Your immediate priorities are to avoid the initial blast, get inside a sturdy building within 10 minutes, and stay sheltered for at least 24 hours. A nuclear detonation is survivable for people outside the immediate fireball zone, but what you do in the first seconds and hours determines your radiation exposure more than almost any other factor.
The First 10 Seconds: Surviving the Flash and Blast
A nuclear detonation produces an intensely bright flash followed by a blast wave that can destroy structures and cause injuries several miles from the explosion. The flash can cause temporary blindness lasting seconds to minutes, and in some cases up to three hours. During the bombings of Hiroshima and Nagasaki, many people were blinded for several minutes. At night, a relatively small 6-kiloton blast can cause flash blindness up to 20 miles away.
If you see a bright flash or receive a warning of an imminent strike, your actions depend on where you are:
- Outdoors: Get behind anything solid. Lie face down to shield exposed skin from the heat pulse and flying debris. Do not look toward the light.
- In a vehicle: Stop safely and duck below the window line.
- Indoors: Move away from windows immediately. Interior rooms and below-ground spaces offer the best protection from both the blast wave and the thermal pulse.
The blast wave travels slower than the flash. You may have a few seconds between seeing the light and feeling the shockwave, depending on your distance. Stay down until the wave passes.
The Next 10 Minutes: Getting to Shelter
After the blast wave passes, your single most important task is getting inside the best available building before radioactive fallout begins settling from the sky. You generally have 10 minutes or more before fallout arrives, depending on wind conditions and your distance from the detonation. This is your window.
Not all buildings offer the same protection. The difference is enormous. A protection factor tells you how much a structure reduces your radiation exposure compared to standing outside with no cover at all. Here’s what the numbers look like:
- Underground shelters, subbasements of large buildings, tunnels: Protection factor of 1,000 or more. You’d receive less than one-thousandth of the outdoor radiation dose.
- Basements of heavy masonry buildings: Protection factor of 250 to 1,000.
- Basements of brick or concrete multistory buildings (with some exposed walls): 50 to 250.
- Basements of small one- or two-story buildings: 10 to 50.
- Ground floor center of a small brick or concrete building: 2 to 10.
- Aboveground areas of a wood-frame house: 2 or less, meaning almost no meaningful protection.
The takeaway is clear: go underground if you can, and put as much concrete, brick, and earth between you and the outside as possible. If you’re in a wood-frame house, a basement is vastly better than the ground floor. If you have no basement, the center of the ground floor away from windows and exterior walls is your best option. A nearby commercial building, parking garage, or school with a basement is worth moving to if you can reach it within a few minutes.
Staying Sheltered: How Long and Why
Fallout radiation drops rapidly following a predictable pattern called the 7:10 rule. For every sevenfold increase in time after the blast, the radiation intensity drops by a factor of 10. So if the radiation level one hour after detonation is 1,000 units, it drops to 100 units after 7 hours and to 10 units after 49 hours (about two days).
This means the first 24 hours are by far the most dangerous. Staying sheltered during this period avoids the worst of the exposure. Official guidance recommends remaining inside for at least 24 hours unless local authorities give different instructions over emergency broadcast channels. If your shelter is poor (a wood-frame house without a basement, for example), and a much better shelter is nearby, it may be worth relocating early, but only if you can get there quickly.
While sheltering, keep windows and doors closed. Turn off ventilation systems that pull in outside air if you can. Seal gaps around doors and windows with wet towels or tape if available.
Decontamination If You Were Outside
If you were outdoors when fallout began settling, radioactive particles may be on your skin, hair, and clothing. Removing these particles significantly reduces your continued exposure.
Before entering your shelter, remove your outer layer of clothing. This alone can eliminate up to 90% of surface contamination. Place the clothing in a plastic bag and leave it outside or as far from living spaces as possible.
If you can shower, use warm water and plenty of soap. Wash your hair with shampoo or soap, but do not use conditioner, which can bind radioactive particles to hair. Do not scrub or scratch your skin. Cover any cuts or open wounds before washing so radioactive material doesn’t enter. If you can’t shower, wash your hands, face, and any exposed skin at a sink. Gently blow your nose and wipe your eyelids, eyelashes, and ears with a damp cloth or wet wipe, then seal those materials in a bag.
Food, Water, and Supplies
Any food or water that was inside a building or in sealed containers before the blast is safe to consume. You don’t need to worry about canned goods, bottled water, or packaged food that was indoors. If containers were outside during fallout, wipe or rinse them off before opening.
For planning purposes, the International Commission on Radiological Protection recommends stocking 2 liters of drinking water per person per day, plus another 2 to 4 liters for hygiene and decontamination. A supply lasting one to two weeks is strongly recommended. Long-life foods that don’t require cooking or refrigeration are ideal, since power may be out and you want to minimize trips outside your shelter. Don’t forget pet food if applicable.
Potassium Iodide: What It Does and Doesn’t Do
Potassium iodide (KI) protects against exactly one thing: radioactive iodine accumulating in your thyroid gland. A nuclear blast releases radioactive iodine into the air, and your thyroid absorbs it readily, which increases your risk of thyroid cancer, especially in children. Taking KI floods the thyroid with stable iodine so it doesn’t take up the radioactive form.
KI does not protect any other part of your body and is not a general anti-radiation pill. It’s most critical for children, adolescents, pregnant women, and adults under 40. The FDA-recommended adult dose is 130 mg (one standard tablet). Children ages 3 through 18 take 65 mg, and younger children take proportionally less. Adults over 40 have a much lower risk of radiation-induced thyroid cancer and need a higher exposure threshold before KI is recommended.
Timing matters. KI works best when taken shortly before or soon after exposure to radioactive iodine. It’s less effective the longer you wait.
Recognizing Radiation Sickness
Radiation sickness, or acute radiation syndrome, occurs after significant whole-body exposure. The earliest symptoms are nausea, vomiting, and loss of appetite, appearing anywhere from one hour to two days after exposure depending on the dose received.
One of the most deceptive features of radiation sickness is the latent phase. After the initial nausea passes, a person may feel completely normal for hours, days, or even weeks before more serious symptoms emerge. This period of apparent recovery does not mean you’re fine. Anyone who experienced nausea or vomiting within hours of a nuclear detonation should seek medical evaluation when it becomes safe to travel.
At very high doses, symptoms appear within minutes: severe vomiting, diarrhea, confusion, and skin burning. At more moderate doses, early symptoms are milder and take longer to appear. The speed of symptom onset is actually one of the best indicators of how much radiation someone absorbed. Vomiting within an hour suggests a serious exposure. Mild nausea starting a day or two later suggests a lower, more survivable dose.