as many walls as possible between you and the outside ²³. If a large brick or concrete building or a basement is within a short walk, FEMA's planning figures suggest moving within about 30 minutes in a very poor shelter ², or follow what officials tell you.

Do I need to tape windows or seal the room?

Not for fallout. FEMA notes that structures do not need to be airtight to protect against fallout radiation, and that buildings with minor damage can be used if they are structurally sound ². Closing windows and doors helps limit dust and smoke ².

Should I take potassium iodide?

Only if public health authorities tell you to. The Government of Canada lists taking potassium iodide pills as one of the instructions authorities may give ⁵. The same page says to take them only when public health authorities directly instruct you to ⁵.

When is it safe to go outside?

When officials say so. Radiation levels drop quickly after the first hours ², but Ready.gov advises staying in your shelter until authorities say it is safe to leave ¹. Stay tuned to official broadcasts ¹².

Make your plan before you need it

Ready.gov counts on 10 minutes or more before fallout arrives ¹: little time to choose a building on the day. Identify the best nearby building or basement at home and at work, and plan to stay at least 24 hours ¹³. Start your Phaenyx plan.

Sources

1. Ready.gov (FEMA) — Radiation Emergencies — https://www.ready.gov/radiation (accessed 2026-10-20)
2. FEMA — Planning Guidance for Response to a Nuclear Detonation, Third Edition (2022), §1.1, §1.3, Chapter 2, §3.2-3.3 (PDF pp. 26-27, 31, 33, 43, 53-55, 75-78) — https://www.fema.gov/sites/default/files/documents/fema_nuc-detonation-planning-guide.pdf (accessed 2026-10-20)
3. CDC — Preparing for a Radiation Emergency — https://www.cdc.gov/radiation-emergencies/safety/index.html (accessed 2026-10-20)
4. REMM, U.S. Department of Health and Human Services — Nuclear Detonation: Weapons, Improvised Nuclear Devices — https://remm.hhs.gov/nuclearexplosion.htm (accessed 2026-10-20)
5. Government of Canada — Radiological and nuclear emergencies – Get prepared — https://www.canada.ca/en/services/policing/emergencies/preparedness/get-prepared/hazards-emergencies/radiological/how-prepare.html (accessed 2026-10-20)